Pub

licat

ions

of t

he In

stitu

te fo

r Bio

dyna

mic

Res

earc

h, v

ol. 8

, Dar

mst

adt 1

996 Fertilization Systems in Organic Farming

(concerted action AIR3-CT94-1940)

Symbiotic nitrogen fixation in croprotations with manure fertilization

Proceedings of the third meeting in Copenhagen, March4th to 5th, 1996,

edited by Joachim Raupp.

The concerted action is supported by the European Com-munity.

Publications of the Institute for Biodynamic Researchvol. 8

© 1996 by Institute for Biodynamic ResearchBrandschneise 5D- 64295 Darmstadt

Phone: +49 6155 84210Fax: +49 6155 842125

The printed version of this volume was published in 1996 (ISBN 3-928949-07-1);out of print.

Fertilization Systems in Organic Farming(concerted action AIR3-CT94-1940)

Symbiotic nitrogen fixation in croprotations with manure fertilization

Proceedings of the third meeting in Copenhagen, March4th to 5th, 1996,

edited by Joachim Raupp.

Preface

Research on symbiotic nitrogen fixation and on the effective useof fixed nitrogen has a long tradition in several countries. It cov-ers various aspects of crop production, plant nutrition, microbiol-ogy, genetics, biochemistry and mathematics, the latter e.g. bymodelling the enzymatic, physiological and soil biological pro-cesses involved.

These fields of research received a new impetus from organicfarming in which symbiotic nitrogen fixation and N transfer toother crops are basic elements of fertilization. To deal with fertil-ization systems in organic farming is therefore impossible with-out discussing the questions of how to quantify and to managelegume nitrogen during the entire crop rotation period. In thatregard the role of farmyard manure also has to be discussedwith regard to its effects on soil organic matter, nutrient dynam-ics and plant growth.

This was the task of our meeting at the Royal Veterinary andAgricultural University, Copenhagen. We thank Prof. Dr. Niel-sen, Section of Soil, Water and Plant Nutrition, for hosting themeeting at his institute. We are grateful to Henning Høgh-Jensen and Dr. Torsten Müller for organizing the meeting and tothe European Community for its financial support of our project.

Joachim Raupp

Table of contents

1. Symbiotic nitrogen fixation in the context of our project on fertilization systems(J. Raupp) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Symbiotic N2 fixation in clover-grass mixtures and nitrogen transfer fromclovers to the accompanying grass

(H. Høgh-Jensen) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3. Legume nitrogen in crop rotation: Reducing losses - increasing precrop effects(U. Köpke) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4. Residual nitrogen effect of clover-ryegrass sward on a subsequent cereal cropas studied by 15N methodology and mathematical modelling

(H. Høgh-Jensen & J.K. Schjoerring) . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5. Strategies to improve yield and crop quality by different distribution of limitedamounts of farmyard and liquid manure applied to subsequent crops aftergrass-clover

(Karin Stein-Bachinger) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

6. Nitrogen cycling in an organic dairy crop rotation. Effects of organic manuretype and livestock density

(J. Eriksen, M. Askegaard & F.P. Vinther) . . . . . . . . . . . . . . . . . . . . . . . 74

7. Discussion: Nitrogen management in crop rotation. Symbiotic nitrogen fixationand the role of farmyard manure

(summarized by J. Raupp) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

Institute for Biodynamic Research, Brandschneise 5, D-64295 Darmstadt (Germany)

3

1. Symbiotic nitrogen fixation in the context of our project onfertilization systems

By J. Raupp1

Just about 100 years ago, the ability of legumes to use atmospheric nitrogen by means ofa symbiosis with bacteria in their roots was discovered in controlled experiments byHermann Hellriegel. This event was part of a very interesting scientific developmentlasting many decades during the last century and investigating different practical andscientific aspects of nitrogen fertilization (Böhm, 1986). In the light of history this develop-ment shows characteristic phenomena concerning the competition between different orcontradictory scientific conceptions.

The fact of symbiotic nitrogen fixation is of outstanding importance in organic farmingsystems, as the fixed quantities are the only nitrogen source of an organically managedfarm (apart from purchased fertilizers and feedstuffs, whose amounts are very limited, andatmospheric deposition). Whereas the efficiency of asymbiotic nitrogen fixation is very low(Dunger & Fiedler, 1989), by means of the symbiosis considerable but rather varyingamounts can be accumulated, e.g. 31 to 469 kg N/ha in the above-ground parts of differ-ent legume species as main crops (Heinzmann, 1981). The legume nitrogen is availableto the other crops in the crop rotation as green manuring or mulching and, in farms withlivestock, after being used as feed protein and transformed into farmyard manure.

Farmyard manure is of central importance for fertilization in organic farming systems andhas been investigated in some long-term field experiments. Because of these reasonsone of the main objectives of the present project is to evaluate the long-term effects ofmanure fertilization on soil properties, crop yield and product quality. At the beginning ofour project the whole subject was subdivided into certain aspects (Figure 1.1) which areelaborated in the course of our cooperation.

The project in 1995

In the first year our work attended to the evaluation of the basic effects of manure fertiliza-tion on physical, chemical and biological soil parameters and on crop yield (see Figure1.1, sections 1.1, 1.2, 1.3, 1.5 and 3.1). As regards the characteristics of the

MANURE FERTILIZATION SYSTEMS:

A. Type of manure B. Handling method C. Application regimes

A.1 farmyard manure B.1 storage time C.1 quantityA.2 liquid manure/urine B.1.1 storage 1-3 months C.1.1 lowA.3 slurry B.1.2 storage 3-6 months C.1.2 high

B.1.3 storage >6 months C.2 frequencyB.2 conditions C.2.1 once a yearB.2.1 aerobic C.2.2 to a certain cropB.2.2 anaerobic

3. Products

3.1 yield3.1.1 yield formation process

and parameters3.1.2 final yield quantity3.2 quality3.2.1 single parameters3.2.2 complex indicators3.2.3 composed indices

2. Plant health

2.1 microbial antagonists2.2 soil macro fauna

1. Soil organic matter contentand turnover

1.1 C- and N-dynamics1.1.1 C-mineralization1.1.2 N-mineralization1.1.3 org. C-fractions1.2 biochem. parameters1.2.1 enzyme activities1.3 soil conditions1.3.1 physical structure1.3.2 pore volume1.3.3 total C content1.3.4 total N content1.4 energetic aspects1.5 microbial parameters1.5.1 microbial C- biomass1.5.2 microbial N- biomass1.5.3 community structure

4. Environm. impact

4.1 nutrient losses4.2 soil fertility deve-

lopment

5. Methods to investigate sections A to C and 1 to 4

Fig. 1.1: Relevant topics of manure fertilization

5

manure (sections A, B and C), there are some topics on which only few experimentalresults are available, in particular B.1 and C. This means that the main effects of manurefertilization can be predicted, but not the effects of each single combination of the factorsA, B and C on each single unit of the areas 1 to 4.

In general, manure compared with mineral fertilization increases the following parametersof soil biological activity: microbial biomass, enzyme activities, soil respiration, earthwormnumber and/or activity. In most cases (but not as a rule) manure fertilization also in-creases the organic carbon (humus) content in soils or keeps it constant for many years(further main effects are summarized in Raupp, 1995, chapter 2).

At our second meeting (summarized by Raupp et al., 1995) it was concluded that micro-bial biomass, basal respiration and enzyme activities in soils are adequate parameters toassess fertilization systems, compared to physical and chemical soil properties. Thebiological parameters indicate the intensity of nutrient turnover in soils. Depending uponsite conditions fresh or composted farmyard manure seems to be more effective in thatregard.

Concerning soil biological parameters, however, the choice of analytical methods can playa significant role. The chloroform fumigation extraction method (FEM) or the substrateinduced respiration method (SIR) have been regarded as adequate methods to character-ize soil microbe pool sizes. Soil enzyme activities, usually measured under controlledconditions, reflect potentials of activities occurring in the field. The question remains if themeasured potentials really exist in situ or only under the artificial circumstances of the test(Raupp et al., 1995).

Furthermore, we elaborated suggestions for the improvement of methods, identified gapsin knowledge - e.g. concerning the interaction of some parameters of soil biology, planthealth and product quality with the effects of biodynamic preparations - and discussedrelevant questions for future research.

The project in 1996

In many (if not in all) of the processes of soil biology and plant growth and issues of yieldand product quality, a very close involvement of nitrogen can be observed. Transformationand management of nitrogen fractions are the major tasks of fertilization. To be precise:the matter is one of setting the circumstances for organisms or biological processes thatactually execute this. In organic farming systems, as indicated above, nitrogen fertilizationis above all concerned with operationalizing the

6

symbiotically fixed nitrogen by proper agronomic techniques in the entire crop rotation andby an effective use of farmyard manure as a nitrogen vehicle (apart from its other func-tions). These questions are the subject of the present meeting dealing with

- estimation of the fixed N amounts, the N transfer to the accompanying grass and theuse of residual N (Jensen),

- legume nitrogen usage and efficiency in the crop rotation (Köpke),- effects of different manure application systems, varying in time and quantity (Stein-

Bachinger),- nitrogen flows in treatments with different types and quantities of manure (Eriksen).

Most of these contributions take up again elements of the areas A, B and C of Fig. 1.1using mainly crop and environmental parameters (sections 3.1 and 4.1) as criteria.

In addition to that, the other major subject in this year will be the relationship betweenfertilization and the quality of plant products, considering also the role of nitrogen and theinteractions with agronomic techniques and with quality-stimulating preparations. How-ever, we first have to agree on suitable and meaningful parameters, concepts and testsfor food quality. All that will be discussed at our next meeting.

REFERENCES

Böhm, W. (1986) Die Stickstoff-Frage in der Landbauwissenschaft im 19. Jahrhundert. Z. Agrargesch.Agrarsoz. 34, 31-54

Dunger, W. & Fiedler, H.J. (1989) Methoden der Bodenbiologie. G. Fischer V., Stuttgart, New York

Heinzmann, F. (1981) Assimilation von Luftstickstoff durch verschiedene Leguminosenarten und dessenVerwertung durch Getreidenachfrüchte. PhD Thesis Hohenheim

Raupp, J. (1995) (Ed.) Main effects of various organic and mineral fertilization on soil organic matter turnoverand plant growth. Proc. 1st Meeting Concerted Action "Fertilization Systems in Organic Farming", Darmstadt,May 1995

Raupp, J.; Mäder, P.; Fließbach, A. (1995) Summarizing statements of the discussion. In: Mäder, P.; Raupp,J. (Eds.) Effects of low and high external input agriculture on soil microbial biomass and activities in view ofsustainable agriculture. Proc. 2nd Meeting Concerted Action "Fertilization Systems in Organic Farming",Oberwil, September 1995; 76-78

Plant Nutrition Laboratory, Department of Agricultural Sciences, Royal Veterinary and Agricultural

University, DK-1871 Thorvaldsensvej 40, Copenhagen, Denmark

7

2. Symbiotic N fixation in clover-grass mixtures and nitrogen2transfer from clovers to the accompanying grass

By H. Høgh-Jensen 1

SUMMARY

Organic farms normally have large amounts of clover-grass mixture in their rotation system.For temperate grassland, white clover (Trifolium repens) is generally the most importantlegume species. In Denmark 66% of the total organically farmed area is grassland and"green fodder" and only 22% is used for cereals. Of these 66% approximately half is clover-grass in rotation in a typical organic dairy farming system. More precise estimates of thecontribution of symbiotic N fixation and transfer of nitrogen from clovers to grass in both2

cut-and-carry and grazed systems are most certainly needed.

The ability of the total N difference method, the acetylene reduction method, and the N15

isotope dilution method to give accurate estimates of N fixation and transfer is briefly2

reviewed. Evaluating the role of biological N fixation in Danish organic farming systems2

quantities of fixed N in temperate clover-grass mixtures was found to range from 128-3052

kg ha year . This value may be underestimated by 18-25% because fixed-N in root and-1 -12

stubble must also be included in the calculations. Transfer of atmospherically derivednitrogen from clover to the associated grass was found to range from 0-44 kg ha year-1 -1

in cut-and-carry systems. In grazed systems under temperate conditions transfer has beenfound to be as high as 130 kg ha year .-1 -1

The influence of management and environmental parameters on N fixation is discussed2

with special emphasis on age of pasture, access to water, light, and nitrogen. Theseparameters are discussed in relation to theories of N fixation and theories of intercropping.2

Finally, the role and possibilities of using dynamic mathematical simulation in relation to N2

fixation and intercropping are discussed. This includes a summary of the data needed torun the simulation model DAISY.

INTRODUCTION

The fixation of N from the atmosphere is the original source of virtually all the N in soil-2

plant-animal systems, since igneous rocks contain only 10-50 mg N kg (Stevenson, -1

8

1965). In agricultural terms, the most important N fixing bacteria are those of the genus2

Rhizobium, which live symbiotically in the roots of leguminous plants such as clovers, peas,and beans. Of the 90-200 Mt N symbiotically fixed annually in terrestrial ecosystems on aglobal scale (Hauck, 1986; Haynes, 1986) approximately half is fixed by grain legumes andthe other half is fixed by fodder legumes.

Fodder legumes are fairly new agricultural plants in North-western Europe, and systemsincluding clovers have only been of major importance in Denmark for the last 200 years(Kjærgaard, 1995). For temperate grassland, white clover (Trifolium repens) is generallythe most important legume species, especially under grazing conditions. However, also redclover (Lolium perenne) and lucerne (Medicago sativa) are of some importance, althoughmostly in cut-and-carry systems (Frame & Newbould, 1986; Kristensen et al., 1995;Lampkin, 1990; Whitehead, 1995). Especially white clover grows well together with grass,it is tolerant to grazing, it grows well under most temperate climatical conditions, and itcolonizes open gaps in the sward.

Organic farms normally have a large amount of clover-grass mixture in their rotationsystem (Halberg et al., 1995) as it can give a high quality forage (Parsons et al., 1991b)and accumulate organic nitrogen that can benefit subsequent crops (Evans et al., 1992;Høgh-Jensen & Schjoerring, 1996; Könnecke, 1967). Symbiotic dinitrogen fixation istherefore an important source in the nitrogen cycle in organic farming systems, and theclover-grass systems are also called the "fertilizer factory".

In Denmark 66% of the total organically farmed area is grassland and "green fodder" andonly 22% is used for cereals. Of these 66%, approximately half is clover-grass in rotationin a typical organic dairy farming system. This is almost the same as in conventionalsystems; the difference is that the number of animal units is higher in conventional systems(Halberg et al., 1995).

The objective of this paper is to review the methodology of estimating N fixation under field2

conditions. The role of biological N fixation in Danish organic farming systems will briefly2

be evaluated. Finally, the possibilities and requirements for creating mathematicalsimulation models of grassland systems including fodder legumes will be discussed.

METHODS FOR QUANTIFYING N FIXATION2

The amount of N fixed by legumes that is entirely dependent on fixation can be measured2

simply by analysing the accumulation of total N. Three main types of methods have been

Ndfa � total�Nclover�grass � total�Ngrass�only

%Ndfa �

Ndfatotal�Nclover

9

developed to quantify the amount of atmospherically derived N in the legumes when thelegumes are absorbing nitrate or ammonium from the soil: the total N difference method,the acetylene reduction method, and the N isotope dilution method. 15

Total N difference

In the field, atmospherically derived N has traditionally been estimated as the total Nharvested in a clover-grass mixture minus the total N harvested in an adjacent grass grownin monoculture (grass-only) (Eqs. 1 and 2; Munroe & Davies, 1974).

(1)

(2)

As estimates of N fixation often are based on measurements of shoot material (Ta & Faris,2

1987), this implies that the shoot:root ratio between the compared systems must be similar.This further means that the combined plants in the clover-grass mixture compared to thegrass-only must have the same ability to extract soil N and accumulate nitrogen over time.

Using the total N difference method is particularly complicated when dealing withintercropped clovers, or any other intercropped legumes, because intercrop competitionmay affect the ability of the clover and the grass to access soil nitrogen. This has beenobserved under low soil nitrogen conditions (Chalk & Smith, 1994; Høgh-Jensen &Kristensen, 1995).

Of course, these assumptions can never be fully met in biological systems (Rennie, 1984).However, they can be approached under low N conditions.

The method can be improved by extending it from only using shoot nitrogen to includenitrogen in the root system and leaf litter (Hauser, 1992). However, the extraction of fineroots and nodules from the soil system is very difficult. This is especially the case in semi-perennial systems such as clover-grass swards because the root system will have had acertain turnover (Robertson et al., 1993) leading to pools of more or less degraded root-derived organic matter.

%Ntrans � total�Ngrass�mix � total�Ngrass�only

10

Finally, including considerations of differences in soil nitrate (Hauser, 1992) and soilammonium content between the time of seeding and harvest can improve the method.This, however, can in no way be taken to be caused by plant uptake, as immobiliza-tion/mobilization processes and leaching will influence these calculations. It can beassumed that differences related to these processes are equal under the clover-grassmixture and grass-only (Hauser, 1992). This assumption, however, will most likely beinvalid when comparing clover-grass and grass-only systems as a consequence of the verybasic idea of build-up of organic nitrogen under clover-grass swards (Clement & Williams,1964; 1967).

Estimation of transfer of atmospherically derived nitrogen from clover to the accompanyinggrass can be estimated as:

(3)

In Eq. 3 it is assumed that the ability to search for nutrients for grass in mixture and grass-only is identical. This assumption has been observed to be invalidated under a wide rangeof soil N conditions (Høgh-Jensen, unpublished data).

Acetylene reduction

Since Dilworth (1966) discovered that acetylene is reduced to ethylene by nitrogenase, thisassay has been a very popular technique for measuring nitrogenase activity. The acetylenereduction technique for measuring N fixation is based on the fact that the nitrogenase2

enzyme system, in addition to catalysing the chemical reduction of gaseous N to2

ammonia, also catalyses the reduction of acetylene to ethylene (Hardy et al., 1968). Whenthis method is used in controlled environmental conditions, whole plants or detached rootsystems are enclosed in a vessel containing acetylene, usually at an enzyme-saturatingconcentration of 10%, and subsequent accumulation of ethylene is determined by gaschromatography.

The technique is based on the assumption that nitrogenase activity is not affected by thesubstitution of acetylene for N . This may be valid initially but activity has been found to2

decline in many legume species after a few minutes' exposure to acetylene. This declineis now attributed to a depression of the oxygen diffusion in the nodule (Witty et al., 1984;Drevon et al., 1988). However, it may be feasible to obtain a value for a longer period bymaking repeated short-term measurements of nitrogenase activity to allow for diurnal andseasonal fluctuations, but, in general, conclusions drawn from experiments in which theclosed vessel approach has been used must be questioned (Minchin et al., 1985). Todayflow-through systems are used.

11

An additional problem is that, when calculating results, it is necessary to assume a specificratio between the acetylene reduction and N fixation. The reduction of N to ammonia2 2

requires 8 electrons (including 2 electrons for the associated reduction of 2 protons to H ),2

while the reduction of acetylene to ethylene requires 2. This gives a theoretical ratio of 4:1.However, measured values vary a lot and are often greater than 4:1 (Minchin et al., 1983).

Measuring under field conditions requires to dig-up the plants’ root system and exposingit to the gas. Complete recovery of the root system with all nodules attached is not possible. Furthermore, disturbance of the root system influencesthe diffusion barrier of the nodule. In agreement with abundant literature, it can beconcluded that this method is not usable for obtaining quantitative estimates of N fixation2

under field conditions. Furthermore, the method can give no estimate of transfer.

N isotope dilution15

In studies of nitrogen fixation and nitrogen transfer, stable nitrogen isotopes are the mostreliable tools. The two stable nitrogen isotopes used as tracers are N with a natural14

atmospheric abundance of 99.6337% and N with a natural atmospheric abundance of15

0.3663 ± 0.0004 atom%. The use of techniques based on application of N-enriched15

nitrogen compounds to the soil in small plots has increased in recent years, starting withMcAuliffe et al. (1958). Costs of enriched N material restrain the use of this method under15

field conditions.

Alternatively, the natural difference in N abundance between atmospheric nitrogen and15

soil nitrogen may be used to assess nitrogen fixation (see review by Shearer & Kohl, 1986).This technique relies on the fact that most transformations in the nitrogen cycle showisotopic fractionation causing a slight N enrichment (5-10‰) of soil nitrogen relative to15

atmospheric nitrogen (Handley & Raven, 1992). Until recently, the lack of fast and preciseanalytical techniques limited the use of the natural N abundance technique, and it has so15

far only been used in few experiments (Bergersen et al., 1990; Høgh-Jensen &Schjoerring, 1994; Ledgard & Peoples, 1988; Sanford et al., 1994; Shearer & Kohl, 1986).

Both the N enriched and the natural 15N abundance techniques are based on the15 15

comparison of the N composition of clover and grass, where the difference is assumed15

to be caused by N fixation (Peoples et al., 1989). Both methods assume that grass and2

clover roots recover nitrogen with similar isotopic ratios from the same soil pools, and thatany isotopic discrimination is similar in grass and clover.

%Ndfa � [ 1 �

�15Nclover

�15Ngrass � B

] × 100

%Ntrans � [�

15Ngrass�mix

�15Ngrass�only

] × 100

12

The fraction of nitrogen derived from the atmosphere in harvested clover material (%Ndfa)is calculated as (Ledgard & Peoples, 1988):

(4)

where � N is the per mille N enrichment relative to atmospheric nitrogen (Shearer &15 15

Kohl, 1986), and B is the N enrichment, relative to atmospheric dinitrogen, of the clover15

grown solely on atmospheric nitrogen.

A problem with the N techniques is the selection of a proper reference plant with a growth15

pattern and a rooting distribution that match that of the legume. This can be alleviated byusing slow-release N fertilizers (Giller & Witty, 1987) or, even better, by using N-labelled15 15

organic matter applied to the experimental area years in advance.

Such precautions will also improve estimates of transfer of atmospherically derivednitrogen. The fraction (%Ntrans) of atmospherically derived nitrogen transferred from cloverto grass can be calculated as (Vallis et al., 1967):

(5)

Other approaches that can be used to calculate %Ntrans have been proposed as well(Chalk & Smith, 1994; Ta & Faris, 1987). They tend to become increasingly complicatedbut accuracy is not significantly improved. This, however, is mostly because manyexperiments are based on application of N-enriched fertilizer to the experimental area the15

same or the previous year of the measurements.

QUANTITIES OF FIXED N AND TRANSFER FROM CLOVER TO GRASS IN2

TEMPERATE GRASSLAND SYSTEMS

Estimation of N fixation from fodder legumes varies widely. This reflects not only2

difficulties of precise measurement and differences in the fixation ability of different fodderlegumes but the impact of different farming practices (Sprent & Sprent, 1990). For whiteand red clovers grown in mixtures with grasses under low N input and temperate conditionsestimates range from 128-305 kg N ha year (Boller & Nösberger, 1987; Hoglund &-1 -1

Brock, 1978; Hoglund et al., 1979; Nesheim & Øyen, 1994) and, specifically, under Danish

13

conditions from 138-208 kg N ha year (Høgh-Jensen & Schjoerring, 1994; Koefoed &-1 -1

Klausen, 1969; Kristensen et al., 1995; Pedersen & Møller, 1976).

The above mentioned estimates are solely based on above-ground shoot material. Thesevalues have been calculated to underestimate N fixation by 18-25% when including the2

fixed N in stubble and roots (Høgh-Jensen & Kristensen, 1995). 2

Studies of transfer of atmospherically derived nitrogen from clover to grass are few (seediscussion of Høgh-Jensen & Schjoerring, 1994), which to a large extent reflects themethodological difficulties (see Chalk & Smith, 1994). Mostly these studies have beenconducted under controlled conditions in pots (e.g. McNeill & Wood, 1990). Under fieldconditions, 0 to 37 kg N ha of transfer N was observed in the first production year-1

whereas the transfer can amount to as much as 18-44 kg N ha in the second production-1

year (Table 2.1). These values, however, depend on the calculation approach asmentioned in relation to equation 5. The application of N to the growth media is the key15

factor in order to get reliable estimates of %Ntrans (Chalk & Smith, 1994; Høgh-Jensen &Schjoerring, 1994) because an isotopic equilibium is an assumption in relation to equation4 and 5. Such an equilibrium is of course difficult to obtain.

Tab. 2.1: Symbiotic N fixation (%Ndfa) as estimated by the enriched N dilution technique and the transfer215

of atmospherically derived nitrogen in the associated grass (%Ntrans) as estimated by the natural N dilution15

technique.

Preceding crop 1992 1993%Ndfa %Ntrans %Ndfa %Ntrans

Clover-grass 9cuts+400N 64 10 83 n.d.Clover-grass 9cuts+0N 96 9-37 97 18-19Clover-grass 6cuts+400N 50 6 77 n.d.Clover-grass 6cuts+0N 94 0 96 0-44

Tab. 2.2: Amounts of atmospherically derived nitrogen fixed in white clover shoots and transferred from theclover to the associated grass as estimated by the enriched N technique (from Ledgard, 1991).15

Source of nitrogen kg N ha year-1 -1

N fixation (in clover shoots) 2692

Transfer of fixed-N via dung and urine 602

Transfer of fixed-N via root/nodule/shoot turnover 702

Chalk & Smith (1994) caution that the grass component might benefit from a N-sparringeffect of the legume; this will occur if less N-labelled fertilizer is used by the grass in15

14

mixture compared to grass-only, assuming that grass has the same plant density in the twosystems. Nevertheless, this has not earlier been observed (Høgh-Jensen & Kristensen,1995) and, furthermore, the technique based on natural N abundance will eliminate this15

potential error.

Tab. 2.3: Amounts of N transferred from clover to grass in cut and grazed white clover-ryegrass mixtures asestimated by the total N difference method (from Shaw et al., 1966).

Age of pasture Cut Grazedkg N ha year kg N ha year-1 -1 -1 -1

3 43 674 79 89

The seasonal pattern of fixation activity in white clover has seldom been studied. However,Masterson and Murphy (1976), using acetyle reduction methodology, studied this underIrish conditions and found soil temperature to be the dominant factor regulating noduleactivity. They observed fixation activity down to soil temperatures of 4-5°C. This is inagreement with Gordon et al. (1989) who found that it was poor growth that restricted plantperformance - not a low fixation rate.

Most studies have been performed in cut-and-carry systems. This is a consequence of thedynamic grazing animal induce on the systems, including preferential grazing, trampling,urine and dung, among others. In an impressive study Ledgard (1991) has addressed thisproblem, observing that the recycling of atmospherically derived nitrogen was much largerthan in cut-and-carry systems (Table 2.2 and 2.3), mainly via dung and urine but also fromleaf litter following trampling.

INFLUENCE OF ENVIRONMENTAL PARAMETERS ON N FIXATION2

Generally we assume that change in species composition in ryegrass and white clovercommunities is caused by changes in the ratio of white clover growing points to ryegrasstillers. This ratio is dependent on temperature (Clark et al., 1995) and influenced by accessto inorganic nitrogen. It is most likely that access to other nutrients will influence the ratioas well (Marschner, 1995). Access to water might also influence the ratio but this isbasically not known. All together, this means that clover-grass mixtures are intrinsicallyunstable (Parsons et al., 1991a; Turkington & Mehrhoff, 1990) where the species thatpossesses a physiological advantage is shown progressively to dominate the mixture. Asneither white clover nor ryegrass have a mechanism to escape grazing or cutting this isconsidered the case under all management types.

15

Assuming that N fixation is a parameter dependent on yield (Hoglund & Brock, 1978;2

Köpke, 1995) old Danish experiments were re-investigated using the total N differencemethod (Kristensen et al., 1995). This investigation clearly revealed that the clover contentof the harvested herbage explained most of the variation in %Ndfa (Table 2.4) and that theage of pasture was significant negatively correlated to %Ndfa.

These observations lead to the proposal of a simple empirical model of %Ndfa asdetermined by pasture age and clover content of the herbage (Table 2.5). It must be notedthat this type of model has its relevance under practical circumstances; not in researchdetermining N fixation. This is clearly demonstrated by the variation in N fixation per ton2 2

clover DM: 27 (Boller & Nösberger, 1987), 45-55 (Hoglund & Brock, 1978), 14-41 (Høgh-Jensen & Kristensen, 1995).

Tab. 2.4: The influence of soil type, water access, and age of sward on clover yield and N fixation based on2

a re-analysis of old Danish experiments (modified after Gregersen, 1980; Koefoed & Klausen, 1969;Kristensen et al., 1995).

Experimental factors DM yield %Clover %Ndfa %Ndfacontent uncorrected corrected

ton ha (DM basis) for %clover for %clover-1

Soil type:Sandy 7.6 18 82 108Sandy loam 9.5 29 126 117P-value 0.01 0.02 0.63

Irrigation:Non-irrigated 7.2 19 81 104Irrigated 9.9 28 127 120P-value 0.002 0.01 0.37

Pasture age:1 10.9 29 156 1362 9.5 33 159�3 7.9 25 83 � 4 7.3 17 73 � 89

5 7.3 12 53 � P-value 0.01 0.10 0.004 0.04

Influence of inorganic and organic nitrogen on N fixation2

Much theoretical work to date has focused on competition for light and the supply of carbon

substrate for growth (e.g. Thornley & Johnson, 1990) but little attention has been paid to

the competition for inorganic nitrogen (but see Ross et al., 1972).

16

Whether the N fixation is decreased by limited supply of photosynthetic products to the2

nodule or decreased by an inhibitory nitrogen effect has been a standing debate for years

(Streeter, 1988; Parsons et al., 1993c) without a satisfying theory that can explain all the

observed phenomena in different legumes. A recent contribution by Sinclair & Serraj (1995)

might lead to further clarification as it now seems likely that N fixation in white clover can2

be restricted by limited supply of photosynthetic products. Whatever the outcome of this

debate will be it has often been demonstrated that access to inorganic nitrogen will

decrease N fixation through a substitution effect between atmospheric N and soil N /2

fertilizer N.

For a given external concentration of nitrogen the uptake rate depends on the nutrient

demand of the plant as demonstrated in the classic study of Clement et al. (1978) implying

that the plant regulates the nitrogen uptake in agreement with the Dijkshoorn-Ben Zioni

model (Ben Zioni et al., 1971; Dijkshoorn et al., 1968). Originally, they proposed that the

carboxylate (mainly malate) ions synthesized in shoot during NO assimilation are3-

transported to the root where decarboxylation leads to the formation of HCO ions, which3-

are released into the external solution (the so-called pH stat). Furthermore, a cycling-loop

of K ions supplies the accompanying cations for NO in the xylem and the carboxylate+ -3

anions in the phloem.

Tab. 2.5: Atmospherically derived nitrogen (kg N ha year ) at varying clover contents in herbage dry matter-1 -1

and pasture age based on a re-analysis of old Danish experiments as modified after Kristensen et al. (1995)

and Høgh-Jensen & Kristensen (1995).

Pasture age Clover content (% of dry matter)3-16 17-29 above 29

1-2 year:Høgh-Jensen & Kristensen (1995) 72 127a

Koefoed & Klausen (1969) 54 115 172b

Pedersen & Møller (1976) 74 117 208b

Gregersen (1980) 63 123 193b

3-5 year:Gregersen (1980) 47 84 128b

) Estimated by the enriched N dilution technique.a 15

) Estimated using the total N difference method (Kristensen et al., 1995).b

17

The importance of the Dijkshoorn-Ben Zioni model is that it proposes a mechanism

whereby products of NO reduction in the shoot might control NO uptake by the roots, by3 3- -

providing HCO ions for exchange against the NO absorbed (Touraine et al., 1994). The3 3- -

idea that the phloem-translocated amino acids control NO uptake is currently acknowl-3-

edged (Marschner, 1995; Touraine et al., 1994) but the precise nature of the regulatory

signals remains to be discovered, as does the mechanism by which the amino acids

repress the NO uptake system. However, it is very interesting to note how this regulatory3-

mechanism, through which the intensity of cycling of elements in the plant generally

integrates nutritional status of the whole plant (Touraine et al., 1994), is similar to the

model proposed by Parsons et al. (1993c) in the regulation of N fixation. 2

It is often generally assumed that the ability to fix atmospheric nitrogen will reduce the

legumes competitive strength towards its non-fixing partners in the intercropping system

(Vandermeer, 1989). This assumption is probably based on the frequently observed

phenomena that in competition with e.g. grass the fixing clover will not absorb significant

amounts of inorganic nitrogen. However, this assumption is not empirically based; e.g.

interactions between dinitrogen fixation and access to other nutrients (P, K), water, and

energy are not well understood in intercropping systems.

When comparing the effects of liquid manure, solid fertilizer and an aqueous fertilizer

solution, all containing the same amounts of N and K and resulting in similar DM yields, the

liquid manure gave the highest - but unexplained - clover content in the herbage (Drysdale,

1965; Schechtner et al., 1980). Similar effects have only been found on K-deficient soil in

a Danish investigation (Iversen, 1943), whereas on K-sufficient soils a decrease in clover

content was observed due to the nitrogen stimulation of the grass component. Liquid

manure N and urea N apparently inhibit clover less than mineral fertilizer N (Drysdale,

1966; Frame & Newbould, 1986). However, Iversen (1943) attributes such effects to the

gaseous loss of N when applying urine N.

On-going experiments with application of solid manure to clover-grass mixture on Risø

National Laboratory suggest that the grass component will absorb the main body of the

mineralized nitrogen (Finn Jørgensen, personal communication). As further information on

this subject is not available in existing literature it can be speculated that the smaller

inhibitory effect of organic nitrogen compared to similar amounts of inorganic nitrogen is

18

caused by the slower release. It would consequently mean that application of organic

nitrogen will be a source for competition between grass and clover after mineralization

similar to indigenous soil organic nitrogen.

Influence of water availability on N fixation2

Availability of water in the soil can decrease both photosynthetic activity and dinitrogen

fixation rate. It is not known which process first influences the plant growth under drought

conditions. Over the past ten years many studies of empirical and simulation character have

showed that legume nodules exercise physiological control over their permeability to O2

diffusion, thereby protecting nitrogenase from O inactivation following drought stress (e.g.2

Walsh, 1995). It can be suggested, that white clover belongs to the non-sensitive group

(Sinclair & Serraj, 1995) implying that even under drought conditions, white clover does not

experience N scarcity but that the limitations on growth are on the photosynthetic process.

If we turn towards the aspects related to growing different species intermixed, the inclusion

of another species can modify the water environment. However, how this happens and to

what extent will depend much on the chosen system and its management.

Influence of light on the N fixation2

Information about canopy structure is necessary in order to understand the processes in

mixtures of morphologically different species as the structure indirectly can affect such

processes as photosynthesis, transpiration, cell enlargement, pathogens and insects,

residue decomposition, soil evaporative losses, soil temperature, and photomorphogenesis.

Therefore, the canopy structure affects the competition between two species in a very

dynamic manner and as N fixation in white clover is affected by the competitive pressure2

by grass light and canopy structure might also indirectly affect N fixation.2

Such canopy structures are often described statistically as time or space averages

(Campbell & Norman, 1989). As plants grow, they may begin to overlap so that it is difficult

to discern the outline of a particular plant or row of plants. This is termed 'canopy closure'

where-after most processes related to photosynthetic processes can be treated using one-

dimensional theory, which allows us to dramatically simplify the convective and radiative

19

processes (i.e. the "superleaf" approach).

Canopies of agriculturally important crops are often described in terms of two parameters;

the average leaf area and the angle distribution of the leaves (Campbell & Norman, 1989).

The latter is very difficult to measure and consequently simplified or idealized leaf angle

distribution functions have been used widely to approximate actual leaf angle distributions

(e.g. Campbell, 1986). Nevertheless, direct measurements of both leaf area and leaf angle

have been conducted (e.g. Lemeur, 1973) dividing the canopy in up to six layers.

It is often attempted to describe radiation through a canopy by a negative exponential

function (Monsi & Saiki, 1953; Russell et al., 1989) (called Beer's law or the Monsi-Saiki

equation), which can be measured directly by placing 5-10 point sensors below the canopy.

Following this approach the canopy can be described by leaf area index and the extinction

coefficient (k). The k value is normally higher for white clover than for grass (0.8 vs. 0.5)

because of the more prostrate nature of clover leaves (Davidson et al.,1982; Thornley &

Johnson, 1990). However, for mixed clover-grass swards that have adapted to either

continuous grazing or frequent cutting, the relative leaf distribution is approximately

homogeneous (Johnson et al., 1989). As grass and clover have similar photosynthetic

characteristics (Woledge & Dennis, 1982) the effect of nitrogen on the photosynthetic

capacity is expected to be found in grass only.

THEORIES ON COMPETITION

It is generally stated in literature that different morphologies are important traits in the

competition for resources (Grime, 1993; Tilman, 1990). Of course physiological traits also

influence plants' ability to compete for nutrients. Two aspects of the physiology are thought

to be important, namely the kinetics of nitrogen uptake and the place of absorption in the

plant.

An attempt to provide a definitive statement on the role of competition in structuring pastures

is difficult, although evidently it is important (de Wit et al., 1966; Grime, 1993; Haynes, 1980;

Menchaca & Connolly, 1990; Tilman, 1990; Vandermeer, 1989). Evolutionary theory (Lawlor

and Smith, 1976; Pianka, 1983) predicts that inter- and intraspecific competition results in

selection that decreases competition by either niche divergence or niche expansion,

20

respectively. Alternate theories predict that interspecific competition leads to increased

competitive ability through niche convergence or increased interference mechanisms

(Berendse & Elberse, 1990; Lüscher et al., 1992). What few appropriate data are available

tend to support the more traditional view of interspecific competition leading to niche

differentiation (Turkington & Mehrhoff, 1990). However, pasture communities have other

complicating and interacting factors such as pathogens, mycorrhiza, and Rhizobium strains.

Concerning the competition and the balance between white clover and ryegrass in the sward

then it is likely that the competitive ability mainly is determined by a lower specific leaf area

after canopy closure for clover above-ground and a smaller nitrogen uptake efficiency (kg

N per kg root dry matter per time unit) of the nodulated clover root than of the ryegrass root,

as proposed by Thornley et al. (1995). However, as indicated in this brief review, this will

only be rough generalizations that in no way properly reflect the dynamic nature of plant

adaptations to their environment combined with adaptations of the environment by the plant

community.

Traditional statistical models are not sufficiently able to provide an understanding of the

processes within the system and the complex interactions between plant processes and

plants and their environment. Therefore, research is moving towards more holistic and to

some extent multidisciplinary studies. Systems theory seems especially well suited as a

scientific approach in these studies (Høgh-Jensen, 1996). Systems theory and computer

simulation are tools that enable us to adopt a multidisciplinary approach and deal with

complex interactions and systems dynamics.

DYNAMIC MODELLING

Science is concerned with explanation and to some extent with prediction. This we can only

do by virtue of having models or conceptual schemes of the world. Broadly speaking, plant

and crop modelling has two aims: 1) to increase knowledge in science and 2) to find

strategic solutions of current problems. The mechanistic model DAISY (see further

description in chapter 4 of this volume) operates with three different crop production levels

following the approach of de Wit (1982) and Penning de Vries (1982). Level 1 is the potential

production where the crop is supplied with ample water and nutrients during the whole

21

growth period. Level 2 incorporates water limitations, and level 3 incorporates nitrogen

limitations. De Wit also proposed a level 4 incorporating P-limitations or other nutrient

deficiencies. For further understanding of the role of modelling in science see Thornley &

Johnson (1990) or one of several contributions on this topic from the Production Ecology

Graduate School at Wageningen Agricultural University (e.g. de Wit, 1982; Penning de

Vries, 1982; Rabbinge et al., 1990).

In the following we will provide a summary of the data needed for running the model DAISY,

which is developed and very much used at our department. However, other models are

under development based on soft-ware that enables the "non-computer-freak" to work with

dynamic modelling.

Site-specific data

1. Meteorological data, that is date, global radiation (W m ), air temperature in 2 m height-2

(�C), precipitation (mm), and preferably also potential evapotranspiration (mm).

2. Soil definitions/characteristics, that is

2a) soil retention curve for each horizon (or at least field capacity and wilting point, clay,

silt, sand content, porosity, particle density, number of and depths of horizons), and

2b) soil hydraulic conditions, as a minimum K , K , ground water table (depth,sat pf=2

constant, fluctuating),

2c) max. rooting depth for that soil.

3. Inorganic initialisation, that is number of horizons (no connection to point 2) and initial

conditions of nitrate and ammonium content.

4. Organic initialization, that is C:N in soil microbial biomass and total C and total N

content in user-defined horizons.

Management-specific data

5. Tillage operations, that is date, type, and depth of tillage.

6. Fertilization, that is

6a) Inorganic: application method (surface, broadcast, incorporation depth) and amount

(kg N ha , per cent NH ),-1 +4

6b) Organic: incorporation depth, dry matter, %C and %N in dry matter, % NH in4+

22

applied total N.

Crop-specific data

7. Management of the specific crops, that is max. rooting depth for crop, stubble dry

matter for cereals and grass, harvest index for cereals, photosynthetically active

stubble, root/shoot ratio or ratio allocated to roots, primary yield, harvest index, dry

matter and N content of different plant parts.

In order to create a mechanistic sub-module in DAISY for N fixation we need to understand2

how water limitations and access to inorganic nitrogen influence N fixation in legumes. This2

information is not available today and a module will therefore consequently be based on

empirical correlations.

To further develop the model to include intercropping we need to achieve knowledge of the

turnover of root systems - especially in semi-perennial systems like clover-grass - and

consequently incorporate a kind of "dead-rate" per time unit in the module. As intercropping

means competition or coexistence this can be incorporated in the model as a mechanistic

module based on eco-physiological understanding of the competition between plants based

on distribution of light, water, and nutrients between the species, but also the way in which

species utilize the amounts taken up for production (Kropff & van Laar, 1993).

Furthermore, in future developments of the simulation model it will be relevant to include the

so-called "fourth production level" of P- and/or K-deficiency as agricultural science

increasingly must address problems associated with low-input conditions.

CONCLUSIONS

The preferential choice of methodology when estimating N fixation is N isotope dilution2 15

techniques. However, this requires N labelling of the growth media long time in advance.15

Alternatively, the natural N abundance technique can be utilized.15

N fixation seems less affected by application of organic fertilizer than inorganic fertilizer.2

23

However, this must be investigated further. We need greater knowledge about variation in

dinitrogen fixation under grazing conditions. Knowledge about turnover of root systems in

semi-perennial systems like clover-grass must be generated.

Generally, application of fertilizer beyond 100 kg N ha will increase the competition from-1

grass for energy and nutrients with the consequence that the amount of fixed N is reduced2

because the more fertile the soil is, the more the grass component will contribute to the total

yield of the pasture and thus increase the competition pressure on the clover component.

Transfer of atmospherically derived nitrogen from clover to grass is not likely to exceed 20-

40 kg N ha year in cut-and-carry systems. This, however, can be 50% of the N-1 -1

accumulation in the grass. Transfer is most likely to increase under grazing conditions as

a consequence of trampling and animal excreta. Estimates of transfer of atmospherically

derived N to the accompanying crop must be based on N isotopes. Natural abundance of15

N is a potential powerful tool for getting more accurate estimates of transfer.15

Modelling N fixation must be based on empirical correlations as we so far haven't achieved2

an adequate eco-physiological understanding of limitation and regulation of the N fixation2

process. On the other hand, modelling intercropping and competition can be based on

mechanistic approach.

REFERENCES

Ben Zioni, A., Vaadia Y. & Lips, S.H. (1971) Nitrate uptake by roots as regulated by nitrate reduction products

of the shoot. Physiologia Plantarum, 24, 288-290.

Berendse, F. & Elberse, W.T. (1990) Competition and nutrient availability in heathland and grassland

ecosystems. In: Perspectives on Plant Competition (Grace, J.B. & Tilman, D., eds.). San Diego: Academic

Press, Inc. pp. 93-116.

Bergersen, F.J., Peoples, M.B., Herridge, D.F. & Turner, G.L. (1990) Measurement of N natural abundance2

in the management of legume crops: roles and precautions. In: Nitrogen Fixation: Achievements and Objectives

(Greshoff, P.M., Roth, L.E., Stacey, G. & Newton, W.E., eds.). Proc. 8th Int. Congress on Nitrogen Fixation.

Knoxvill. Tenn. USA. 1990. New York: Chapman and Hall. pp. 315-322.

24

Boller, B.C. & Nösberger, J. (1987) Symbiotically fixed nitrogen from field-grown white and red clover mixed with

ryegrasses at low levels of N-fertilization. Plant and Soil, 104, 219-226.15

Campbell, G.S. (1986) Extinction coefficients for radiation in plant canopies calculated using an ellipsoidal

inclination angle distribution. Agriculture and Forestry Meterology, 36, 317-321.

Campbell, G.S. & Norman, J.M. (1989) The description and measurement of plant canopy structure. In: Plant

canopies: their growth, form and function (Russell, G., Marshall, B. & Jarvis, P.G., eds.). Cambridge: Cambridge

University Press. pp. 1-20.

Chalk, P.M. & Smith, C.J. (1994) N isotope dilution methodology for evaluating the dynamics of biologically15

fixed N in legume-non-legume associations. Biolology and Fertility of Soils, 17, 80-84.

Clark, H., Newton, P.C.D., Bell, C.C. & Glasgow, E.M. (1995) The influence of evaluated CO and simulated2

seasonal changes in temperature on tissue turnover in pature turves dominated by perennial ryegrass (Lolium

perenne) and white clover (Trifolium repens). Journal of Applied Ecology, 32, 128-136.

Clement, C.R. & Williams, T.E. (1964) Leys and soil organic matter. I. The accumulation of organic carbon in

soils under different leys. Journal of Agricultural Science, 63, 377-383.

Clement, C.R. & Williams, T.E. (1967) Leys and soil organic matter. II. The accumulation of nitrogen in soils

under different leys. Journal of Agricultural Science, 69, 133-138.

Clement, C.R., Hopper, M.J. & Jones, L.H.P. (1978) The uptake of nitrate by Lolium perenne from flowing

nutrient solution. I. Effect of NO -concentration. Journal of Experimental Botany, 29, 453-464.3

Davidson, I.A., Robson, M.J. & Dennis, W.D. (1982) The effect of nitrogenous fertilizer on the composition,

canopy structure and growth of a mixed grass-clover sward. Grass and Forage Science, 37, 178-179.

Dijkshoorn, W., Larthwell, D.J. & de Wit, C.T. (1968) Temporal changes in carboxylate content of ryegrass with

stepwise change in nutrition. Plant and Soil, 29, 369-390.

Dilworth, M.J. (1966) Acetylene reduction by nitrogen-fixing preparations from Clostridium pasteurianum.

Biochim. Biophys. Acta, 127, 285-294.

Drevon, J-J., Kalia, V.C., Heckmann, M-O. & Pedelahore, P. (1988) In situ open-flow assay of acetylene

25

reduction activity by soybean root nodules: influence of acetylene and oxygen. Plant Physiology and

Biochemstry, 26, 73-78.

Drysdale, A.D. (1965) Liquid manure as a grassland fertilizer. 3. The effect of liquid manure on the yield and

botanical composition of pasture and its interaction with nitrogen, phosphate and potash fertilizers. Journal of

Agricultural Science, Cambridge, 65, 333-340.

Evans, D.R., Williams, T.A. & Mason, S.A. (1992) Residual N effect of grazed white clover (Trifolium

repens)/ryegrass (Lolium perenne) swards on subsequent yield of spring barley. Journal of Agricultural Science,

118, 175-178.

Frame, J. & Newbould, P. (1986) Agronomy of white clover. Advances in Agronomy, 40, 1-88.

Giller, K.E. & Witty, J.F. (1987) Immobilized N-fertilizer sources improve the accuracy of field estimates of N -152

fixation by isotope dilution. Soil Biology & Biochemisty, 19, 459-463.

Gordon, A.J., Michell, D.F., Ryle, G.J.A. & Powell, C.E. (1989) White clover N -fixation in response to root2

temperature and nitrate. II. N -fixation, respiration and nitrate reduction activity. Journal of Experimental Botany,2

40, 527-534.

Gregersen, A.K. (1980) Vand og kvælstofgødskning til flerårig græs og kløvergræs. Tidskrift for Planteavl,

84,191-208.

Grime, J.P. (1993) Stress, competition, resource dynamics and vegetation processes. In: Plant Adaptations to

Environmental Stress (Fowden, L., Mansfield, T. & Stoddart, J., eds.). London: Chapman & Hall. pp. 45-63.

Halberg, N., Kristensen, E.S. & Kristensen, I.S. (1995) Nitrogen turnover on organic and conventional mixed

farms. Journal of Agriculture and Environmental Ethics, 8, 30-51.

Handley, L.L.; & Raven, J.A. (1992) The use of natural abundance of nitrogen isotopes in plant physiology and

ecology. Plant, Cell and Environment, 15, 965-985.

Hardy, R.W.F., Holsen, R.D., Jackson, E.K. & Burn, R.C. (1968) The acetylene - ethylene assay for N fixation:2

laboratory and field evaluation. Plant Physiology, 43, 1185-1207.

26

Hauck, R.D. (1986) A human ecosphere perspective of agricultural nitrogen cycling. In: Advances in Nitrogen

Cycling in Agricultural Ecosystems (Wilson, J.R., ed.). Wallingford: CAB International, pp. 3-19.

Hauser, S. (1992) Estimation of symbiotically fixed nitrogen using extended N difference methods. In: Biological

Nitrogen Fixation and Sustainability of Tropical Agriculture (Mulongoy, K., Gueye, M. & Spencer, D.S.C., eds.).

Chichester: John Wiley & Sons, pp. 309-321.

Haynes, R.J. (1980) Competitive aspects of the grass-legume association. Advances in Agronomy, 33, 227-261.

Haynes, R.J. (1986) Origin, distribution, and cycling of nitrogen in terrestrial ecosystems. In: Mineral nitrogen

in the plant-soil system (Haynes, R.J., ed.). Orlando: Academic Press, Inc., pp. 1-51.

Hoglund, J.H. & Brock, J.L. (1978) Regulation of nitrogen fixation in a grazed pasture. New Zealand Journal

of Agricultural Research, 21, 73-82.

Hoglund, J.H., Crush, J.R., Brock, J.L., Ball, R. & Carran, R.A. (1979) Nitrogen fixation in pasture. XII. General

discussion. New Zealand Journal of Experimental Agriculture, 7, 45-51.

Høgh-Jensen, H. (1996) Systems theory as a scientific approach towards organic farming. Submitted to

Biological Agriculture & Horticulture.

Høgh-Jensen, H. & Kristensen, E.S. (1995) Estimation of biological N fixation in a clover-grass system by the2

N dilution method and the total-N difference method. Biological Agriculture & Horticulture, 11, 203-219.15

Høgh-Jensen, H. & Schjoerring, J.K. (1994) Measurement of biological dinitrogen fixation in grassland:

Comparison of the enriched N dilution and the natural N abundance method at different nitrogen application15 15

rates and defoliation frequencies. Plant and Soil, 166, 153-163.

Høgh-Jensen, H. & Schjoerring, J.K. (1996) Residual nitrogen effect of clover-ryegrass sward on a subsequent

cereal crop as studied by N methodology and mathematical modelling. This volume.15

Iversen, K. (1943) Forsøg med ajle til bælgplante-græsblanding 1937-1940. Tidskrift for Planteavl, 47, 272-286.

Johnson, I.R., Parsons, A.J. & Ludlow, M.M. (1989) Modelling photosynthesis in monocultures and mixtures.

Australian Journal of Plant Physiology, 16, 501-516.

27

Kjærgaard, T. (1995) Agricultural development and nitrogen supply from an historical point of view. Biological

Agriculture & Horticulture, 11, 3-14.

Koefoed, A.D. & Klausen, P.S. (1969) Kvælstofgødskning til kløvergræs og rent græs. Tidsskrift for Planteavl,

73, 203-246.

Könnecke, G. (1967) Fruchtfolgen. 2. ed. Berlin: VEB Deutscher Landwirtschaftsverlag.

Köpke, U. (1995) Nutrient management in organic farming systems: the case of nitrogen. Biological Agriculture

& Horticulture, 11, 15-29.

Kristensen, E.S., Høgh-Jensen, H. & Kristensen, I.S. (1995) A simple model for estimation of atmospheric-

derived nitrogen in grass-clover systems. Biological Agriculture & Horticulture, 12, 263-276.

Kropff, M.J. & van Laar, H.H. (1993) Modelling Crop-Weed Interactions. Wallingford: Cab International.

Lampkin, N. (1990) Organic Farming. Ipswich: Farming Press Books.

Lawlor, L.R. & Smith, J.M. (1976) The coevolution and stability of competition species. American Nature, 110,

79-99.

Ledgard, S.F. & Peoples, M.B. (1988) Measurement of nitrogen fixation in the field. In: Advances in Nitrogen

Cycling in Agricultural Ecosystems (Wilson, J.R., ed.). Wallingford: Cab Int. pp. 351-367

Ledgard, S.F. (1991) Transfer of fixed nitrogen from white clover to associated grasses in swards grazed by

dairy cows, estimated by using N methods. Plant and Soil 131, 215-223.15

Lemeur, R. (1973) A method for simulating the direct solar radiation regime in sunflower, Jerusalem artichoke,

corn and soybean canopies using actual stand structure data. Agric. Met., 12, 229-247.

Lüscher, A., Connolly, J. & Jacquard, P. (1992) Neighbour specificity between Lolium perenne and Trifolium

repens from a natural pasture. Oecologia, 91, 404-409.

Marschner, H. (1995) Mineral nutrition of higher plants. 2nd. ed. London: Academic Press Ltd.

Masterson, C.L. & Murphy, P.M. (1976) Application of the acetylene reduction technique to the study of nitrogen

28

fixation by white clover in the field. In: Symbiotic Nitrogen Fixation in Plants (Nutman, P.S., ed.). Cambridge:

Cambridge University Press, pp. 299-316.

McAuliffe, C., Chamblee, D.S., Uribe-Arango, H. & Woodhouse, W.W.Jr. (1958) Influence of inorganic nitrogen

on nitrogen fixation by legumes as revealed by N. Agronomy Journal, 50, 334-337.15

McNeill, A.M. & Wood, M. (1990) N estimates of nitrogen fixation by white clover (Trifolium repens L.) growing15

in mixture with ryegrass (Lolium perenne L.). Plant and Soil, 128, 265-273.

Menchaca, L. & Connolly, J. (1990) Species interference in white clover-ryegrass mixtures. Journal of Ecology,

78, 223-232.

Minchin, F.R, Witty, J.F., Sheehy, J.E. & Muller, M. (1983) A major error in the acetylene reduction assay.

Journal of Experimental Botany, 34, 641-649.

Minchin, F.R., Sheehy, J.E. & Witty, J.F. (1985) Factors limiting N fixation by the legume-Rhizobium symbiosis.2

In: Nitrogen Fixation Recent Progress (Evans, H.J, Bottomly, P.J. & Newton, W.E., eds.). Proceedings of VI.

International Symposium on Nitrogen Fixation, Corvallis, USA. pp. 285-291.

Monsi, M. & Saiki, T. (1953) Über den Lichtfaktor in den Pflanzengesellschaften und seine Bedeutung für die

Stoffproduktion. Japan Journal of Botany, 14, 22-52.

Munroe, J.M.M. & Davies, D.A. (1974) Potential pasture production in the uplands of Wales. 5. The nitrogen

contribution of white clover. Journal of the British Grassland Society, 29, 213-238.

Nesheim, L. & Øyen, J. (1994) Nitrogen fixation by red clover (Trifolium pratense L.) grown in mixtures with

timothy (Phleum pratense L.) at different levels of nitrogen fertilization. Acta Agriculturæ Scandinavica, 44, 28-

34.

Parsons, A.J., Harvey, A. & Johnson, I.R. (1991a). Plant-animal interactions in a continuously grazed mixture.

II. The role of differences in the physiology of plant growth and of selective grazing on the performance and

stability of species in a mixture. Journal of Applied Ecology, 28, 635-658.

Parsons, A.J., Orr, R.J., Penning, P.D. & Lockyer, D.R. (1991b) Uptake, cycling and fate of nitrogen in grass-

clover swards continously grassed by sheep. Journal of Agricultural Science, 116, 47-61.

29

Parsons, R., Stanforth, A., Raven, J.A. & Sprent, J.I. (1993c) Nodule growth and activity may be regulated by

a feedback mechanism involving phloem nitrogen. Plant, Cell and Environment, 16, 125-136.

Pedersen, E.J.N. & Møller, E. (1976) Almindeligt rajgræs og kløver i renbestand og i blanding. 6. Beretning fra

fællesudvalget for Statens Planteavls- og Husdyrbrugsforsøg. (English subtitles).

Penning de Vries, F.W.T. (1982) Systems analysis and models of crop growth. In: Simulation of Plant Growth

and Crop Production (Penning de Vries, F.W.T. & van Laar, H.H., eds.). Simulation Monographs, Wageningen:

Pudoc pp. 9-19.

Peoples, M.B., Faizah, A.W., Rerkasem, B., Herridge, D.F. (1989) Methods for evaluating nitrogen fixation by

nodulated legumes in the field. Canberra: ACIAR.

Pianka, E.R. (1983) Evolutionary Ecology. 3rd. ed. New York: Harper & Row.

Rabbinge, R., Goudriaan, J., van Keulen, H., Penning de Vries, F.W.T. & van Laar, H.H. (1990) Theoretical

production ecology: Reflections and prospects. Simulation Monographs 34, Pudoc, Wageningen.

Rennie, R.J. (1984) Comparison of N balance and N isotope dilution to quantify N fixation in field-grown152

legumes. Agronomy Journal, 76, 785-790.

Robertson, F.A., Myers, R.J.K. & Saffigna, P.G. (1993) Dynamics of carbon and N in long-term cropping system

and permanent pasture system. Australian Journal of Soil Research, 45, 211-221.

Ross, P.J., Henzell E.F. & Ross D.R. (1972) Effects of nitrogen and light in grass-legume pastures - a systems

analysis approach. Journal of Applied Ecology, 9, 535-556.

Russell, G., Jarvis, P.G. & Monteith, J.L. (1989) Absorption of radiation by canopies and stand growth. In: Plant

canopies: their growth, form and function (Russell, G., Marshall, B. & Jarvis, P.G., eds.). Cambridge: Cambridge

University Press. pp. 21-39.

Sanford, P.; Pate, J.S. & Unkovich, M.J. (1994) A survey of proportional dependence of subterranean clover

and other pasture legumes on N fixation in South-west Australia utilizing N natural abundance. Australian215

Journal of Agricultural Research, 45, 165-181.

Schechtner, G., Tunney, H., Arnold, G.H. & Keuning, J.A. (1980) Positive and negative effects of cattle manure

30

on grassland with special reference to high rates of application. In: The Role of Nitrogen in Intensive Grassland

Production (Prins, W.H. & Arnold, G.H., eds.). Wageningen: Pudoc, pp. 77-93.

Shaw, P.G.; Brockman, J.S. & Wolton, K.M. (1966) The effect of cutting and grazing on the response of

grass/white clover swards to fertilizer nitrogen. Proc. 10th Int. Grassland Congr., Helsinki, 1966; pp. 240-244.

Shearer, G. & Kohl, D.H. (1986) N -Fixation in field settings: estimations based on natural N abundance.215

Australian Journal of Plant Physiology, 13, 669-756.

Sinclair, T.R. & Serraj, R. (1995) Legume nitrogen fixation and drought. Nature, 378, 344.

Sprent, J.I. & Sprent, P. (1990) Nitrogen fixing organisms. Pure and applied aspects. Cambridge: Cambridge

University Press.

Stevenson, F.J. (1965) Origin and distribution of nitrogen in soil. In: Soil Nitrogen (Bartholomew, W.W. & Clark,

F.E., eds.). Madison, Wisconsin: American Society of Agronomy, pp. 1-42.

Streeter, J. (1988) Inhibition of legume nodule formation and N fixation by nitrate. CRC Critical Reviews in Plant2

Science, 7, 1-23.

Ta, T.C. & Faris, M.A. (1987) Species variation in the fixation and transfer of nitrogen from legumes to

associated grasses. Plant and Soil, 98, 265-274.

Thornley, J.H.M. & Johnson, I.R. (1990) Plant and crop modelling. A mathematical approach to plant and crop

physiology. Oxford: Clarendon Press.

Thornley, J.H.M., Bergelson, J. & Parsons, A.J. (1995) Complex dynamics in a carbon-nitrogen model of a

grass-legume pasture. Annals of Botany, 75, 79-94.

Tilman, D. (1990) Mechanisms of plant competition for nutrients: The elements of a predictive theory of

competition. In: Perspectives on Plant Competition (Grace, J.B. & Tilman, D., eds.). San Diego: Academic

Press, Inc. pp. 117-141.

Touraine, B., Clarkson, D.T. & Muller, B. (1994) Regulation of nitrate uptake at the whole plant level. In: A whole

plant perspective on carbon-nitrogen interactions (Roy, J. & Garnier, E., eds.). The Hague: SPB Academic

Publishing. pp. 11-30.

31

Turkington, R. & Mehrhoff, L.A. (1990) The role of competition in structuring pasture communities. In:

Perspectives on Plant Competition (Grace, J.B. & Tilman, D.). San Diego: Academic Press, Inc. pp. 307-340.

Vandermeer, J. (1989) The ecology of intercropping. Cambridge: Cambridge University Press.

Vallis, I., Haydock, K.P., Ross, P.J. & Henzell, E.F. (1967) Isotopic studies on the uptake of nitrogen by pasture

plants. III. The uptake of small additions of N-labelled fertilizer by rhodes grass and townsville lucerne.15

Australian Journal of Agricultural Research, 18, 865-877.

Walsh, K.B. (1995) Physiology of the legume nodule and its response to stress. Soil Biology & Biochemistry,

27, 637-655.

Whitehead, D.C. (1995) Grassland Nitrogen. Wallingford: CAB International.

Wit, C.T.de, Tow, P.G. & Ennik, G.C. (1966) Competition between legumes and grasses. Wageningen:

Agricultural Research Report 687.

Wit, C.T.de (1982) Simulation of living systems. In: Simulation of Plant Growth and Crop Production (Penning

de Vries, F.W.T. & van Laar, H.H., eds.). Simulation Monographs, Wageningen: Pudoc. pp. 3-8.

Witty, F.R., Minchin, F.R., Sheehy, J.E. & Minquez, M.I. (1984) Acetylene-induced changes in the oxygen

diffusion resistance and nitrogenase activity of legume root nodules. Annals of Botany, 53, 13-20.

Woledge, J. & Dennis, W.D. (1982) The effect of temperature on photosynthesis of ryegrass and white clover

leaves. Annals of Botany, 50, 25-35.

Institute of Organic Agriculture, University of Bonn, Katzenburgweg 3, D-53115 Bonn1

32

3. Legume nitrogen in crop rotation: Reducing losses - in-creasing precrop effects

By U. Köpke1

AIMS AND PRINCIPLES OF NITROGEN MANAGEMENT IN ORGANIC AG-RICULTURE

The main approach of Organic Agriculture is the conduct of a mixed farm practice asfar as possible within a closed system cycle. The production system demands imple-mentation within its own local, ecological, socio-economic, and cultural setting. Sincesite conditions are individual properties by definition, a "farm organism" can be con-ceived as an individual entity. Compared to other types of agriculture, Organic Farm-ing depends more on specific site conditions and is therefore forced to combine thebest adapted elements to a holistic approach which under aspects of nitrogen man-agement in practice means:

- Nearly closed cycles of nutrients and organic matter within the farm.

- Predominantly farm produced manures and composts, maintaining and im-proving productivity of land and animals as far as possible by encouraging andenhancing biological processes such as N -fixation by leguminous crops.2

In relation to nutrient management in contrast to Conventional Agriculture, OrganicAgriculture has to deal with scarcity of nutrients. From this point of view managementhas to be considered as the optimized combination of resources that are restricted orhave to be unlocked by achieving the optimized utilization (e. g. via increased rootdensity and efficiency of nutrient absorption). Therefore strategies making the nitrogenin the system internally available (e. g. N -fixation) or keeping nitrogen potentially2

available in the long term have to be used efficiently. Nutrient management can there-fore be defined as a systematic target-oriented organisation of nutrient flows (Köpke,1993, Köpke, 1995).

CROP ROTATION AND NITROGEN FLOW OF LIQUID PHASE

Main streams of nitrogen flow are fixed in the long term by organising and optimisingthe site-adapted crop rotation. Positive precrop effects are mainly determined by avail-able nitrogen. Following scientifically based steps of optimisation and problems of de-

33

signing an optimised rotation, this contribution will deal with the rotation of the experi-mental farm for Organic Agriculture "Wiesengut"/Hennef, Germany:

1. Faba beans with underseeds/catch crops oil radish or mustard2. Spring wheat3. Winter rye with undersown grass-clover mixtures4. Grass/red clover mixture5. Potatoes6. Winter wheat

REGULATION OF THE N-SOURCE

Besides directly applied farm produced manures and composts optimisation of nitrogenflows is regulated by the amount of nitrogen fixed by legumes and via adequate sinksfor nitrogen in non-leguminous crops (Fig. 3.1). Residual nitrogen, i. e. especiallynitrate in the liquid phase has to be minimized for avoiding nitrate losses via leachingduring periods where no plant sinks can act.

Fig. 3.1: Flow of nitrogen in the liquid phase

34

Under the growing conditions of Central Europe pulses and fodder legumes act asmain sources for nitrogen. In grain legumes the amount of symbiotically fixed nitrogenis closely correlated to grain yield and to the amount of N in grains (Hauser, 1992;Köpke, 1987). The amount of symbiotically fixed nitrogen is nearly equal to theamount of nitrogen bound in the grains. The amount of symbiotically fixed nitrogen cantherefore be derived from the amount of grains produced by pulses. Highest amountsof N-fixed can be achieved by selecting those species which are best adapted to givensite conditions and by further selection of the highest yielding variety within the highestyielding species. Farmers ought to decide on an individual site basis whether they mayexpect higher yields from lupines, peas or from faba beans. All other agronomic strate-gies which can increase grain yields of pulses will simultaneously further increase ni-trogen fixation (Köpke, 1995).

Similar strategies can be applied for fodder legumes. N fixation can be maximized by2

selecting those species and cultivars which are best adapted to given site conditions.Clear relationships between dry matter yield and nitrogen fixation in fodder legumesexist. The higher the percentage of clover in grass-clover mixtures and the yield percut of those stands, the higher the amount of symbiotically fixed nitrogen (Boller,1988).

In the case of pulse crops which can fix up to 70 - 80 % of total nitrogen uptake theamount of symbiotically fixed nitrogen is close to the amount of grain exported nitro-gen. By using such charts as have been presented in the literature (Boller, 1988;Hauser, 1987; Köpke, 1987) the amount of nitrogen which is symbiotically fixed bypulses can be estimated on the base of grain yield and mean nitrogen content of grainyield; in the case of fodder crops by estimating or measuring the amount of dry matteryield produced and the share of clover in the yield mass.

STRATEGIES TO REDUCE NITRATE LEACHING

Faba beans and pulse crops in general

By selecting the appropriate site-specific strategy an efficient use of N-fixed and soil-borne N is ensured by sinks of following non-legumes (Fig. 3.1). Compared to non-legumes pulses leave reasonable higher amounts of nitrate in the soil after harvest.These amounts of rest nitrate are suspected to be leached during winter and earlyspring to get lost for the farming system. In Figure 3.2 residual nitrogen of faba beansand non-leguminous reference crops, oats, rapeseed, and marrowstem kale wasshared into the fractions

- NSN: Soil nitrate N,- NL: Nitrogen in litter (dropped leaves),

35

- NR: Nitrogen in roots,- NS: Nitrogen in shoot rest,- N: Nitrogen in nodules.

Fig. 3.2: Residual nitrogen in autumn and soil nitrate in spring after faba beans and non-legumes (Köpke,1987)

Higher amounts of soil nitrate in spring after beans compared to soil nitrate after oatsand brassicas were closely related to higher amounts of residual N in autumn afterbeans compared to non-legumes (Köpke, 1987). On the other hand spring nitratecontents of soil were generally lower in 1985 compared to 1984, although amounts ofresidual N were higher in autumn 1984 compared to amounts of residual nitrogen inautumn 1983. With data from Table 3.1 and Figure 3.2 it is quite obvious that thiseffect is a function of weather conditions during winter and effects of soil milieu, i. e.mobilisation or immobilisation conditions modifying the nitrogen based precrop effectof pulses which is potentially high, but cannot be predicted precisely in terms of plantavailable nitrogen and its effect on yield on the long term.

36

Tab. 3.1: Precipitation and sum of temperature between ploughing time in autumn and sampling time for

soil nitrate in spring (Köpke, 1987)

Period No. of days Precipitation Sum of temperature

(mm) (°C)

Oct. 10, 1983 - March 1, 1984 142 212 435

Oct. 21, 1984 - Febr. 4, 1985 106 150 213

All strategies to hinder nutrients to disappear out of the farm are based on- contra-movement- retardation or - fixation.

In the liquid phase oriented on nitrate or ammonia N vertical downward flow has to behindered by creating sinks, for instance via efficient nitrogen uptake by growing plants.Leaching is avoided by orienting vertical downward flow vertical upward (contra-move-ment). Accumulating soil nitrate under pulse crops is a function of relatively low rootdensity of legumes and their heterogenous root distribution. In field experiments ofKönings (1987) root length density (cm/cm ) of oats was 5 fold higher compared to3

faba beans. When including maximum root depth root length density (km/m ) of oats2

was 12 fold higher compared to faba beans.

Due to the non-homogeneous horizontal root distribution a remarkable nitrate gradientoccurred under wide-row faba bean pure stands with up to twice the amount of soilnitrate between the rows than directly under the rows. No remarkable gradient wasdetermined under pure stands with narrow row distances and intercrops due to a morehomogeneous root distribution and a more homogeneous uptake of soil nitrate re-spectively (Justus & Köpke, 1992). As a function of soil nitrate uptake and presumablydifferent mineralizing milieus after faba bean harvest as well, the efficiency of the strat-egies tested to reduce residual nitrate content at the beginning of winter varied signifi-cantly (Fig. 3.3).

Faba beans sown with narrow row distances having a more homogeneous root distri-bution tended to reduce residual nitrate content in the soil ranging from 8 - 41 % de-pending on year and location. Due to the additional nitrate uptake faba bean cerealintercrops were considerable more efficient and showed a more constant reduction ofresidual soil nitrate content (Fig. 3.3). Intercropping faba beans with summer barleyreduced soil nitrate N between 37 - 50 %. Due to the higher nitrogen uptake of oatscompared to summer barley, faba bean/oats intercrops reduced soil nitrate contentbetween 45 - 59 % (Justus & Köpke, 1995). The efficiency of the undersown crop andstubble catch crop treatments was significantly correlated to the accumulation of nitro-gen in the shoots. In all trials the brassica species were the most efficient underseed

-110

-80

-50

-20

10

40

70

100

130

Row distance55 cm

kg / ha

0 - 30

60 - 90

Faba beans Stubble-catch crop

May 29 July 11 July 26 July 11 July 26 August 27

LSD 5%

LSD 5 %

Row distancecm

Beans / Oil radish

Faba beans with underseedsSowing dates

Soildepth(cm)

55 27.5 B B

Beans / Barley Beans / Oats

B Oa R O R O M R O M R O M R O M R O M

NO

3 - N

soil

Nitr

ogen

in s

hoot

s(u

nder

seed

s / c

atch

cro

ps)

Beans / Rye grass Beans / White mustard

80

110

50

20

010

40

70

100

130

©IOL

purecrop

intercrop

30 - 60

Row distance27.5 cm

37

treatments reducing soil nitrate contents up to 90 %. If these undersown crops were establishedsuccessfully, sowing dates showed only minor effects. Under practical conditions undersowncrops can be sown in combination with the last mechanical weed control. Establishment of earlysown undersown crops can cause problems in years which have considerable water deficits incombination with high shading by the faba beans. Under these conditions brassica speciesshould be sown as stubble crops after the faba bean harvest. Since nitrogen uptake of ryegrasswas low this strategy was less efficient compared to oil radish and white mustard. Ryegrassefficiency was considerably affected by sowing dates and ranged from 76 %, when sowndirectly after faba beans to 70 %, when sown late at pod setting of the faba beans (Justus &Köpke, 1995). Due to the short time for the development in all trials stubble catch crops wereless efficient compared to undersown crops but represent a suitable technique if the latter havefailed.

Fig. 3.3: Soil nitrate content and nitrogen accumulation of undersown crops and stubble catch crops asaffected by faba bean row distances, intercropping with cereals, sowing dates and species of underseedsand stubble seeds. Date of determination: November 4, 1991. Date of faba bean harvest: August 23, 1991.(Justus & Köpke, 1995)

38

Table 3.2 shows the faba bean grain yields and the amounts of symbiotically fixed ni-trogen.

Tab. 3.2: Grain yield and N fixation of faba beans (Justus, 1996)2

Row distance Treatment Grain yield N fixation2 (cm) (t/ha) (kg/ha)

55 Pure stand 3.46 124

27.5 Pure stand 3.82 131

27.5 Intercropping with oats 1.17 47

27.5 Intercropping with s.barley 1.88 83

55 Underseed ryegrass 3.02 95

55 Underseed oil radish 3.32 124

55 Underseed mustard 3.51 119

Due to a minor intraspecific competition faba bean pure stands with narrow row dis-tances yielded higher and showed a higher N fixation than pure stands with wide2

rows. Intercropping with cereals generally caused significant reduction with regard toboth parameters. Oats were determined to be more competitive for water and proba-bly also for light than summer barley and reduced grain yields of faba beans drasti-cally. According to minor grain yields N fixation was reduced simultaneously.2

Undersown crops caused no significant reduction of faba bean grain yield and N fixa-2

tion. Brassicas gave higher faba bean grain yield and higher nitrogen fixation com-pared to ryegrass.

Grass-clover mixtures and green fallows

Besides faba beans the second main source for nitrogen in the above mentioned rota-tion is red clover combined with ryegrass. Grass-clover mixture is undersown in earlyspring in winter rye stands. During the following 2 years of soil tranquillity soil fertilitybased on high amounts of roots and crop residues is increased. Approximately 60 %of organic farms in Northrhine Westfalia grew winter wheat after ploughing grass-clo-ver in autumn (Hess et al., 1990).

As shown above for faba bean strategies to use nitrogen sinks to take up soil-bornenitrogen are exemplified well by grass-clover mixtures. Under standing grass-clovermixtures grown as green fallows in set-aside programs of the EC, soil nitrate N was

Soil

dept

h (c

m)

kg NO3--N . ha-1 kg NO3

--N . ha-1

0306090

120150

0 20 40

03 06 0

9 01 2 0

1 5 0

0 2 0 4 0

03 06 09 0

1 2 01 5 0

0 2 0 4 0

03 06 09 0

1 2 01 5 0

0 2 0 4 0

03 06 09 0

1 2 01 5 0

0 2 0 4 0

30 Oct 85

11 Dec 85

30 Jan 86

27 Apr 85

19 Mar 85

03 06 09 0

1 2 01 5 0

0 2 0 4 0

25 May 85

early ploughing(21 Oct 85)

early ploughing (20 Sep 85)+ catch crop

late ploughing (4 Dec 85)

NO3-N kg/ha NO3-N kg/ha

39

always measured to be lower than 20 kg/ha (Dreesmann, 1993). Early primary cultivation ofgrass-clover in summer or autumn can result in high amounts of soil nitrate that can be leachedduring winter. Early ploughing combined with white mustard sown as a catch crop or lateploughing in the beginning of winter can reduce nitrate accumulation and leaching duringwinter. Concepts to reduce nitrate losses when growing grass-clover mixtures have thereforeto focus on post-harvest and post-ploughing conditions. Transfer of symbiotically fixed nitrogento following crops after ploughing grass-clover mixtures can be optimized by minimizing pre-winter mineralisation, in particular by:

- proper timing of ploughing - for example delaying the primary cultivation of grass-clover inautumn; and/or

- reducing the tillage intensity (depth, frequency), while still maintaining the standard croprotation (wheat following grass-clover); or

- changing the crop rotation using catch crops as nitrate sinks following grass-clover mixtureto fix pre-winter mineralized nitrogen followed by summer crops; or

- cultivating main crops that show a high nitrogen uptake before winter (e. g. rapeseed)(Hess, 1989).

Fig. 3.4: Soil nitrate content after grass-clover as a function of the date of ploughing, cultivation of a catchcrop, soil depth and time (Hess, 1989).

Edge M ulch

-stronger, shorter culm s-reduced lodging

-weed control -release of nutrients-no transport of m anure /nutrients

-precrop effect-soil structure

Crop Sowing date

Strip cropping of spring wheat

Previous crop

©IOL

mulch mulch mulch

grass-clover mixture when early ploughing in autumn was combined with the cultiva-

40

Figure 3.4 shows for example reduced soil nitrate contents and winter wheat following

tion of a catch crop (white mustard) or after ploughing later in the beginning of winter,respectively.

Green fallows consisting of grass and clover can be used in set-aside programs of theEC for the transition to Organic Farming. Amounts of up to 275 kg N/ha have beenaccumulated with grass-red clover or lucerne mixtures with just two cuts(Dressemann, 1993; Dreesmann & Köpke, 1990). Since the shoot mass of green fal-low can not be harvested, mineralisation of green fallow residues produced soil nitratecontents of more than 200 kg nitrate N/ha in a 0 - 90 cm soil profile in June under fol-lowing sugar beets. One can suspect considerable amounts of this nitrate will beleached in rainy summers. Beside the above grass-clover strategies, management ofgreen fallows has to pay special attention either to the competition of stands in orderto control the source (N supply by reducing the percentage of clover) or by usinggrass-clover cuts for manuring and soil covering in a joining strip cropping cerealstand. Strip cropping of spring wheat where residues of grass-clover are used asmulch for weed control and N source gave higher yields and protein contents of grainsand therefore higher cooking quality by avoiding lodging due to thicker and shorterculms as a function of higher number of margin rows when sown in east-west direction(Fig. 3.5) (Schulz-Marquardt et al., 1994; Schulz-Marquardt et al., 1995a; Schulz-Marquardt et al., 1995b; Weber et al., 1994; Weber et al., 1995). The latter procedureought to be legalized as it might be a truly efficient way to produce organic cerealswhich do not pollute the ground water with any "untimely" mineralized N outside thegrowing season.

Fig. 3.5: Strip cropping of spring wheat

41

PRECROP EFFECTS

The most common subsequent crop after faba beans in Germany is winter wheat.Direct precrop effects can be measured by comparing yields of following crop winterwheat after legumes and non-legumes. In experiments performed by Köpke (1987)yields of winter wheat after faba beans (cultivars: Minica and Kristall) were up to 92 %higher compared to precrop oats. Means of plus 30 and 64 % higher yields weredetermined for winter wheat grain yields in year 1984 and 1985, respectively (Tab. 3.3).

Tab. 3.3: Relationship of winter wheat grain yields after faba beans and non-legumes oats and marrowst-em kale

As shown in Figure 3.6 the fertilizer nitrogen equivalent of faba beans was quantifiedwith 84 and 108 kg mineral N fertilizer per ha compared to marrowstem kale or oats,respectively, as preceding crops to winter wheat.

Based on winter wheat as the first following crop and winter barley as the secondfollowing crop Köpke (1987) calculated a mean fertilizer nitrogen equivalent of 100 kgN/ha for faba beans when used as a preceding crop to cereals on loess soils, withoutusing any strategy for saving nitrate N. Fertilizer nitrogen equivalent was drasticallyreduced to 47 or 65 kg N/ha when maximum yields are realized by mineral N fertilizerinputs (Fig. 3.6). Derived from the curves given in Figure 3.6 we can conclude thatconventional agriculture wastes positive precrop effects when using mineral N torealize maximum yields.

Fig. 3.6: Grain yield of winter wheat as a function of preceding crop and amount of mineral nitrogenfertilizer (Köpke, 1987)

To avoid a high pre-winter nitrogen mineralization, and in order to gain maximumnitrogen uptake by undersown crops and stubble crops, oats were chosen as thesubsequent summer crop after faba beans instead of a winter cereal (formally winterwheat). As a function of different nitrogen availability oats grown after faba beans withbrassica undersown crops showed darker green foliage colours and yielded up to 1t/ha higher compared to all other treatments that gave lighter green leaves (Fig. 3.7).

Dark green plots also showed higher nitrate contents of the stem base of oats. Thatindicated that these plots were apparently facing the highest amounts of availablenitrogen due to minor nitrate losses during winter and a faster and higher mineraliza-tion of incorporated and soil-borne nitrogen resulting in highest grain yields of oats.Obviously due to a temporary N-immobilisation, oats grown after faba bean/cerealintercrops and also oats after faba beans with undersown ryegrass yielded significantlylower compared to oats grown after faba beans pure stand. Due to higher pricesmeanwhile oats have been substituted by spring wheat in the rotation of the Experi-mental Farm Wiesengut.

42

43

Fig. 3.7: Relationship between grain yield of oats as affected by different preceding faba bean treatments(Visual valuation of oats by colour by using an aerial picture: May 20, 1990) (Justus & Köpke, 1992)

Undersowing faba beans with brassica species like oil radish or white mustard repre-sent the most efficient strategy to reduce nitrate losses and to increase grain yield ofthe following crop considerably. Furthermore this strategy has to be recommendedsince in contrast to the other strategies it did not affect grain yield and N2 fixation offaba beans.

Under the site conditions of the Experimental Farm Wiesengut yields of spring wheatfollowing faba beans with brassicas as underseeds or stubble catch crops are up to 2t/ha higher compared to winter wheat following potatoes (Köpke et al., 1995). Further-more in contrast to winter wheat spring wheat has a higher content of grain protein anda higher baking quality.

44

As shown above (Fig. 3.1) legume-borne nitrogen flow is based1. on residual N derived from crop residues and2. on soil-borne N, that is nitrogen mineralized and nitrified under growing legumes

and which is not taken up by legumes and therefore accumulated in the soilprofile.

Compared to direct manuring efficient use of positive precrop effects is much moreimportant to realize high and stable yields in Organic Agriculture. Precrop effects andtheir efficient use are the base for optimizing crop rotations. Not to forget the appliedmaterial added and its own compounds. As shown by Figure 3.8 the crude fibre tonitrogen relationship of residues is much closer in fodder legumes compared to pulsecrops resulting in higher yields of wheat after fodder legumes instead of pulseprecrops.

Fig. 3.8: Grain yield of winter wheat as a function of crude fibre/nitrogen relationship of pulse and fodderlegumes (Heinzmann, 1981).

These results confirm data which show that amount and quality of harvest residues area function of species and plant age (stage of plant development) (Tab. 3.4).

45

Tab. 3.4: Roots and harvest residues of faba beans and lucerne. Parameters of composition and chemi-

cal characteristics (Klimanek et al., 1988)

Variety Growth stage Decay N % C/N C ) % in dry matter1) hws2

(%) (dm) Lignine Carbo- Starchhydratesws3)

young 44.9 3.8 11.5 21.1 8.0 3.7 2.3

Faba beans

ripening 31.1 1.2 41.2 10.4 22.8 3.7 0.9

1 cut 63.2 2.1 21.0 36.7 4.8 15.1 21.8st

Lucerne (1 year)st

3 cut 73.6 1.9 23.4 40.6 5.6 16.3 21.9rd

1) % decay at the end of incubation (70 - 90 days, 25 °C, 60 % max. water capacity)

2) C : Carbon soluble in hot waterhws3) Water soluble carbohydrates

THE PROBLEM: ACCUMULATED SURPLUS OF NITRATE IN THE LIQUIDPHASE

As mentioned above the problem of accumulating nitrate after ploughing grass-cloverbeside other strategies can be solved by cultivating potatoes as first following crop af-ter ploughing grass-clover in early spring. Using this strategy time of soil tranquillity isextended by 6 months compared to ploughing of grass-clover in autumn. Additionallya fourth grass-clover cut yields in a higher gain of nitrogen. Potatoes can use positiveprecrop effects of grass-clover efficiently. Winter wheat as second following crop canuse direct and indirect precrop effects of grass-clover and potatoes efficiently.

Figure 3.9 shows that compared to the standard rotation spring ploughing of grass-clover to potatoes can minimize soil nitrate accumulation during winter. Accumulatingsoil nitrate under potatoes is restricted to the upper soil layers (0 - 50 cm soil depth)and is reduced immediately in the time of tuber setting between May and July due tohigher nitrogen uptake. Since sink capacity of following winter wheat sown early in au-tumn is low, higher soil nitrate contents were measured under early sown winter wheatcompared to winter rye of the standard rotation. On the other hand, rearrangement ofthe standard rotation results in only 1 winter period endangering nitrate to be leachedin contrast to 2 winter periods with danger to leaching of nitrate in the standard rota-tion. Amounts of accumulated nitrate can further be reduced by growing a catch cropcombined with postponed tillage and sowing date of winter wheat (Fig. 3.9).

46

Fig. 3.9: Soil nitrate N as a function of rotation and time (Hess, 1993)

Results shown in Figure 3.9 were gained on the Experimental Farm Dikopshof whereon loess soils in contrast to the site conditions of the Experimental Farm Wiesengutthe amount of soil-borne nitrate is generally higher. Under the site conditions of theExperimental Farm Wiesengut with sandy/loamy soils after ploughing grass-clover topotatoes in early spring, soil nitrate amounts up to 150 kg N/ha in June. Theseamounts were reduced by plant uptake within 4 months to only 20 kg NO3-N/ha andwere sufficient for 32 t/ha tuber yield. Additionally applied farmyard manure amountingbetween 40 - 240 kg total N/ha did not increase tuber yield but increased spoilage oftubers significantly (Fig. 3.10). Investigations recently performed in the Institute of Or-

47

ganic Agriculture (Schulz, unpublished) have shown that poor fragrance, taste, andcolour deficiency of potato tubers were caused by increased application of farmyardmanure after the precrop grass-clover mixture. Derived from these results and basedon the above mentioned rotation and site conditions, farmyard manure should not beapplied to potatoes after grass-clover mixture to avoid increased nitrate potential to beleached. Farmyard manure should primarily applied to following crop winter wheatbefore sowing. Liquid manure should be applied during tillering stage to increase grainyield and cooking quality (Schenke, 1993; Stein-Bachinger, 1993).

Fig. 3.10: Potatoes after grass-clover ploughed in spring: Tuber yields and spoilage as a function of theamount of farmyard manure. Experimental Farm Wiesengut (Stein-Bachinger, 1993)

INTEGRATION OF VEGETABLES - USE OF SOLID MANURE

Sustainable soil fertility in Organic Agriculture is mainly based on optimal and in-creased production of soil organic matter. Self-reliance regarding humus production isbased on the cultivation of crops producing high amounts of roots and harvest resi-

48

dues and the application of farmyard manure. All calculations to quantify reproductionof soil organic matter as developed in the former GDR (Asmus, 1992; Kundler et al.,1981) were oriented on farmyard manure as the reference.

Fig. 3.11: Efficient export of nitrogen via cultivating vegetables.

Increasing the amount of soil organic matter results in higher Ct- and Nt-contents ofthe soil. Higher or increased rates of mineralization result in higher soil nitrate con-tents. The extended soil nitrate pool can increase nitrate losses when plant sinks, i. e.efficient nitrate uptake by plants, act inadequate. Strategies to use brassica under-seeds or catch crops act merely temporary. Uptake of cereals and potatoes is oftenlimited as an effect of "untimely" plant available nitrogen and limited N sink of theshoots and harvested organs. All the mentioned strategies act therefore suboptimal, ifthe soil nitrate pool is steadily increased. Alternatively a part of the nitrogen in theliquid phase can be bound into the solid phase via transferring for instance brassicafodder crops to farmyard manure (Fig. 3.11). Farmyard manure can bind nitrogen insolid less reactive nitrogen compounds. Nevertheless also this strategy is only partlyeffective on the long term. The high sink capacity of brassica catch crops can focusattention on brassica cash crops grown as vegetables acting as efficient sinks fornitrogen and realizing high export of nitrogen when sold. Two strategies of growingvegetables integrated or apart from rotation are proposed to be investigated rotation-oriented

49

in field experiments. It is assumed that a rotation-integrated vegetable production isoptimal for conditions, where based on site and rotation the need for increased soilorganic matter is high on all fields used by the rotation of the farm. For farms havingconditions of high soil fertility, i. e. high nitrogen mineralization rates, the concept of"rotation external" production (separated vegetable production) should be discussed(Fig. 3.11).

ACKNOWLEDGEMENTS

The author gratefully acknowledges the technical assistance to Ms D. Schulz-Marquardt and Mr C. Dahn.

REFERENCES

Asmus, F. (1992) Einfluß organischer Dünger auf Ertrag, Humusgehalt des Bodens und Humusrepro-

duktion. Berichte über Landwirtschaft, 206. Sonderheft, Bd. 4, Humushaushalt; S. 127-139.

Boller, B. (1988) Biologische Stickstoff-Fixierung von Weiß- und Rotklee unter Feldbedingungen.

Landwirtschaft Schweiz, 1 (4), 251 - 253.

Dreesmann, S. & Köpke, U. (1990) Vorfruchteffekte einjähriger Leguminosen-Grünbrachen zu Senf und

Zuckerrüben. Mitteilungen der Gesellschaft für Pflanzenbauwissenschaften 4, 45 - 48.

Dreesmann, S. (1993) Pflanzenbauliche Untersuchungen zu Rotklee- und Luzernegras-Grünbrachen in

der modifizierten Fruchtfolge Zuckerrüben - Winterweizen - Wintergerste. Ms Thesis, University of Bonn.

Hauser, S. (1987) Schätzung der symbiotisch fixierten Stickstoffmenge von Ackerbohnen (Vicia faba L.)

mit erweiterten Differenzmethoden. Ms Thesis, University of Göttingen.

Hauser, S. (1992) Estimation of symbiotically fixed nitrogen using extended N difference methods. In:

Biological nitrogen fixation and sustainability of tropical agricultue (Mulongoy, K., Gueye, M. & Spencer,

D. S. C, eds.), 309 - 321.

Heinzmann, F. (1981) Assimilation von Luftstickstoff durch verschiedene Leguminosenarten und dessen

Verwertung durch Getreidearten. Ms Thesis, University of Hohenheim.

Hess, J. (1989) Kleegrasumbruch im Organischen Landbau: Stickstoffdynamik im Fruchtfolgeglied

Kleegras - Kleegras - Weizen - Roggen. Ms Thesis, University of Bonn.

Hess, J., Pauly, J. & Franken, H. (1990) Standorterhebungen zur Stickstoffdynamik nach

Kleegrasumbruch. Mitteilungen der Gesellschaft für Pflanzenbauwissenschaften 3, 269 - 272.

50

Hess, J. (1993) Residualer Stickstoff aus mehrjährigem Feldfutterbau: Optimierung seiner Nutzung

durch Fruchtfolge und Anbauverfahren unter Bedingungen des Ökologischen Landbaus. Habilitation

Thesis, University of Bonn.

Justus, M. & Köpke, U. (1992) Strategies to reduce nitrogen losses and to increase precrop effects

when growing faba beans in humid climates. In: Proceedings of the 9th International IFOAM-Conference

1992, Sao Paulo, Brazil, 59 - 66.

Justus, M. & Köpke, U. (1995) Strategies to reduce nitrogen losses via leaching and to increase precrop

effects when growing faba beans. Biological Agriculture and Horticulture, Vol. 11, Nos 1-4, 145 - 155.

Justus, M. (1996) Optimierung des Anbaus von Ackerbohnen: Reduzierung von Nitratverlusten und

Steigerung der Vorfruchtwirkung zu Sommergetreide. Ms Thesis, University of Bonn.

Klimanek, E.-M., Körschens, M. & Eich, D. (1988) Menge und Qualität von Ernte- und Wurzelrückstän-

den ausgewählter Pflanzenarten als Parameter für das Modell der Umsetzung organischer Substanz.

FZB-Report 1988; Wiss. Jahresbericht des Forschungszentrums für Bodenfruchtbarkeit Müncheberg

der Akad. der Landw.wiss der DDR; S. 64-72.

Könings, J. (1987) Durchwurzelung des Bodens von Hafer, Erbsen, Ackerbohnen und im Boden

verbleibende Nitratmengen. Beobachtungen im Feldversuch 1986. Diploma Thesis, University of

Göttingen.

Köpke, U. (1987) Symbiotische Stickstoff-Fixierung und Vorfruchtwirkung von Ackerbohnen (Vica faba

L.). Habilitation Thesis, University of Göttingen, Germany.

Köpke, U. (1989) Körnerleguminosen: N -Fixierung, Vorfruchtwirkung und Fruchtfolgegestaltung -2Auswirkung auf die Belastung von Agrar-Ökosystemen. In: Körnerleguminosen, Schriftenreihe des

Bundesministers für Ernährung, Landwirtschaft und Forsten, Reihe A: Angewandte Wissenschaft, Heft

367, 52 - 63.

Köpke, U. (1993) Nährstoffmanagement durch acker- und pflanzenbauliche Maßnahmen. Berichte über

Landwirtschaft 71, 207. Sonderheft, 181 - 203.

Köpke, U. (1995) Nutrient management in Organic Farming Systems. The case of nitrogen. Biological

Agriculture and Horticulture (BAH), Vol 11, Nos 1-4, 15 - 29.

Köpke, U., Dahn, C., Haas, G., Riebeling, H., Siebigteroth, J., Täufer, F., Tucholla-Haas, M. & Zedow, D

(1995) Referenzflächen des Versuchsbetriebs für Organischen Landbau "Wiesengut": Konzept zur

Erfassung und Analyse der gesamtbetrieblichen Entwicklung. Tagungsband 3. Wiss. Fachtagung zum

Ökologischen Landbau 1995, Wissenschaftlicher Fachverlag Gießen, 329 - 332.

Substanz zum Boden - Wesentliche Voraussetzungen für einen hohen Leistungsanstieg in der

51

Kundler, P., Eich, D. & Liste, J. et al. (1981) Mehr tun als nur ersetzen: Verstärkte Zufuhr organischer

Pflanzenproduktion. Neue Deutsche Bauernzeitung 22, 36, 8 - 9.

Schenke, H. (1993) Anbautechnik von Winterweizen im Organischen Landbau: Unkrautaufkommen und

Ertragsbildung in Abhängigkeit von mechanischer Unkrautregulierung, Saatgutqualität, Standraumzu-

messung und organischer Düngung. Ms Thesis, University of Bonn.

Schulz-Marquardt, J., Weber, M. & Köpke, U. (1994) Anbau von Grünbrachestreifen im Wechsel mit

Sommerweizen zur Erzeugung von Qualitäts-Backweizen. VDLUFA-Schriftenreihe, Kongreßband 1994,

549 - 552.

Schulz-Marquardt, J., Weber, M. & Köpke, U. (1995a) Streifenanbau von Sommerweizen im Wechsel

mit Futterleguminosen zur Erzeugung von Qualitäts-Backweizen im Organischen Landbau.

Tagungsband 3. Wiss. Fachtagung zum Ökologischen Landbau 1995, Wissenschaftlicher Fachverlag

Gießen, 109 - 112.

Schulz-Marquardt, J., Weber, M. & Köpke, U. (1995b) Streifenanbau von Sommerweizen mit

Futterleguminosen - Nutzung von Grünbrachemulch zur Steigerung der Backqualität von

Sommerweizen im Organischen Landbau. Mitteilungen der Gesellschaft für Pflanzenbauwissenschaften

8, 57 - 60.

Stein-Bachinger, K. (1993) Optimierung der zeitlich und mengenmäßig differenzierten Anwendung von

Wirtschaftsdüngern im Rahmen der Fruchtfolge organischer Anbausysteme. Ms Thesis, University of

Bonn.

Weber, M., Schulz-Marquardt, J. & Köpke, U. (1994) Grünbrachestreifen im Wechsel mit

Sommerweizen im Organischen Landbau. VDLUFA-Schriftenreihe, Kongreßband 1994, 139 - 142.

Weber, M., Schulz-Marquardt, J. & Köpke, U. (1995): Streifenanbau von Sommerweizen mit

Futterleguminosen - Wirkung auf Unkrautentwicklung und Krankheitsbefall. Mitteilungen der

Gesellschaft für Pflanzenbauwissenschaften 8, 61 - 64.

Plant Nutrition Laboratory, Department of Agricultural Sciences, Royal Veterinary and AgriculturalUniversity, DK-1981 Thorvaldsensvej 40, Copenhagen, Denmark

52

4. Residual nitrogen effect of clover-ryegrass sward on asubsequent cereal crop as studied by N methodology and15

mathematical modelling

By H. Høgh-Jensen & J.K. Schjoerring1

SUMMARY

The residual effect of 2-year-old sward of respectively clover-ryegrass mixture andryegrass-only on yield and N uptake in a subsequent winter wheat crop were investigatedby use of the N dilution method and mathematical modelling. The amount of N derived15

from clover-grass residues was 25 to 43% higher than that derived from residues ofryegrass-only. Expressed in absolute values the N uptake in the subsequent winter wheatcrop was 23-28 kg N ha higher after clover-ryegrass than after ryegrass. Up to about 54-1

kg N ha of the N mineralised from the clover-ryegrass crop was leached, whereas only-1

11 kg N ha was leached following grass in monoculture.-1

When simulating a delay in ploughing-in of the residues the poorer crop developmentbefore winter was counteracted by the higher absorption of inorganic N the subsequentspring because N leaching was reduced to 30 kg N ha . When further delaying ploughing-1

to spring then N absorption was simulated to be even higher than in an autumn-sowncereal and N leaching was reduced to 16 kg N ha .-1

INTRODUCTION

Clover-grass swards have a dual purpose in low input agricultural systems: One purposeis to produce fodder for the livestock and the other is to provide N for succeeding crops viamineralization of organic N. The latter is commonly called the preceding crop effect.

Swards with a substantial amount of clover are often ploughed-in in the autumn followedby sowing of a winter cereal crop. The effect of clover-based swards contra swardsconsisting of ryegrass-only on the succeeding cereal crop has been evaluated on basis ofyield differences (e.g. spring barley; Evans et al., 1992), assuming that the difference inyields were caused by differences in mineralization of organic N (Ladd et al., 1986).

Measurements of differences in N enrichment (Hauck & Bremner, 1976) are valuable15

tools in quantifying the role of legumes in providing N for succeeding crops. However, thefocus has mostly been on the effect of green manure or grain legume crops (Chalk et al.,1993; Power, 1990) whereas studies on the effect of including clovers in swards on thefollowing crop have been few (Dou & Fox, 1994; Evans et al., 1992), and none haveutilised the N methodology. 15

Beneficial effects of the preceding crop on water use efficiency and reduction in cropdiseases can in some cases account for up to 50% of the yield response of the succeeding

53

crop (Reeves et al., 1984; Russelle et al., 1987, Harper et al., 1995). However, relativelylittle information is yet available on these effects; also the actual differences in dry matterand total N of the ploughed-in plant residues (root and stubble) between productionsystems have seldom been considered (Papastylianou, 1993). Similarly, the problem ofpotential losses of nutrients related to the timing of the ploughing-in of plant residues andestablishment of a new crop is gaining interest but very little data are available.

Mathematical models can be useful tools to investigate the interaction between differentprocesses contributing to the overall effect of different treatments on crop growth(Gliessman, 1990). A mathematical model called DAISY has recently been developed(Hansen et al., 1990; 1991) and shown to be able to reliably simulate N turnover andbiomass production in agroecosystems (Vereecken et al., 1991). Among other things theDAISY model can predict mineralization of organic N from crop residues on basis of theirdry matter and total N content.

The objective of the present study was to investigate N differences in the preceding cropeffect between 2-year-old swards consisting of either clover-ryegrass with a known historyof N fixation or of ryegrass in monoculture. The simulation model DAISY was used to2

further elucidate the N dynamics after incorporation of the crop residues.

MATERIALS AND METHODS

Experimental site

The experimental plots were established in spring 1991 at an experimental field, located18 km west of Copenhagen, that had been farmed organically for the previous five years.The total N and clay content in soil from the experimental site was on average 0.23% and10.2%, respectively, in 0-25 cm depth. A seed mixture consisting of 3 kg ha red clover-1

(Trifolium pratense L. cv. Rajan), 3 kg ha white clover (Trifolium repens L. cv. Milkanova)-1

and 20 kg ha of ryegrass (Lolium perenne L. cvs Pimpernel, Tonga, Tove, and Merlinda)-1

was undersown a spring barley crop. The three factorial experimental design includedthree replicates of 15m plots that consisted of either a clover-grass mixture or ryegrass-2

only. In the first production year the plots were cut either three or six times and receivedeither 0 or 400 kg N ha . In the second production year, all plots were cut three times and-1

received no fertilizer.

Six days after the last cut, the area was ploughed and the cereal was established at 25September 1993.

In brief, an increase in defoliation frequency reduced the total production of dry matter inboth years, but increased the total accumulation of N in shoots (Table 4.1). Application ofinorganic N fertilizer increased dry matter production as well as N accumulation in shoots.However, the proportion of atmospheric derived N in the clover-ryegrass mixture wasreduced by N fertilizer application (P<0.05) as described by Høgh-Jensen & Kristensen(1995).

54

Tab. 4.1: Dry matter and N content in clover and ryegrass shoots in two production years preceding the cerealcrop. The amount of nitrogen derived from symbiotic N fixation in the clover fraction, as estimated by the N2

15

dilution technique, is shown in brackets.

Preceding crop 1992 1993DM N DM N

(kg ha ) (kg ha ) (kg ha ) (kg ha )-1 -1 -1 -1

Clover-grass 9cuts + 400N 10936 412 (70) 7420 196 (102)Grass 9cuts + 400N 10924 366 4231 67Clover-grass 9cuts + 0N 8171 261 (161) 8913 277 (214)Grass 9cuts + 0N 4486 90 2327 38Clover-grass 6cuts + 400N 13235 349 (73) 9025 281 (156)Grass 6cuts + 400N 12243 284 4213 81Clover-grass 6cuts + 0N 10431 228 (157) 9613 290 (223)Grass 6cuts + 0N 4771 64 2617 40

Application of N fertilizer and non-labelled fertilizer 15

The plot with clover-grass mixture and ryegrass-only received N fertilizer in the firstproduction year labelled by N, as described in details by Høgh-Jensen & Schjoerring15

(1994). No N was applied the second production year. 15

One week after establishing the cereal N-labelled fertilizer, equivalent to 1.0 kg N ha ,15 -1

with an enrichment of 99 atom% was applied to the plots not previously labelled with N,15

i.e. those that had not received N fertilizer. No other fertilizer was applied to the cereal. TheN-labelled fertilizer was applied to the central 2 m of the plot following the procedure15 2

described by Høgh-Jensen and Schjoerring (1994).

Sampling and analysis of plant material

Wheat plants from a 1 m subplot out of the 2 m that was previously N-labelled were2 2 15

separated in grain, straw, and stubble (stubble was defined as straw material from soilsurface to cutting height of 8 cm). Furthermore, after harvesting the cereal, duplicate soilcores with a diameter of 7 cm were taken to a depth of 50 cm, the root mass was washedfree from soil, dried and weighed. All plant material was dried to constant weight at 70�C,ground to a fine powder and analysed simultaneously for N and total N in an ANCA-MS15

system (Carlo Erba Strumentazione, Milan, Italy and Europa Scientific Ltd., Crewe, UK).

Sampling and analysis of soil material

To assess the content of NO and NH in the soil below the cereal, soil cores were taken3 4- +

to a depth of 50 cm at 7 April and 22 June 1994. Immediately after sampling, the coreswere divided into sections of 25 cm and kept frozen (-20�C) until analysis. Wet soilsamples were shaken for one hour with 2M KCl and filtered. NO and NH in the extracts3 4

- +

were determined spectrophotometrically.

Modelling of N turnover

A mathematical model called DAISY (Hansen et al., 1990; 1991) was used for simulatingthe N turnover. DAISY has five interfaces to the surrounding environment. These are threeinput parameters: daily values of global radiation, air temperature and precipitation, andtwo output parameters: nitrate leaching and water percolation.

%Nrel � ( �15N wheat at harvest

�15N grass before ploughing

) × 100

%Ndiff � ( 1 �

�15N wheat after clover�grass mixture�

15N wheat after ryegrass�only) × 100

%Nrec � ( �15N wheat × N wheat

�15N fertiliser × N applied fertiliser

) × 100

55

The content of the system to be modelled is described by 4 modules: a hydrologicalmodule, a soil temperature module, a soil N module, and a crop module. Four differentcrop modules were used in the simulations, namely modules for winter wheat, springbarley, and ryegrass-only (Hansen et al., 1990), and a module for clover-ryegrass mixtureunder low N input condition (Høgh-Jensen, 1996). Important is that the four crop moduleswere run with standard parameterization. This implies that parameters characterising theplant residues at the time of ploughing-in in terms of partitioning between organic poolsand the turnover rates of these pools were similar for residues following ryegrass-only andclover-ryegrass mixture.

Calculations

When attempting to utilise the residual N enrichment of the organic matter as measured15

in the last harvest of the grass before ploughing, evidently the residual N enrichment was15

different in ryegrass-only and grass in mixture, thus complicating the use of traditionaldilution calculation (Equation 2; Danso & Papastylianou, 1992; Senaratne & Hardarson,1988). However, using different the different values for the N enrichment of the ryegrass15

obtained in the individual treatments, the contribution from the N-labelled organic pool in15

the sward was calculated as (Senaratne & Hardarson, 1988):

(1)

By applying N-labelled fertilizer at the establishment of the wheat crop, the extra15

amount of N coming from the clover-grass compared to grass in monoculture can becalculated as (Senaratne & Hardarson, 1988):

(2)

In a similar way recovery of the N that was applied to the winter wheat crop was15

calculated as (Zapata, 1990):

(3)

RESULTS AND DISCUSSION

Residual N effect of swards and N uptake by succeeding wheat crop

Wheat following clover-ryegrass in all cases produced more shoot dry matter than wheatfollowing ryegrass-only (Table 4.2). In plots previously grown with a mixture of clover andryegrass, the highest wheat yields occurred in non-fertilised plots that had been harvested

56

nine times (P<0.05), whereas no effect of cutting frequency or N fertilizer treatment wasobserved in plots following ryegrass in monoculture. Increased cutting frequency andapplication of inorganic N fertilizer lead to a reduced content of clover in the herbage(Høgh-Jensen & Kristensen, 1995) and thereby to a smaller residual N effect (Table 4.2).The management of the preceding sward thus greatly influences the yield of thesucceeding crop.

The amount of N mineralised from ryegrass residues and absorbed by the wheat crop(%Nres) amounted to 11-16% of the initial N content of the ryegrass (Table 4.3), which isin the range that could be expected (Giller & Cadish, 1995). Correcting for the estimatedamounts of N mineralised from clover residues this corresponded to an uptake by thewheat crop of respectively 5 and 10 kg N ha derived from ryegrass-only or ryegrass in-1

mixture.

The amount of N in the wheat crop that originated from the residues of the ploughed-insward was 25 to 43% higher in clover-ryegrass plots than in plots following ryegrass-only(%Ndiff; Table 4.3). %Ndiff was slightly higher than that generally found for legumeresidues benefiting the following crop under temperate conditions as reviewed by Giller &Cadish (1995). It was also higher than the 20%-24% found by Müller & Sundman (1988)using decomposing clover material in mesh bags. However, earlier experiments with clover(Dou & Fox, 1994; Müller & Sundman, 1988) and other legumes have all been based onthe residual N effect of a one-season crop including legumes, e.g. peas (Jensen, 1994),field bean (Senaratne & Hardarson, 1988), lentil (Bremer & van Kessel, 1992), soybean(Bergersen et al., 1992), and Medicago litoralis (Ladd et al., 1983; Ladd & Amato, 1986).A higher preceding crop effect in terms of residual N must be expected in biannual or olderswards compared to annual crops because of a potential N immobilisation in many swards(Goodman, 1988; Høgh-Jensen & Kristensen, 1995; Robbins et al., 1989; Robertson et al.,1993a; 1993b).

Tab. 4.2: Yield (tons dry matter ha ) and N content (kg N ha ) of the different plant parts of winter wheat.-1 -1

Preceding crop Grain Straw Stubble TotalDM N DM N DM N DM N

Clover-grass 9cuts + 400N 2.3 32.0 3.1 9.2 2.2 18.1 7.6 59.3Grass 9cuts + 400N 2.0 29.7 2.1 7.8 2.2 18.6 6.3 56.1Clover-grass 9cuts + 0N 4.0 54.0 4.4 13.3 3.0 24.9 11.4 92.2Grass 9cuts + 0N 1.9 24.3 2.7 10.5 2.1 17.9 6.7 53.7Clover-grass 6cuts + 400N 2.9 38.9 4.5 11.2 2.5 18.9 9.9 69.0Grass 6cuts + 400N 1.9 24.5 2.6 8.4 2.3 24.9 6.8 57.8Clover-grass 6cuts + 0N 2.7 36.4 3.2 7.7 2.4 19.6 8.3 63.7Grass 6cuts + 0N 2.0 27.9 1.6 4.0 1.5 13.5 5.1 45.4 LSD 0.3 4.0 0.4 1.2 0.2 0.6 0.8 5.20.95

A difference between the clover-ryegrass and the ryegrass-only sward in an unlabelledinorganic N pool available for the wheat crop (%Ndiff) can be caused either by atmosphericderived N following incorporation and mineralization of clovers as discussed above orhigher amounts of inorganic N under the clover-ryegrass system compared to the ryegrass-only. The latter have been termed an "N sparing effect" of the legume (Chalk et al., 1993;Evans et al., 1989; Senaratne & Hardarson, 1988). However, there was no indication of adifferent ability to absorb inorganic N in clover-ryegrass and in ryegrass-only (Høgh-Jensen& Kristensen, 1995). It can consequently be assumed that mineralization of atmosphericderived N caused the observed differences in N absorption by the wheat crop, equivalent

57

to 23-28 kg N ha (%Ndiff × total-N; Tables 4.2 and 4.3). This value, calculated on basis-1

of the N dilution method, agrees with measurements of Lindén & Wallgren (1993) and15

varied less than the corresponding value calculated by the traditional N difference method,ranging between 18 and 39 kg N ha (Table 4.3), but still the two estimates were within the-1

same range.

Tab. 4.3: Content of N in winter wheat at anthesis (whole shoots above stubble height) and at maturity15

(weighted average of grain, straw, stubble and roots) compared with the recovery of applied N after clover-15

ryegrass or ryegrass-only swards.

Preceding crop Content of N15

anthesis harvest %Nrel %Nrec %Ndiffa b c

(atom%) (atom%) (%) (%) (%)Clover-grass 9cuts + 400N 0.38897 0.38078 16.3 -Grass 9cuts + 400N 0.37880 0.37686 11.3 -Clover-grass 9cuts + 0N 0.52636 0.50706 - 14.2 24.8Grass 9cuts + 0N 0.58796 0.55376 - 19.1Clover-grass 6cuts + 400N 0.38232 0.38159 15.0 -Grass 6cuts + 400N 0.3771 0.37975 12.0 -Clover-grass 6cuts + 0N 0.4831 0.46426 - 9.9 43.4Grass 6cuts + 0N 0.57562 0.53926 - 17.5a) Per cent N released from the labelled organic pool (Eq. 1). N enrichment of the ryegrass was 0.45516,15

0.45970, 0.46809, and 0.47856 atom% at the last harvest in treatments clover-ryegrass 9cuts+400N,ryegrass-only 9cuts+400N, clover-ryegrass 6cuts+400N, and ryegrass-only 6cuts+400N, respectively.

b) Per cent recovery of applied N (Eq. 3).15

c) Per cent N extra released from the organic pool following clover-ryegrass mixture compared to ryegrass-only (Eq. 2).

Tab. 4.4: Inorganic N content (kg N ha ) in 0-25 and 25-50 cm depth of the soil profile below the winter wheat-1

crop.

Preceding crop 0-25 cm 25-50 cm 7 April 22 June 7 April 22 June

Clover-grass 9cuts + 400N 14.6 28.6 13.3 16.4Grass 9cuts + 400N 15.4 20.2 16.4 19.1Clover-grass 9cuts + 0N 18.4 19.9 27.1 22.6Grass 9cuts + 0N 19.6 21.9 12.7 15.9Clover-grass 6cuts + 400N nd 42.5 nd 23.4Grass 6cuts + 400N 21.4 26.2 20.8 19.5Clover-grass 6cuts + 0N 16.9 23.1 21.9 13.7Grass 6cuts + 0N 14.0 21.9 14.9 14.9 LSD 1.8 5.4 2.8 2.60.95

The content of N in the winter wheat crop was measured twice during the growing15

season, i.e. at anthesis and at final harvest. In agreement with Ladd et al. (1986) only avery limited uptake of N took place after anthesis (Table 4.3).

The inorganic N content was measured twice during the growing season to a depth of 50cm (Table 4.4). No significant difference between plots following ryegrass-only or plotsfollowing clover-ryegrass mixtures was found (P>0.05).

Yield differences between cereal crops following a legume or a non-legume can often bemade up by N fertilizer, suggesting the yield difference to be due to improved soil Nconditions after incorporation of legume residues. However, sometimes the gap cannot beclosed completely, suggesting the involvement of other factors than improved N availability(Rowland et al., 1994; Strong et al., 1986). Assessed on basis of carbon isotope ( C/ C)13 12

58

ratios (Farquhar & Richards, 1984) in the grain dry matter no differences in water useefficiency between wheat plants following clover-ryegrass and ryegrass-only were observedin the present experiment (data not shown).

N balance after sward incorporation

Plots harvested nine times without receiving inorganic N fertilizer, i.e. plots with the largesteffects of clover on the N turnover, were selected for modelling the N balance of theexperimental winter wheat plots.

The predicted input of organic N in residues of stubble and roots was 156 kg N ha higher-1

after clover-ryegrass than after ryegrass-only. These inputs are in a reasonable range ofmeasured values from soil scores taken mid-summer (data not shown). These inputs ofresidues were leading to a predicted net N mineralization of 140 kg N ha after clover--1

ryegrass against only 63 kg N ha after ryegrass-only (Table 4.5). Despite this very-1

substantial difference in net N mineralization, N uptake by the succeeding cereal onlydiffered with as little as 39 kg ha when determined experimentally by the difference-1

method (Table 4.2), 28 kg ha when determined by the N isotope dilution method (Tables-1 15

4.2 and 4.3), and 33 kg ha when simulated (Table 4.5). -1

The simulated N balance sheet for the cereal crop indicate that a substantial amount ofinorganic N (54 kg N ha ) was leached after clover-ryegrass, whereas very little N (11 kg-1

N ha ) was leached after ryegrass-only (Table 4.5) when ploughed-in at 20 October. In-1

agreement, %Nrec was lower in the plots following clover-ryegrass mixture compared toryegrass-only (Table 4.3). The major part of the N leaching occurred during the winterseason (Figure 4.1). The sward was ploughed-in in mid-September when soil temperaturewas still relatively high (Fig. 4.2), enabling substantial N mineralization. The resulting Nleaching was clearly related to the soil temperature (Fig. 4.2; Table 4.5) and the size of theorganic N pool (Table 4.5) as the two crop modules (ryegrass-only and clover-grassmixture) were initialised with identical parameters for relative pool sizes of organicresidues with identical C:N ratios and turnover rate. Preliminary results in our lab clearlyindicate that this is indeed not the case. White clover residues mineralize significantlyfaster than ryegrass residues (data not shown).

Tab. 4.5: A N balance sheet for the root zone (1 m depth) of a cereal following 2-year-old swards of clover-ryegrass or ryegrass-only as simulated by the mathematical model DAISY (kg ha ). Both swards had not been-1

fertilised and had been cut 9 times totally in the two production years.

Preceding crop Inorganic Incorporated Minera- Plant Leach- Inorganic N start crop residues lisation uptake ing N end

Clover-grass- ploughed 20 Sep 7 200 140 92 54 14 - ploughed 20 Oct 9 200 140 117 30 13- ploughed 20 Mar 9 198 140 133 16 17

Ryegrass-only- ploughed 20 Sep 3 44 63 58 11 9

Sep Nov Jan Mar May Jul

0

10

20

30

40

50

60

Following ryegrass in monocultureFollowing clover/grass in mixture

Sep Nov Jan Mar May Jul-5

0

5

10

15

20

25

59

Fig. 4.1: Simulated nitrate content in the root zone (1 m depth) of a cereal crop following a 2-year-old swardof clover-ryegrass mixture or ryegrass-only. Both swards had not been fertilised and had been cut 9 timestotally in the two production years.

Fig. 4.2: Simulated soil temperature as an average of 2-20 cm depth.

Sep Nov Jan Mar May Jul

0

10

20

30

40

50

60

70

ploughed-in 20 Sepploughed-in 20 Octploughed-in 20 Mar

60

Fig. 4.3: Simulated accumulated leaching from the root zone (1 m depth) of a cereal crop following a 2-year-old sward of clover-ryegrass mixture ploughed-in at 20 Sep, 20 Oct, and 20 Mar, respectively. The valuesdecrease at the end of the growth season as DAISY calculate an upwards movement of water and nitrate.

The leaching following ploughing a sward in autumn is, of course, also related to the timeof ploughing. Francis et al. (1995) and Lindén & Wallgren (1993) showed that earlyploughing of a sward in autumn increased N mineralization compared to late ploughing,and that the released N largely was lost by leaching during the winter season. Breland(1994) recommended that N-rich materials such as clover should be incorporated into thesoil as late as possible before establishing the new crop because they decompose rapidlyeven at temperatures below 5�C. When simulating that the ploughing-in of residues wasdelayed by one month (to 20 October) the leaching from the clover-grass mixture wasreduced from 54 kg N ha to 30 kg N ha . When further delaying the ploughing-in to spring-1 -1

(19 March) the leaching was reduced to 16 kg N ha (Figure 4.3)-1

Only by including limitation on plant growth by N and water deficiencies DAISY was ableto simulate dry matter and N accumulation of the cereal crop. This suggests that thepreceding crop effect under these experimental conditions only consisted of an N effect(Table 4.5; Webb & Sylvester-Bradley, 1994). Based on fertilizer N response curvesRowland et al. (1994) were not able to predict the response of cereal crops to a precedinglegume. This is not surprising as no information on the amount and utilisation of residualN after the preceding legume, nor information about the composition of the incorporatedN compounds, were used as basis for predicting this response. The present work showsthat a simulation model containing modules for the carbon and N dynamics of the soil canpredict the N effect of preceding legumes on succeeding crops (Figure 4.1; Table 4.5).

In the near future, DAISY will be expanded to simulate the growth and competition ofdifferent intercropped species. This will open the possibility for further analysing the

61

contribution of different species to the residual N effect.

ACKNOWLEDGEMENTS

Financial support from the Danish Ministry of Agriculture under the research programme"Organic Farming" is gratefully acknowledged.

REFERENCES

Bergersen, F.J., Turner, G.L., Gault, R.R., Peoples, M.B., Morthorpe, L.J. & Brockwell, J. (1992) Contributionof nitrogen in soybean crop residues to subsequent crops and to soils. Australian Journal of AgriculturalResearch, 43, 155-160.

Breland, T.A. (1994) Measured and predicted mineralization of clover green manure at low temperatures atdifferent depths in two soils. Plant and Soil, 166, 13-20.

Bremer, E. & van Kessel, C. (1992) Plant-available nitrogen from lentil and wheat residues during asubsequent growing season. Soil Science Society of America Journal, 56, 1155-1160.

Chalk, P.M., Smith, C.J., Hamilton, S.D. & Hopmans, P. (1993) Characterisation of the N benefit of a grainlegume (Lupinus angustifolius L.) to a cereal (Hordeum vulgare L.) by an in situ N isotope dilution technique.15

Biology and Fertility of Soils, 15, 39-44.

Danso, S.K.A. & Papastylianou, I. (1992) Evaluation of the nitrogen contribution of legumes to subsequentcereals. Journal of Agricultural Science, 119, 13-18.

Dou, Z. & Fox, R.H. (1994) The contribution of nitrogen from legume cover crops double-cropped with winterwheat to tilled and non-tilled maize. European Journal of Agronomy, 3, 93-100.

Evans, J., O'Connor, G.E., Turner, G.L., Coventry, D.R., Fettell, N., Mahoney, J., Armstrong, E.L. & Walsgott,D.N. (1989) N fixation and its value to soil N increase in lupin, field pea and other legumes in south-eastern2

Australia. Australian Journal of Agricultural Research, 40, 791-805.

Evans, D.R., Williams, T.A. & Mason, S.A. (1992) Residual N effect of grazed white clover (Trifoliumrepens)/ryegrass (Lolium perenne) swards on subsequent yield of spring barley. Journal of AgriculturalScience, 118, 175-178.

Farquhar, G.D. & Richards, R.A. (1984) Isotopic composition of plant carbon correlates with water useefficiency of wheat genotypes. Australian Journal of Plant Physiology, 11, 539-552.

Francis, G.S., Haynes R.J., Speir, T.W. & Williams, P.H. (1995) The effect of a nitrification inhibitor onleaching losses and recovery of mineralised nitrogen by a wheat crop after ploughing-in temporaryleguminous pastures. Fertilizer Research, 41, 33-39.

Giller, K.E. & Cadish, G. (1995) Future benefits from biological nitrogen fixation: An ecological approach toagriculture. Plant and Soil, 174, 255-277.

Gliessman, S.R. (1990) Agroecology: Researching the ecological basis for sustainable agriculture. In:Researching the ecological basis for sustainable agriculture. Part 1: Basic ecological concepts inagroecosystems (Gliesman, S.R., ed.). New York: Springer-Verlag Inc., pp. 3-10.

Goodman, P.J. (1988) Nitrogen fixation, transfer and turnover in upland and lowland grass-clover swards,using N isotope dilution. Plant and Soil, 112, 247-254.15

62

Hansen, S., Jensen, H.E., Nielsen, N.E. & Svendsen, H. (1990) DAISY - Soil Plant Atmosphere SystemModel. NPO-Research Report No. A10. The National Agency of Environmental Protection. Denmark.

Hansen, S., Jensen, H.E, Nielsen, N.E. & Svendsen, H. (1991) Simulation of N dynamics and biomassproduction in winter wheat using the Danish simulation model DAISY. Fertilizer Research, 27, 245-259.

Harper, L.A., Hendrix, P.F., Langdale, G.W. & Coleman, D.C. (1995) Clover management to provide optimumnitrogen and soil water conservation. Crop Science, 35, 176-182.

Hauck, R D. & Bremner, J.M. (1976) Use of tracers for soil and fertilizer nitrogen research. Advances inAgronomy, 28, 219-226.

Høgh-Jensen, H. (1996) Simulation of nitrogen dynamics and biomass production in a low nitrogen input,water limited clover/grass mixture. In: Dynamics of roots and nitrogen in cropping systems of the semi-aridtropics (Ito, O., ed.). Proc. Int. Workshop at ICRISAT, India, at 21-25 Nov. 1994. In press.

Høgh-Jensen, H. & Kristensen, E.S. (1995) Estimation of biological N fixation in a clover-grass system by2

the N dilution method and the total-N difference method. Biological Agriculture and Horticulture, 11, 203-219.15

Høgh-Jensen, H. & Schjoerring, J.K. (1994) Measurement of biological dinitrogen fixation in grassland:comparison of the enriched N dilution and the natural N abundance methods at different nitrogen15 15

application rates and defoliation frequencies. Plant and Soil, 166, 153-163.

Jensen, E.S. (1994) Availability of nitrogen in N-labelled mature pea residues to subsequent crops in the15

field. Soil Biology & Biochemistry, 26, 465-472.

Ladd, J. & Amato, M. (1986) The fate of nitrogen from legume and fertilizer sources in soils successivelycropped with wheat under field conditions. Soil Biology & Biochemistry, 18, 417-425.

Ladd, J., Amato, M., Jackson, R.B. & Butler, J.H.A. (1983) Utilisation by wheat crops of nitrogen from legumeresidues decomposing in soils in the field. Soil Biology & Biochemistry, 15, 231-238.

Ladd, J., Butler, J.H.A. & Amato, M. (1986) Nitrogen fixation by legumes and their role as sources of N for soiland crop. Biological Agriculture and Horticulture, 3, 269-286.

Lindén, B. & Wallgren, B. (1993) Nitrogen mineralization after leys ploughed in early or late autumn. SwedishJournal of Agricultural Research, 23, 77-89.

Müller, M.M. & Sundman, V. (1988) The fate of nitrogen ( N) released from different plant materials during15

decomposition under field conditions. Plant and Soil, 105, 133-139.

Papastylianou, I. (1993) Estimating total N fixation by legumes in long-term rotation studies. European2

Journal of Agronomy, 2, 1-10.

Power, J.F. (1990) Legumes and crop rotation. In: Sustainable agriculture in temperate zones (Francis, C.A.,Flora, C.B., King, L.D., eds). New York:Wiley & Sons, pp. 178-204.

Reeves, T.G., Ellington, A. & Brooke, H.D. (1984) Effects of lupin-wheat rotations on soil fertility, crop diseaseand crop yields. Australian Journal of Experimental Agriculture and Animal Husbandry, 24, 595-600.

Robbins, G.B., Bushell, J.J. & McKeon, G.M. (1989) Nitrogen immobilisation in decomposing litter contributesto productivity decline in ageing pastures of green panic (Panicum maximim var. trichoglume). Journal ofAgricultural Science, 113, 401-406.

Robertson, F.A., Myers, R.J.K. & Saffigna, P.G. (1993a) Carbon and nitrogen mineralisation in cultivated andgrassland soils in subtropical Queensland. Australian Journal of Soil Research, 31, 611-619.

Robertson, F.A., Myers, R.J.K. & Saffigna, P.G. (1993b) Dynamics of carbon and N in long-term croppingsystem and permanent pasture system. Australian Journal of Soil Research, 45, 211-221.

63

Rowland, I.C., Mason, M.G., Pritchard, I.A. & French, R.J. (1994) Effect of field peas and wheat on the yieldand protein content of subsequent wheat crops grown at several rates of applied N. Australian Journal ofExperimental Agriculture, 34, 641-646.

Russelle, M.D., Hesterman, O.B., Shaeffer, C.C. & Heichel, G.H. (1987) Estimating N and rotation effects inlegume-corn rotations. In: The role of legumes in conservation tillage systems (Power, J.F., ed.). Ankeny: SoilConservation Society of America, pp. 41-42.

Senaratne, R. & Hardarson, G. (1988) Estimation of residual N effect of faba bean and pea on two succeedingcereals using N methodology. Plant and Soil, 110, 81-89.15

Strong, W.M., Harbison, J., Nielsen, R.G.H., Hall, B.D. & Best, G.K. (1986) Nitrogen availability in a DarlingDowns soil following cereal, oilseed and grain legume crops. 2. Effects of residual soil N and fertilizer N onsubsequent wheat crops. Australian Journal of Experimental Agriculture, 26, 353-359.

Vereecken, H., Jansen, E.J., Broeke, M.J.D.H., Swerts, M., Engelke, R., Fabrewiz, F. & Hansen, S. (1991)Comparison of simulation results of five nitrogen models using different data sets. In: Soil and groundwaterresearch report II: nitrate in soils. Final Report on Contracts EV4V-0098-NL and EV4V-00107-C. DG XII.Commission of the European Communities. pp. 321-338.

Webb, J. & Sylvester-Bradley, R. (1994) Effects of fertilizer nitrogen on soil nitrogen availability after grazedgrass ley and on the response of the following cereal crop to fertilizer N. Journal of Agricultural Science, 122,445-457.

Zapata, F. (1990) Isotope techniques in soil fertility and plant nutrition studies. In: Use of nuclear techniquesin studies of soil-plant relationships (Hardarson, G., ed.). Vienna: IAEA. pp. 61-127.

64

5. Strategies to improve yield and crop quality by different distribution of limited amounts of farmyard and liquid manure applied to subsequent crops after grass-clover

By Karin Stein-Bachinger 1

SUMMARY In Organic Agriculture the proportion of legumes in the rotation and their ability to accumulate N2 determine the potential productivity of the whole system decisively (Köpke, 1990; Reents, 1991). Because the amount of additional fodder from outside the farm is restricted, only a certain maximum number of animals can be kept. Hence, the amount of farmyard manure, which can be used flexible in the crop rotation, is limited. Therefore its storage and application has to be carefully planned to improve yield and crop quality and to minimise nutrient-losses within the system. Field trials were conducted in 1989-1991 on the organically run experimental farm at the University of Bonn (Wiesengut) to determine the optimal distribution of organic manure (farmyard manure compost (FYMC) and liquid manure) in the first part of a six year crop rotation cycle (grass-clover - potatoes - winter wheat - beans - spring cereals - winter rye with grass-red clover undersown) after ploughing grass-clover in spring. The amounts of organic manure applied during the cycle were limited by the nutrient equivalent of the manure produced by 0.6-1.4 livestock units (LU) per hectare. Increasing amounts of FYMC to potatoes caused no significant difference in total yield after grading, but the number of rotted tubers as well as big tubers (> 65 mm) increased. With the 'Granola' potato variety, there was no relationship between the nitrate content of the tubers and the amount of fertiliser applied nitrate. The concentration in all fertilising variants remained below 100 mg NO-

3/kg. The yield as well as the protein content of winter wheat increased more following the direct application of FYMC before sowing. The use of liquid manure to winter wheat showed that the application during tillering increased the yield, while the application during flush resulted in a higher protein content. The comparison of differently treated FYMC (3-4 and 12 months respectively on a

1 Institute for Plant Nutrition, University of Bonn, Meckenheimer Allee 176, D-53115 Bonn,

Germany; present address: Centre for Agricultural Landscape and Land Use Research, Institute for Land Use Systems and Landscape Ecology; Eberswalder Str. 84, D-15374 Müncheberg, Germany

65

heap) showed no effects on yield and crop quality because of the equal levels of N. However, when the FYMC was composted for 12 month, 50 % higher rates were required in order to reach the equivalent fertilisation level. The results show that when rotation is carried out under similar conditions, it is recommended that limited amounts of organic manure should be applied to winter wheat. INTRODUCTION In Organic Agriculture the principle of operating within a system of crop and livestock nutrition, which is as closed as possible, is aspired to. This requires minimising the use of nutrient inputs from outside the farm as well as losses within the farming system, in order to optimise the productivity and to avoid environmental problems such as nitrate leaching. As mineral nitrogen fertilisers are not used in Organic Agriculture, crop production relies principally upon the efficiency of legumes to accumulate nitrogen. Because the amount of additional fodder from outside the farm is restricted, animal husbandry must match the fodder basis of the farm. Therefore, in organically managed farms about 0.3 - 1.4 LU/ha can be kept. Hence, the amount of farmyard manure is limited and its application must be calculated over the whole crop rotation cycle. In order to prevent nitrate leaching and to optimise yield and crop quality, it is very important to determine the optimal combination of farmyard manure and legumes as pioneer crops to retain nitrogen. Apart from the quantitative limitation of organic manure, it is also difficult to predict the N mineralisation of manure and compost resulting from the different methods of keeping, feeding and stocking animals. Studies of organically managed farms show that there is a lower nutrient level compared to conventional farming systems as well as unfavourable availability of N in different parts of the system (nutrients in soil and manure, nitrate content in the grain) (Brandhuber & Hege, 1991; Hess, 1989; Hess & Klein, 1987; Piorr, 1992; Piorr et al., 1991). Consequently a lot of organic farms have slightly negative to slightly positive N-levels (Hege & Weigelt, 1991; Kaffka et al., 1988; Nolte, 1989). Nitrogen can therefore be a minimal factor in these systems. By compensation of the N-level over the whole rotation cycle there can be a local, short-term N-accumulation on the grass-clover ley (Hess, 1989; Köpke, 1990; Reents, 1991). Obviously there is a need to minimise the N losses through leaching, in order to have a nutrient cycle within the farm system, which is as closed as possible and to avoid environmental problems.

66

In the following investigations the possibility of a spring ploughing of grass-clover, followed by the cultivating of fallow crops, e.g. potatoes, was tested as an alternative method to autumn ploughing following winter wheat in order to improve the nitrogen exploitation of light soils. In addition, the question of the most favourable distribution of limited amounts of farmyard manure to potatoes and winter wheat in the rotation was important with respect to yield and quality. Regarding these different requirements, the main targets of this project were to develop strategies for an efficient use of the limited amount of organic manure during the first part of a six year crop rotation cycle (grass-clover - potatoes - winter wheat - beans - spring cereals - winter rye with grass-clover undersown) by variation of the date of application as well as the amount and quality of organic manure (short and long composted farmyard manure and liquid manure). MATERIAL AND METHODS From 1989-1991 the following field experiments were held with regard to the subsequent two crops (potatoes and winter wheat) following ploughing of grass-clover in spring (Figure 5.1). Trial I covers the period 1989-1990 (02.05.89: planting of potatoes, 28.10.89: sowing of winter wheat). In 1990 the trial was repeated (trial II, 25.04.90: planting of potatoes, 25.10.90: sowing of winter wheat). The soil type was a light fluvisol (sU). Mean annual temperature during the experimental years was 10.8°C, while the annual rainfall ranged between 719 mm and 800 mm. Fertilisation treatments: (Figure 5.1) * Level of fertilisation: The total amount of manure corresponded with the nutrient

equivalent produced by 0.6 - 1.4 LU/ha (= 4 levels; according to Nt of farmyard manure and NH4

+-N of liquid manure). By calculation over three years, provided that faba beans didn't get any fertilisation, the total level for potatoes and winter wheat varied between 80 to 240 kg N/ha.

* Quality of manure: Two farmyard manure qualities (deep litter composted for 3-4 months (FYMC 1) and 12 months (FYMC 2)) were tested.

* Periodic distribution of FYMC applications: The effects were tested of

a) fertilising one crop only (potatoes or winter wheat) without applying any fertiliser in the succeeding years

b) applying equal amounts of organic manure to potatoes and winter wheat according to the four levels

c) applying different amounts of liquid manure to winter wheat in spring (to tillering or flush), with the aim of improving the protein quality of winter wheat.

67

The time chosen for farmyard manure application was shortly before planting/sowing of the crops. The potato variety was 'Granola', the winter wheat variety was 'Reiher' (B 5).

Fig. 5.1: Fertilisation treatments RESULTS AND DISCUSSION The following results concern the effects of FYMC 1 (deep litter composted for 3-4 months). The main subjects of investigation were yield and quality of potatoes and winter wheat. More details are given in Stein-Bachinger (1993). * First crop after ploughing grass-clover: potatoes

Increasing amounts of farmyard manure compost (0 to 240 kg N/ha) to potatoes had no significant effect on the yield (average 1989: 341,5 dt/ha, 1990: 334,8 dt/ha) but increased the amount of spoilage (table 5.1) as well as the number of large tubers >65 mm (no figure). In trials with three potato-varieties, Hunnius & Munzert (1979) also found an increasing percentage of tubers > 55 mm when using higher N application

68

rates, while the percentage of tubers from 35-55 mm decreased. Wedler (1993), on the other hand, only measured a small increase in big tubers (> 70 mm) in trials with FYMC application to potatoes (240-480 dt/ha). But legumes were not used as pioneer crops in any of these trials, therefore a comparison with the present results is restricted. Tab. 5.1: Effect of increasing FYMC to potatoes on the amount of spoilage (dt/ha, 1989 and 1990) (values not marked with the same letter are significantly different, P <5 %, Tukey Grouping)

kg FYMC-N/ha

0

40

80

120

160

240

Trial I (1989)

26 a

31 ab

40 ab

40 ab

44 b

Trial II (1990)

26 a

33 ab

32 ab

32 ab

35 bc

41 c

The nitrate content of potato tubers is an important aspect of crop quality. In both years, the nitrate content of the tubers was not influenced by high amounts of FYMC following legumes as a pioneer crop (figure 5.2). The content in all treatments remained below 100 mg NO3/kg. Kolbe et al. (1992) and Wedler (1992) found similar results with even higher amounts of FYM (up to 600 dt/ha). According to Munzert &

Lepschy (1983) the crop variety is more important for the nitrate content than the level of fertilisation. The 'Granola' variety, which was used in our trials, showed as well the lowest nitrate content of the tested varieties in trials by the

Landwirtschaftskammer Rheinland (Wedler & Over-beck, 1993) using different fertilisation treatments. Fischer et al. (1992) found similar results by increasing mineral fertilisation up to 220 kg N/ha. But above all it cannot be excluded that

other varieties would show more effects especially in combination with manure and legumes as pioneer crops.

NO3 mg / kg FM

0 80 160 0 80 160 2400

20

40

60

80

100

kg FYMC-N/ha

1989 1990

Fig. 5.2: Nitrate content of tubers (trial I, 1989 and II, 1990)

69

* Second crop after ploughing grass-clover: winter wheat The direct application of FYMC to winter wheat showed significant effects on protein content as well as yield in comparison with the application to potatoes or the split variant (Figure 5.3). But higher yields in trial II resulted in a dilution effect due to the lower nitrogen intake in the grain. Therefore, an increase in the yield above the site-specific optimum leads to a decrease in crop quality. This means that it is necessary to strive for an 'optimal quality-yield'. The effect of increasing amounts of liquid manure in trial I as well as a later application to flush showed that the application to tillering increased the yield (Figure 5.4), while the application to flush resulted in a higher protein content. With an optimal com-bination of FYMC and liquid manure a protein content of 11.9 % was obtained (Table 5.2). Grain investigations, conducted by the Cereal Research Institute in Detmold showed that application of liquid manure (50 kg NH4

+-N/ha) to tillering resulted in a similar protein and gluten content as well as Zeleny value in comparison with a FYMC-application of 160 kg N/ha before sowing (Table 5.2). Stöppler (1988) found, using five winter wheat varieties (B 5) at seven locations over two years, an average protein

.

Yield (dt/ha) Protein content (%)

FYMC appl. to potatoes

FYMC appl. to potat.+ww

FYMC appl. to ww

FYMC appl. to potatoes

FYMC appl. to potat.+ww

FYMC appl. to ww

a

a

b

a

a

b

a

a

b

a

a

b

405060 6 7 98 10 1135455565

Yield (dt/ha) Protein content (%)

.3 0

3 5

4 0

4 5

5 0

5 5

6 0

6 5

5

6

7

8

9

1 0

1 1

1 2

Y i e ld ( d t /h a ) P r o t e in c o n t e n t ( % )

0 2 5 * 5 0 * 2 5 ** 0 2 5 * 5 0 * 2 5 * *k g N H - N /h a4

a b c a a a b c b c

Y ie ld ( d t /h a ) P ro t e i n c o n t e n t ( % )

Fig. 5.3: Effects of different applications of FYMC on yield and proteincontent of winter wheat (trial I:1990; trial II: 1991) (appl. = applied;potat. = potatoes; ww = winter wheat)

Fig. 5.4: Influence of different liquid manure applications (* to tillering, to flush) on yield and protein content of winter wheat (trial I: 1990)

70

content of 10.9 %, while the Zeleny value which is a criterion for the amount and quality of gluten (Dreyer 1984), was lower in the present trials at 21 ccm than in Stöppler=s trials (1988, up to 44). Liquid manure and FYMC increased the three quality parameters furthermore (Table 5.2). A lower protein content also resulted in a decrease in the volume. Therefore the values of the 'Rapid-Mix-Test' were on average 54 ml/100 g flour higher in the fertilised variants than in the control. Tab. 5.2: Grain-quality of winter wheat (four fertilisation treatments) (FYMC before sowing winter wheat; 50 kg NH4

+-N/ha liquid manure to tillering)

0 kg FYMC-N/ha

160 kg FYMC-N/ha

without

liquid manure

with

liquid manure

without

liquid manure

with

liquid manure Protein content (%) Zeleny value (ccm) Gluten content (%) Rapid-Mix-Test Volume (ml/100 g flour) Wholemeal-test Volume (ml/100 g flour)

9.7

16

20.4

524

312

10.8

18

23.3

580

317

10.8

18

23.1

583

307

11.9

21

26.3

571

312

Considering both years of rotation the nitrogen-exploitation of the FYMC was between 5 and 21 % because of the high mineralisation from grass-clover residues. Similar ranges were found by Görlitz (1983) and Rauhe & Hoberück (1982). Foth (1984, in: Sommerfeldt et al., 1988) detected residual effects after 40 years. Because of the use of legumes as pioneer crops in the present trials there were lower exploitation rates than other authors confirm (Piorr, 1992; Rauhe, 1969). The comparison of differently treated FYMC (3-4 and 12 months respectively on a heap) did not reveal any differences in the yield or quality parameters of potatoes and winter wheat. The main reason is certainly that the application rates were based on the N content, so that same amounts of nitrogen did not show effects on the plant growth. But it is important to note that the long rotted farmyard manure had very low N contents, so that approximately 50 % higher application rates were needed in order to reach the equivalent fertilisation level. Because of the inevitable increase in substance loss dependent on the storage time, a long composting period should be avoided (not longer than 6-8 months) so that the humification processes will take place in the soil rather than in the compost heap. This is especially important considering the limited amount of farmyard manure in organic agricultural systems.

71

CONCLUSIONS The influence of organic manure to soil fertility and plant growth is very complex and it cannot be restricted only to nitrogen nutrition effects. Plant nutrition results indirectly from activity of the soil flora, organic fertiliser, plant residues and humus. It is i.e. very difficult to predict the N mineralisation of manure and compost resulting from different methods of keeping, feeding and stocking animals. Fertilising preferentially winter wheat, as recommended according to the own results, leads to the necessity of applying manure in spring to another crop in the rotation, because long storage times of manure are causing an inevitable increase in substance loss. On the other hand it cannot be excluded that there might be nitrogen losses during the winter period under winter wheat. Therefore, the optimal application of organic manure should be considered regarding the individual farm system as well as its whole crop rotation cycle, animal keeping and storage facilities. Liquid manure applications in organic agricultural systems must be regarded differently. As there is generally a limited amount of organic manure on a farm, higher amounts of liquid manure with a sufficient nitrogen content depend on the farming system in operation, and additionally, the profitability of a late application has to be considered due to a more expensive application technique necessary to avoid volatile N losses, as well as the damage caused by driving over the crops. From liquid manure and slurry a farmer will have instantaneous fertilising effects because of the short-term availability of nitrogen. Therefore it can be recommended to establish both systems: farmyard manure and liquid manure respectively slurry. These aspects must be taken into account for optimising the way how to deal with organic manure. ACKNOWLEDGEMENTS The author gratefully thanks Prof. Dr. U. Köpke and his stuff of the Institute for Organic Agriculture, University of Bonn, for their helpful cooperation and the Ministry of Environment, Regional Planning and Agriculture in North-Rhine-Westfalia, Germany, for their financial support.

72

REFERENCES Brandhuber, R. & Hege, U. (1991) Nitratbelastung des Sickerwassers unter Acker- und Grünland viehhaltender Betriebe - Ergebnisse von Tiefenuntersuchungen. VDLUFA-Schriftenreihe, 33, Kongreßband, 203-208. Dreyer, E. (1984) Untersuchungen über die Eignung verschiedener Weizensorten und Verarbeitungsverfahren für die Herstellung von Vollkornbackwaren. Diss. Gießen. Fischer, J., Behnke, S., Westrum, H. & Przemeck, E. (1992) Auswirkungen einer Stickstoffreihendüngung zu Kartoffeln auf Ertrag, Qualität und Rest-Nmin im Vergleich zur breitwürfigen Düngung. VDLUFA- Schriftenreihe, 35, Kongreßband, 511-514. Görlitz, H. (1983) Wirkung der Nährstoffe aus organischer Düngung. Tag. Ber., Akad. Landwirtsch. Wiss. DDR, 211, 21-30. Hege, U. & Weigelt, H. (1991) Nährstoffbilanzen alternativ bewirtschafteter Betriebe. Bayer. Landwirtsch. Jahrb., 68. Jg., Heft 4, 403-407. Hess, J. & Klein, A. (1987) Möglichkeiten zur Verringerung der N-Frühjahrslücke im Organischen Landbau durch verbesserte Nutzung von Leguminosen-N und systemkonforme Düngungsmaßnahmen. Forschung und Beratung, Reihe B, Heft 36, 42-63. Hess, J. (1989) Kleegrasumbruch im Organischen Landbau: Stickstoffdynamik im Fruchtfolgeglied Kleegras - Kleegras - Weizen - Roggen. Diss. Bonn. Hunnius, W. , Müller, K. & Winner, C. (1978) Düngen wir richtig im Blick auf die Qualität von Hackfrüchten? Landwirtsch. Forschung, 35, 54-71. Hunnius, W. & Munzert, M. (1979) Zur Höhe und Verteilung der Stickstoffgaben bei Kartoffeln in Abhängigkeit von der Sorte. Potato Res., 289-304. Kaffka, S., Koepf, H.H. & Sattler, F. (1988) Nährstoffbilanz und Energiebedarf im landwirtschaftlichen Betriebsorganismus. Lebendige Erde, Heft 4, 232-240. Kolbe, H., Meineke, S. & Zhang, W.-L. (1992) Ertrags- und Qualitätsunterschiede zwischen langjährig organisch und mineralisch gedüngten Kartoffelknollen im Vergleich zu Modellkalkulationen. 104. VDLUFA-Kongreß, Posterpräsentation. Köpke, U. (1990) Pflanzenbauliche Strategien für einen umweltverträglichen und standortgerechten Landbau. In: Vorträge der 42. Hochschultagung der Landw. Fakultät der Univ. Bonn. Hrsg.: FINKE, K. und J., Forschungsbericht 1987-1989, Bonn, 165-182. Munzert, M. & Lepschy, J. (1983) Zur Frage des Nitratgehaltes in Kartoffelknollen. Der Kartoffelbau, 34,163-168. Nolte, Ch. (1989) Bilanzierung des Nährstoffkreislaufes auf dem biologisch-dynamisch bewirtschafteten "Boschheidehof" sowie Untersuchungen zum Phosphor- und Kaliumhaushalt in drei ausgewählten Böden im Vergleich zu drei Böden eines benachbarten konventionellen Betriebes. Diss. Bonn. Piorr, A., Berg, M. & Werner, W. (1991) Stallmistkompost im Ökologischen Landbau: Erhebungsunter-suchung zu Nährstoffgehalten und deren Beziehung zu Aufbereitungsverfahren. VDLUFA-Schriftenreihe, 33, Kongreßband, 335-340. Piorr, A. (1992) Zur Wirkung von residualem Kleegras- und Wirtschaftsdüngerstickstoff auf die N-Dynamik in ökologisch bewirtschafteten Böden und die N-Ernährung von Getreide. Diss. Bonn.

73

Rauhe, K. (1969) Der Einfluß des Futterbaus sowie der organischen und mineralischen Düngung auf C- und N-Gehalt des Bodens im Fruchtfolgeversuch Seehausen. Albrecht-Thaer-Arch., 13, 455-462. Rauhe, K. & Hoberück, J.-M. (1982) Zur Verwertung des Dünger- und Leguminosenstickstoffs im System Boden-Pflanze am Beispiel langjähriger kombinierter Fruchtfolge-Düngungs-Feldversuche auf einem Lehm-Standort. Tag.-Ber.Akad.Landwirtsch.-Wiss.DDR, Berlin 205, 211-220. Reents, H.-J. (1991) Nitratverlagerung nach Leguminosenumbruch in biologisch-dynamisch geführten Betrieben. Lebendige Erde, Heft 6, 303-312. Sommerfeldt, T.G., Chang, C. & Entz, T. (1988) Long-term annual manure applications increase soil organic matter and nitrogen, and decrease carbon to nitrogen ratio. Soil Sci. Soc. Am. J., 52, 1668-1672. Stein-Bachinger, K. (1993) Optimierung der zeitlich und mengenmäßig differenzierten Anwendung von Wirtschaftsdüngern im Rahmen der Fruchtfolge organischer Anbausysteme. Diss. Bonn. Stöppler, H. (1988) Zur Eignung von Winterweizensorten hinsichtlich des Anbaues und der Qualität der Produkte in einem System mit geringer Betriebsmittelzufuhr von außen. Diss. Witzenhausen. Wedler, A. (1992) Einfluß organischer und mineralischer Düngung auf Inhaltsstoffe von verschiedenen Gemüsearten und Speisekartoffeln. Vortrag anläßlich des 102. VDLUFA-Kongresses in Göttingen. Wedler, A. (1993) Pers. com. Wedler, A. & Oberbeck, G. (1993) Kartoffelqualität. Abschlußbericht: Forschungs- und Entwicklungs-vorhaben "Alternativer Landbau Boschheide Hof" 1979-1992, Forschung und Beratung, Reihe C, Heft 49, 211-216.

74

6. Nitrogen cycling in an organic dairy crop rotation. Effects of organic manure type and livestock density

By J. Eriksen, M. Askegaard & F.P. Vinther1 SUMMARY Nitrogen flows were evaluated in an organic dairy crop rotation with different livestock densities (0.9 and 1.4 livestock units ha-1) and different types of organic manure (slurry and deep litter). Nitrate leaching ranged from 12 to 100 kg ha-1 and was highest in the first years after ploughing-in the grass-clover pasture, irrespective of livestock density or type of manure. Nitrogen balances not including atmospheric nitrogen fixation showed a deficit in legume-containing crops. Using the 15N dilution technique in areas without cattle grazing it was estimated that this deficit was easily equalized by atmospheric N2 fixation. However, urinary N excretion by cattle was shown to reduce biological nitrogen fixation. This indicates that the figure of more than 200 kg N ha-1

fixed in clover shoots in an ungrazed pasture may be considerably overestimating that of a grazed pasture. INTRODUCTION The main part of organic farming in Denmark is concerned with dairy production. On such farms the export of nutrients in sales (animal products) is small and there is a nearly closed cycle of nutrients within the farm. The farms have mixed cropping systems (pasture/arable) and during the pasture phase of the rotation, symbiotic N2-fixation by clover can import large amounts of N. Commonly, N2-fixing crops are required in a third of the area in the rotation to compensate for N lost or exported from the agroecosystem (Granstedt, 1992). In order to achieve optimal yields in the arable crops it is important that nitrogen added to the soil in plant residues and animal manure is available at the time of plant growth. Preferably, N mineralization in the soil should be synchronized with the consumption of N by plant uptake. In the first year after ploughing-in of the pasture, N-fertilizer is not required but in the following years gradually more and more fertilizer- N is required to achieve reasonable yields (Francis et al., 1995). Thus, the mixed rotation can be divided into three phases with different specific objectives regarding nutrient management: i) the pasture phase where N2 fixation is maximized to make nitrogen available to the system, ii) the years immediately after ploughing-in of the pasture, where nitrogen consumption by crops is optimized to maximize fodder production and to minimize nitrate leaching and iii) the following years where utilization of nitrogen in organic manures is optimized for the same reasons. The present investigation was initiated in order to evaluate nutrient balances at the field level in four types of organic husbandries with different livestock densities and different types of organic manure (slurry and deep litter). The objective of this ongoing project is to improve the utilization of nutrients in organic dairy farming with special

1 The Danish Institute of Plant and Soil Science, Research Centre Foulum, P.O. Box 23, DK-8830 Tjele, Denmark

75

reference to optimizing plant production and minimizing losses of nutrients to the environment in each phase of the rotation and consequently in the rotation as a whole. This paper will focus on nitrate leaching, nutrient balances and atmospheric nitrogen fixation in the experiment. MATERIALS AND METHODS Site The organic dairy crop rotation is located at Research Centre Foulum, in the central part of Jutland (9°34’E, 56°29’N). The soil is classified as a Typic Hapludult with 7.5% clay and 1.6% carbon. Mean annual rainfall and temperature is 770 mm and 7.7°C, respectively. The fields were converted to organic farming in 1987, where a six-year rotation was introduced as replacement of a conventional cereal rotation (Table 6.1). At the beginning of conversion to organic farming a detailed physical, chemical and biological characterization of the soil was carried out (Heidmann, 1989). Tab. 6.1: History of the organic dairy crop rotation at Research Centre Foulum

Period Agricultural management system

1974-1986 Mostly cereals; straw removal; inorganic fertilizer 1987-1993 Organic dairy crop rotation; cattle slurry (1.3 LU/ha) Barley undersown with grass-clover 1st year grass-clover 2nd year grass-clover Barley/pea/ryegrass (wholecrop) Oats or winter wheat Fodder beets 1994-1997 Organic dairy crop rotation continued with different fertilizer treatments 1) Slurry (0.9 LU/ha) 2) Slurry (1.4 LU/ha) 3) Deep litter + slurry (0.9 LU/ha) 4) Deep litter + slurry (1.4 LU/ha)

Field trail In autumn 1993 and spring 1994 four organic fertilizer treatments in four replicates were established (Table 6.1), replacing the previous slurry application equivalent to 1.3 livestock units (LU) ha-1. The treatments represent two systems of cattle housing based on either slurry alone or a combination of slurry and deep litter at two livestock densities. Organic fertilizer was applied in spring and ploughed into the soil immediately after application, or by using trail hose application to growing plants. Deep litter was applied to winter wheat in the autumn before sowing. The difference between the 0.9 and 1.4 LU ha-1 systems were in average 40 kg of total N (Table 6.2). In the deep litter systems the major part of total N was contained in organic matter and therefore not available to plants.

76

Each of the six fields in the rotation were divided into four blocks, where the four organic fertilizer treatments were present in plots of 15x18m (Fig. 6.1). After one cut, the grass-clover fields were grazed by cattle. In each grass-clover field two groups of four heifers were grazing in treatment 1 and 3 and in treatment 2 and 4, respectively (Fig. 6.1). Fencing allowed the plots grazed by the same group of heifers to be connected. Tab 6.2: Organic fertilizer application to the dairy crop rotation Slurry Slurry + deep litter 0.9 LU ha-1 1.4 LU ha-1 0.9 LU ha-1 1.4 LU ha-1 kg total-N ha-1 1. Barley, grass-clover

undersown Slurry Deep litter

70 100 50 90

80 90

2. 1st year grass-clover Slurry Deep litter

3. 2nd year grass-clover Slurry Deep litter

70 140

4. Barley/pea wholecrop ryegrass undersown

Slurry Deep litter

60 90

5. Winter wheat Slurry Deep litter

140 170 80 90

140 90

6. Fodder beets Slurry Deep litter

210 250 110 110

140 160

Average 82 120 88 132

Nitrate leaching In each of the 16 plots in each field three ceramic suction cups were installed in a depth of 1 m (Fig 6.1). Soil water was sampled and analyzed for nutrient content, including nitrate. Drainage volume was calculated by the EVACROP model (Olesen & Heidmann, 1990) where inputs were daily measurements of meteorological data (precipitation, temperature and evaporation) and type of crop, time of sowing, cutting and irrigation and soil physical parameters. Nitrate leaching was calculated using the trapezoidal rule (Lord & Shepherd, 1993). Calculation of the nitrogen balances at field level Nitrogen balances were calculated from accurately measured in- and outputs. Inputs were nitrogen in organic fertilizer and outputs were nitrogen in crop yields, weight gain of the grazing cattle and leaching losses. Determination of atmospheric nitrogen fixation Input of atmospheric nitrogen through biological nitrogen fixation (BNF) in the grass-clover was determined using the 15N dilution method (McAuliffe et al., 1958). In the spring (April) of 1994 15N-plots (1 m2) in four replicates were established by applying 1 g of 6.2 atomic % 15N as (NH4)2SO4. The plots were harvested seven to eight times

77

during the season, clover and grass was separated, dried and analyzed for 15N and total N, and the BNF was calculated as described by McNeil & Wood (1990). The plots were not grazed, and the estimates of BNF may therefore be considered as maximum values. Effects of urine on the BNF were studied separately. Urine with two levels of N (5 and 7 g N l-1 urine, respectively), resulting from two levels of protein in the supplement fodder, was used. Artificial urine patches (1 m2) were established by applying urine (4 l m-2) mixed with 1 g of 12.4 atom % 15N as (NH4)2SO4. In the control treatment water was used instead of urine. The plots were harvested three times and the dry matter production as well as the proportion of N derived from the atmosphere was calculated.

Fig. 6.1: Design of field experiment with four replicates of the four organic fertilizer treatments and two fields with grazing cattle. Magnified: Treatment plot with three ceramic suction cups inserted.

78

RESULTS AND DISCUSSION Yields In Fig. 6.2 yields of 1994 are presented, the first year after introduction of the different organic fertilizer treatments. One cut has been made for silage in the grass-clover fields followed by cattle grazing. The results confirm the findings from private farms, that yields in forage crops (barley/pea wholecrop and fodder beets) are maintained or only slightly reduced by converting to organic farming, whereas yield reductions can be expected in cereals (Halberg et al., 1994). Differences between the organic fertilizer treatments were only observed in barley and the barley/pea-mixture.

Fig. 6.2: Yields in the organic dairy crop rotation 1994. Organic fertilizer application according to Table 2.

Nitrogen leaching

Twentysix times from the beginning of April 1994 to the end of March 1995 soil water from 1 m depth was analyzed for nitrate. In Fig. 6.3 the nitrate concentrations are shown, the drainage modelled by EVACROP and the accumulated leaching losses from the winter wheat field, as an example. Nitrate leaching occurred all through autumn and winter. Total nitrate leaching from the crops in the rotation, ranging from 12 to 100 kg N ha-1 (Fig. 6.4), indicated that livestock density and organic fertilizer type were of minor importance for nitrate leaching compared to the pulse of nitrogen released from the grass-clover fields. Total annual leaching was lowest in the first year grass-clover (average 14 kg ha-1) increasing to around 50 kg in the second year grass-clover. Although urine-N may contribute to nitrate leaching in grazed pastures (Clough et al., 1996) these findings were to be expected since similar low leaching losses have been found by others when comparing clover-based pastures and fertilized grass pastures (Parsons et al., 1991; Ruz-Jerez et al., 1995). It appears that winter wheat in the first autumn after incorporation of grass-clover could not utilize all mineralized N,

79

Fig. 6.3: Nitrate concentrations in soil water sampled by ceramic suction cups, drainage modeled by EVACROP and accumulated leaching losses from April 1994 to April 1995.

which was therefore susceptible to leaching. The ability of a cover crop to remove nitrogen from the soil in autumn before winter leaching starts is largely determined by the sowing date (Francis et al., 1995). Sowing in early October probably means that soil temperatures were too low for winter wheat to have a significant N-uptake before spring. Under similar conditions it has been shown by Andersen et al. (1994) that winter wheat sown in October took up only 5 kg N ha-1 in autumn. Considerable leaching was also observed in the second autumn after grass-clover incorporation, where the soil was bare from harvest of winter wheat to sowing of fodder beets in spring. The soil was kept bare in this period for the purpose of weed control.

Fig. 6.4: Total annual nitrate leaching under an organic dairy crop rotation.

In the third and fourth year after grass-clover incorporation leaching decreased to around 60 and 40 kg N ha-1 in fodder beets and barley, respectively. A huge effect of increasing organic fertilizer application was only observed in the barley crop in the fourth year after grass-clover. However, leaching losses were generally low under

80

barley, and in this part of the rotation the effect of ploughing-in the pasture must be considered very low.

Nitrogen balance In Fig. 6.5 the nitrogen balance is shown calculated from inputs in organic fertilizer and outputs in crop yields, weight gain of grazing animals and leaching losses. Atmospheric nitrogen fixation was not included in the balance which is the reason for the nitrogen deficit in the grass-clover pastures and the barley/pea mixture. To achieve a positive nitrogen balance atmospheric nitrogen fixation in the grass-clover pasture must exceed 160 kg N ha-1.

Fig. 6.5: Nitrogen balance in the organic dairy crop rotation.

Atmospheric nitrogen fixation By making 7-8 cuts through the year in ungrazed areas in the field where 15N-labelled fertilizer was applied in spring it was shown that fixed N derived from the atmosphere in tops was 200-220 kg ha-1 (Fig. 6.6). The difference between atmospheric fixation in first and second year grass-clover was caused by a higher clover dry matter content in

the second year pasture. Similar experiments were made in the barley/pea mixture,

Fig. 6.6: Accumulated atmospheric nitrogen fixation in ungrazed areas in the 1st and 2nd year grass-clover field.

81

where atmospheric derived nitrogen reached 25-35 kg ha-1 (results not shown). However, determination of fixed atmospheric N in the grass-clover was carried out in a part of the field without grazing cattle. The effect of an artificial urine patch enriched with 15N on biomass production and nitrogen fixation is shown in Fig 6.7.

Fig. 6.7: The effect of an artificial urine patch on the dry matter production and the proportion of N derived from the atmosphere (pNdfa) in clover.

Spatial variability in grazed clover grass is high due to animal excreta. 75- to 95% of N ingested by grazing animals is returned in excreta to localized areas at rates equivalent to up to 1200 kg N ha-1 (Henzell and Ross, 1973). Approximately 50-80% of excreta N is in urine. Biological nitrogen fixation by legumes within urine-affected areas may decline by up to 90% (Ball et al., 1979; Ledgard et al., 1982). The BNF recovers after 30-60 days, when levels of inorganic N in soil fall to “back-ground” levels (Søegaard et al., 1996). However, urinary N induces an increase in associated grass growth and competition with the clover. The net effect of this is that BNF per unit area is reduced for relatively longer periods. Thus, in an intensive dairy farm, where up to 40% of the area may be affected by excreta (Saunders, 1984) and BNF within these areas may decline by an average of about 60%, total BNF may be reduced by up to 20-30% on an annual basis (Ledgard & Steele, 1992). This indicates that the figure of more than 200 kg N ha-1 fixed by clover tops in an ungrazed pasture may be considerably overestimating that of a grazed pasture. Thus, a quantitative measure of atmospheric nitrogen fixation in grazed pastures is essential to be able to evaluate if this rotation is capable of maintaining long term soil fertility. CONCLUSIONS The presented data originate from the first years of an ongoing experiment and conclusions made at this stage need to be confirmed in the following years. However, some general conclusions may be put forward at this point, regarding nitrogen cycling in organic dairy farming:

82

�� In a well-organized rotation including 2 years of grass-clover it is possible to maintain yields in forage crops similar to those of conventional farming. In cereals yields are generally lower.

�� The residual effect of the grazed pasture was more important for nitrate leaching than the type of organic fertilizer and application rate.

�� The usefulness of nitrogen balances was limited by the incomplete knowledge of nitrogen fixation in the grazed clover-grass fields.

In the future we consider major research tasks to be: �� Timing of nutrient release from legume residues and organic manure. �� Quantitative determination of N2 fixation in grazed grass-clover pastures. In the final part of this project and in some future projects we will address these questions. REFERENCES Andersen, A., Olsen, C.C. & Djurhuus, J. (1994) Dyrkning af overvintrende kornarter efter forskellige forfrugter og med forskellig såtid (English summary). Report no. 22. The Danish Institute of Plant and Soil Science. Ball, R., Keeney, D.R., Theobal, P.W. & Nes, P. (1979) Nitrogen balance in urine-affected areas of a New Zealand pasture. Agron. J. 71, 309-314. Clough, T.J., Sherlock, R.R., Cameron, K.C. & Ledgard, S.F. (1996) Fate of urine nitrogen on mineral peat soils in New Zealand. Plant and Soil 178, 141-152. Francis, G.S., Haynes, R.J. & Williams, P.H. (1995) Effects of the timing of ploughing-in temporary leguminous pastures and two winter cover crops on nitrogen mineralization, nitrate leaching and spring wheat growth. Journal of Agricultural Science, Cambridge 124, 1-9. Granstedt, A. (1992) Case studies on the flow and supply of nitrogen in alternative farming in Sweden. I. Skilleby-Farm 1981-1987. Biol. Agric. Hort. 9, 15-63. Halberg, N., Kristensen, E.S. & Kristensen, I.S. (1994) Expected yield loss when converting to organic farming in Denmark. In: Converting to organic agriculture (A. Granstedt & R. Koistinen. eds.). Appendix NJF-Utredning/Rapport nr. 93, pp. 43-47. Heidmann, T. (1989) Startkarakterisering af arealer til systemforskning II. Resultater fra arealet ved Foulum. Report No. S2007, The Danish Institute of Plant and Soil Science. Henzell, E.F. & Ross, P.J. (1973) The nitrogen cycle of pasture ecosystems. In: Chemistry and Biochemistry of Herbage 2. (G. W. Butler & R. W. Bailey, eds.) pp. 227-245. Academic Press, London. Ledgard, S.F. & Steele, K.W. (1992) Biological nitrogen fixation in mixed legume/grass pastures. Plant and Soil 141, 137-153. Ledgard, S.F., Steele, K.W. & Saunders, W.M.H. (1982) Effects of cow urine and its major constituents on pasture properties. N. Z. J. Agric. Res. 25, 61-68.

83

Lord, E.I. & Shepherd, M.A. (1993) Developments in the use of porous ceramic cups for measuring nitrate leaching. J. Soil Sci. 44, 435-449. McAuliffe, C., Chamblee, D.S. Uribe-Arango, H. & Woolhouse, W.W. (1958) Influence of inorganic N on nitrogen fixation as revealed by 15N. Agron. J. 50, 334-337. McNeil, A.M. & Wood, M. (1990) 15N estimates of nitrogen by white clover (Trifolium repens L.) growing in a mixture with ryegrass (Lolium perenne L.). Plant and Soil 128, 265-273. Olesen, J.E. & Heidmann, T. (1990) EVACROP. Et program til beregning af aktuel fordampning og afstrømning fra rodzonen. Version 1.01. Research note no. 9, Dept. of Agrometeorology, Danish Institute of Plant and Soil Science. Parsons, A.J., Orr, R.J., Penning, P.D. & Lockyer D.R. (1991) Uptake, cycling and fate of nitrogen in grass-clover swards continuously grazed by sheep. Journal of Agricultural Science, Cambridge 116, 47-61. Ruz-Jerez, B.E., White, R.E. & Ball, P.R. (1995) A comparison of nitrate leaching under clover-based pastures and nitrogen-fertilized grass grazed by sheep. Journal of Agricultural Science, Cambridge 125, 361-369. Saunders, W.M.H. (1984). Mineral composition of soil and pasture from areas of grazed paddocks, affected and unaffected by dung and urine. N. Z. J. Agric. Res. 27, 405-412. Søegaard, K., Petersen, S.O. & Vinther, F.P. (1996). The effect of urine under grazing on herbage production, N fixation and ammonia volatilization. Proceedings of the 16th European Grassland Federation Meeting, Sept. 1996, Italy, (in press).

Institute for Biodynamic Research, Brandschneise 5, D-64295 Darmstadt (Germany)1

84

7. Discussion: Nitrogen management in crop rotation. Symbioticnitrogen fixation and the role of farmyard manure

Summarized by J. Raupp1

QUANTIFYING SYMBIOTIC NITROGEN FIXATION AND TRANSFER

The fixation of clover-grass mixtures can account for approx. 130-300 kg N ha year (in-1 -1

the above ground plant parts only). To estimate the amounts of symbiotic nitrogen fixationby means of the N isotope dilution method might provide more realistic and reliable15

results compared to the acetylene reduction method and the total N difference method.But the N isotope dilution method requires a relatively high effort. Alternatively, the15

natural N abundance technique can be used. The estimations are accurate only for cut-15

and-carry systems. On pasture land the urine of the animals causes a decrease in nitro-gen fixation. The inhibitory effect of urine seems to be smaller than the reduction causedby equivalent amounts of mineral fertilizer nitrogen; however, mechanisms and extent ofthe urine effect still need to be investigated (see T1).

Transfer of fixed nitrogen from clover to grass is about 20-40 kg N ha year in cut-and--1 -1

carry systems, which is approx. 50% of the N accumulation in the grass. Transfer is likelyto be higher under grazing conditions.

USE OF LEGUME NITROGEN BY FOLLOWING CROPS

The management of N flows can be demonstrated by means of a diagram of Köpke (seeFig. 3.1). In the upper part of this figure the legume-born N flow is shown. The wedgesrepresent two valves adjusting the flow, one at its beginning which regulates the amountof fixed nitrogen, another one at its end which controls the proportion of N uptake to Nleaching. The position of the left valve can be regarded as a function of the efficiency ofthe symbiosis that depends on the site conditions, the symbiotic capacity of the bacteriaand legume species (varieties) as well as on the interactions of all these factors.

The position of the right valve, however, mainly is a function of cultivation techniques andagronomic decisions which can move it in both ways, either up and down or to the rightand left. Moving it down, i.e. reducing N losses is possible e.g. by- ploughing of the legume crop not in the autumn but in the winter or next spring,- cultivating a catch crop in the time between legume and the following crop (e.g.

cruciferous plants),

85

- reducing the tillage intensity when ploughing,- growing a crop with a high N demand (e.g. maize) or an effective root system after

legumes.

Moving the valve to the right or left, i.e. increasing or reducing the available N amount,resp., is possible e.g. by- growing either a legume monoculture or a legume-grass mixture,- ploughing of the legume crop as green manure or cutting it as fodder (transferring N

to the other fields),- using a long growing period of a legume crop (1-3 years) or a short one (some

months to 1 year).

In order to test the effect of N supplied by a legume forecrop, usually yield or N uptake ofthe following crop are compared with the same crop cultivated after a non-legume refer-ence crop. Another method is to use reference treatments with mineral N fertilizationinstead of a legume forecrop. The N content in winter wheat derived from clover-grasswas 25 to 43% higher than that derived from ryegrass residues only; for broad beans aspreceding crop to cereals a fertilizer nitrogen equivalent up to 100 kg N/ha was calculated(see chapters 3 and 4 in this volume).

With indirect approaches it remains unclear, however, which portion of N found in thefollowing crop originates in which N source of the cropping system (legume N, humus N,soil biomass N, manure N). Moreover, the contribution of the different N sources in asequence of years after the legume crop is largely unknown (see T2).

Today only little information is available on this topic, e.g. from tracer experiments com-paring organic and mineral fertilization. Ammonium nitrate fertilization mobilized consider-able amounts of soil-born N, in contrast to slurry fertilization (published in Arch. Acker- u.Pflanzenbau u. Bodenk., 1982). Results from Sweden (on clay soil in cold climate) showthat slurry can have a sufficient mineralization rate which should be used by a crop with ahigh N demand and/or a “late” position in the crop rotation.

FARMYARD MANURE APPLICATION

Concerning symbiotic N fixation farmyard manure has2

- negative effects because its application as such reduces the amount of fixed nitro-gen (see T1);

- positive effects in a long-term view by increasing the availability of phosphorus,calcium and other nutrients and by improving the soil structure.

Farmyard manure recycles organic matter and nutrients coming from the fodder. Thenitrogen content of farmyard manure, however, is not a matter of great significance for thenitrogen supply of crops. The role of farmyard manure in fertilization rather is to offerorganic matter for humus synthesis and to level the nitrogen release of the

86

soil.

In that regard the functions of farmyard manure are to provide- organic matter (carbon) as substratum of an active (microbial) soil life which converts

nitrate into a solid, organically bound type and by this helps to prevent nitrate leach-ing;

- nutrients, in particular phosphorus, and microelements for these processes.

However, the mineralization-immobilization processes are not yet fully understood (seeT3). Climatic influences have a significant impact. Under warm (but not dry) conditionsmineralization is increased with the risk of N leaching, whereas experiments in Swedenshowed a rather weak mineralization and a tendency to humus synthesis without notewor-thy yield effects.

Some experimental results indicate regulating effects of the biodynamic preparations onmineralization intensity (see T4).

The way of sharing out the limited quantity of farmyard manure among the different cropsof a rotation influences yield as well as product quality. These effects vary depending onsite conditions and manure type. Yield potential and yield limiting factors have to be foundspecifically for each crop under existing conditions. In many cases it would be an advan-tage to have diverse types of manure (slurry, solid and liquid farmyard manure, fresh andcomposted). This requires separate housing systems of the livestock being realized in onefarm which can be difficult sometimes because of technical and financial reasons.

NUTRIENT EFFICIENCY

Some strategies for an efficient use of nutrients, in particular nitrogen, based on appropri-ate cultivation techniques are described already in chapter 3. In addition to that, N effi-ciency also depends on certain properties of the cultivated plants which are partly a matterof plant breeding. The following criteria should be given greater priority in breedingprogrammes:- intensive root growth and root density of a genotype;- the capacity of the plant to influence its rhizosphere in order to improve nutrient

availability;- a high harvest index which indicates an efficient nutrient distribution within the plant.

Today some of these aspects are already considered, although they are difficult to bemanaged. Future research could make this easier (see T5).

An important aim of nutrient efficiency is to minimize nitrate leaching after a legume

T 1 The influence of organic nitrogen in solid and liquid manure on the intensity ofsymbiotic nitrogen fixation should be investigated on grazed pastures. The resultswill prove the real contribution of atmospherically derived nitrogen to the Nbalance.

concerning appropriate ploughing date and technique or cultivation of catch crops and

87

crop. A number of strategies have been developed already (see above and in chapter 3)

following cash crops. These strategies and experiences have to be modified consideringthe existing site conditions and the specific requirements of a crop rotation. As a rule theright strategy can reduce nitrate leaching to 20 kg N/ha during a winter period.

TASKS FOR FURTHER RESEARCH

T 2 The quantitative relationship between different organic N pools (soil organic matter,legume and non-legume residues, different types of organic manure) and their sharein the nitrogen cycle of soil-plant systems should be investigated by tracer experi-ments ( N, C) under various site conditions and evaluation more than one year after15 14

legume cultivation.

T 3 The N mineralization-immobilization processes under field conditions should beinvestigated considering different combinations of site conditions and cultivationtechniques. The information gained on this will help to improve the coordinationbetween nutrient release in soils and nutrient demand of crops.

T 4 The effects of biodynamic preparations on the mineralization intensity in soils withand without different organic admixtures should be investigated in field and modelexperiments in order to find out whether the use of preparations might help toimprove the effectiveness of organic fertilizers.

T 5 The nutrient efficiency of crop species and varieties depend on their growth patternand certain abilities to interact with their environment (e.g. rhizosphere, soilorganisms). More research effort should be undertaken to clarify the physiological andgenetic background of these parameters and abilities.