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Editorial

Fixed nitrogen in sustainable farming systems: a symposium examiningfactors influencing the extent of biological nitrogen fixation and its role

in southern Australian agricultural systems. Setting the scene

The principal factor governing the potential productivity ofsouthern Australian agricultural systems is the amount and

timing with which water is made available to growing crops.

Nitrogen supply is arguably the next most limiting factor

with some of the best gains in wheat yield (40 kg ha21 y21)

over the last 60 y occurring in areas where soil fertility has

been enhanced through inclusion of legumes in rotations

(Hamblin and Kyneur, 1993) and more recently by the

application of fertiliser nitrogen (Angus, 2001). Increased

amounts of plant available nitrogen in soils during the

growing season subsequent to legume production attest to

the positive effect that legumes can have on the availability

of nitrogen to subsequent non-leguminous crops (Peoples

et al., 1998; Peoples and Baldock, 2001). Such increases in

availability arise from either or both of the following

processes: (1) an enhanced release of inorganic nitrogen

through the mineralisation of the relatively nitrogen rich

legume residues in response to summer or early season

rainfalls (Peoples et al., 1998) or (2) a nitrogen sparing

effect where inorganic nitrogen not required by the legume

crop accumulates within the soil root zone and remains

available to subsequent crops (Chalk, 1998; Ahmad et al.,

2001).

All legumes (and their rhizobia) that have been

significantly exploited in Australian farming systems are

exotic to the Australian flora. They have been introducedaccidentally, or through plant introduction programs, a

process that continues to this day. Early recordings of exotic

legumes occurred in the late 1800s and by the 1930s early

selections of sub-clover (Trifolium subterraneum), annual

medic (Medicagospp.) and field pea (Pisum sativum) were

being recommended to farmers (Cocks et al., 1980;

Gladstones and Collins, 1983; Hawthorne et al., 2003). A

spectacular adoption of pastures followed in the period from

1940 and 1970 (Donald, 1965; Blyth and Menz, 1987).

More legume options were subsequently provided to

farmers with the introduction and development of several

new genera of pulses including narrow leaf lupin (Lupinus

angustifolius), faba bean (Vicia faba), lentil (Lens culinaris)

and chickpea (Cicer arietinum). In the last decade there has

also been rapid increase in the use of the pasture legumesserradella (Ornithopus spp.) and biserrula (Biser rula

pelecinus), especially on the lighter soils of Western

Australia. Collectively, the pulses are now sown on nearly

2 M ha annually (OConnell, 2001), but still occupy only a

fraction of the pasture area.Hill and Donald (1998)estimate

that sub-clover and annual medics occur on 29 M ha and

24 M ha respectively, although some overlap in the

distribution of the two genera undoubtedly occurs. Since

none of the aforementioned legumes are able to nodulate

with Australias indigenous microflora, compatible strains

of rhizobia have had to be provided to ensure good

nodulation when the legume is sown in a soil for the first

time, or re-sown into a hostile soil (e.g. medics on acidic

soils). High biological nitrogen fixing capacity with the

plant host is a priority in these rhizobial selection programs.

More than 30 strains of rhizobia are currently produced as

commercial inoculants in Australia by Bio-Care Technol-

ogy Pty Ltd (www.bio-care.com.au). There has been a

strong focus on the quality of inoculants. Accordingly, all

batches are tested to ensure they meet standards of efficacy,

rhizobial number and purity set by the Australian Legume

Inoculants Research Unit (NSW Department of Agricul-

ture).

Despite having had the opportunity to carefully manage

the introduction of many of legume and rhizobial genotypesreleased in Australia, the effectiveness of the symbioses that

fix atmospheric nitrogen into plant dry matter is often

suboptimal. It can vary significantly as a function of legume

species, rhizobia ecology, soil properties and environmental

characteristics. Estimates of the amount of biologically

fixed nitrogen for Australian pasture legumes based on the

application of15N natural abundance measurements to shoot

dry matter range from 2153 kg N ha21 for subterranean

clover, 2220 kg N ha21 for annual medics, 72160 for

vetch pastures, and 50195 kg N ha21 for several other

clover species (Peoples and Baldock, 2001). When these

amounts of biologically fixed nitrogen were expressed per

unit of pasture shoot dry matter, values from 9 to 36 kg fixed

N t21 dry matter, with a mean of approximately 25 kg fixed

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N t21 dry matter, were obtained (Peoples and Baldock,

2001) indicating the highly variable efficiency of biological

nitrogen fixation. Additionally, studies also suggest that

4050% of the total nitrogen fixed by pastures may reside

below ground (Zebarth et al., 1991; McNeill et al., 1997;

Jrgensen and Ledgard, 1997). As a result, total inputs and

the variation in inputs of biologically fixed nitrogen could

be up to two times larger than values obtained by measuring

shoots alone. Amounts of biologically fixed nitrogen found

in pulse legumes are even more variable than those noted for

pastures legumes. For example, ranges for the amounts of

nitrogen fixed in the shoots of soybean, faba bean and lupin

were found to be 0450, 12330 and 19327 kg N ha21,

respectively (Unkovich and Pate, 2000).

These data demonstrate the potential beneficial role that

biological nitrogen fixation by the legume/rhizobia sym-biosis can have on the input of nitrogen to Australian

agricultural systems. The data also demonstrate the large

variation in the effectiveness of this symbiosis, suggesting

the presence of gaps in our understanding of the factors

controlling the efficiency of biological N2 fixation and/or

our implementation of management strategies that allow

this symbiosis to operate at its potential capacity.

Contributions of biologically fixed nitrogen can also be

derived from a range of non-symbiotic heterotrophic

organisms. The presence ofnifgenes combined with their

genetic diversity allows these organisms to exist in a broad

range of habitats. Although positive effects of adding

heterotrophic organisms on the growth and N status of avariety of plants have been recorded, there are still no

examples of where these organisms have been exploited or

managed in the agricultural systems of southern Australia.

Important areas of research that will improve our ability

to maximise contributions of biologically fixed N to

agricultural systems include:

(1) Improved methodologies for assessing in situ the

nitrogen fixation capacity of various rhizobia/host

legume combinations.

(2) Selection of legume and rhizobial genotypes that are

better adapted to the various environmental niches

experienced in Australia.(3) Overcoming the competition presented by ineffective

strains of rhizobia.

(4) Understanding the mechanisms that lead to the

development diverse populations of rhizobia in soil.

(5) Development of inoculation technologies that improve

the survival of rhizobia and deliver higher numbers of

viable cells to the rhizosphere.

(6) Quantification of the influence of improved legume/r-

hizobia combinations on the net nitrogen balance of

rotations.

(7) Understanding agronomic practices (e.g. herbicide

application) that depress or enhance the symbiosis.

(8) Understanding the social psychology needed toencourage legume adoption.

(9) Defining tangible opportunities to exploit non-

symbiotic N2 fixers.

We add a note of caution. While there are many

beneficial effects of growing legumes in farming systems,

the asynchrony between the release of fixed nitrogen and its

consumption means that the potential for nitrate leaching

exists in many soils. It has the potential to cause serious off-

site impacts. The scope of the problem is likely to be large

and probably not dissimilar, but less visible, than that of

salinity. This being the case, strategies to utilise the

enhanced soil nitrogen fertility generated by legumes

probably deserves more focus.

This issue of Soil Biology and Biochemistry presents a

collection of some of the papers presented at the 13th

Australian Nitrogen Fixation Conference in Adelaide, SouthAustralia in September 2002 that address the identified areas

of research pertaining to biological nitrogen fixation.

References

Ahmad, T., Hafeez, F.Y., Mahmood, T., Malik, K.A., 2001. Residual effect

of nitrogen fixed by mungbean (Vigna radiata) and blackgram (Vigna

mungo) on subsequent rice and wheat crops. Australian Journal of

Experimental Agriculture 41, 245248.

Angus, J.F., 2001. Nitrogen supply and demand in Australian agriculture.

Australian Journal of Experimental Agriculture 41, 277288.

Blyth, M.J., Menz, K.M., 1987. The role of economics in pasture researchevaluation. In: Wheeler, J.L., Pearson, C.J., Robards, G.E. (Eds.),

Temperate pastures: their production, use and management, Australian

Wool Corporation/CSIRO, Victoria, pp. 586589.

Chalk, P.M., 1998. Dynamics of biologically fixed N in legume-cereal

rotations: a review. Australian Journal of Agricultural Research 49,

303316.

Cocks, P.S., Mathison, M.J., Crawford, E.J., 1980. From wild plants to

pasture cultivars: annual medic and Subterranean clover in Southern

Australia. In: Summerfield, R.J., Bunting, A.H. (Eds.), Advances in

Legume Science, Royal Botanic Gardens, London.

Donald, C.M., 1965. The progress of Australian agriculture and the role of

pastures in environmental change. Australian Journal of Science 27,

187198.

Gladstones, J.S., Collins, W.J., 1983. Subterranean clover as a naturalised

plant in Australia. The Journal of the Australian Institute of AgriculturalScience 49, 191202.

Hamblin, A., Kyneur, G., 1993. Trends in wheat yields and soil fertility in

Australia, Australian Government Publishing Service, Canberra.

Hawthorne, W., Day, T., Pritchard, I., Ali, M., Sykes, J., Armstrong, E.,

Brouwer, J., Bretag, T., Davidson, J., 2003. Pea history calendar of

events. In: Regan, K., Harries, M., Pritchard, I. (Eds.), Proceedings

of the Field Pea Focus 2003, Pulse Australia, West Australia, pp.

5262.

Hill, M.J., Donald, G.E., 1998. Australian Temperate Pastures Database.

Compact Disc, CSIRO Animal Production, Perth.

Jrgensen, R.V., Ledgard, S.T., 1997. Contribution from stolons and roots

to estimates of the total amount of N2fixed by white clover (Trifolium

repens L). Annals of Botany 80, 641648.

McNeill, A.M., Zhu, C., Fillery, I.R.P., 1997. Use of in situ 15N-labelling to

estimate the total below-ground nitrogen of pasture legumes in intactsoil/plant systems. Australian Journal of Agricultural Research 48,

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OConnell, L., 2001. In: OConnell, L., (Ed.), Australian Grain Yearbook,

Australian Grain, Toowoomba, p. 4.

Peoples, M.B., Baldock, J.A., 2001. Nitrogen dynamics of pastures:

nitrogen fixation inputs, the impact of legumes on soil nitrogenfertility, and the contributions of fixed nitrogen to Australian

farming systems. Australian Journal of Experimental Agriculture 41,

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Peoples, M.B., Gault, R.R., Scammell, G.J., Dear, B.S., Virgona, J.,

Sandral, G.A., Paul, J., Wolfe, E.C., Angus, J.F., 1998. The effect of

pasture management on the contributions of fixed N to the N-economy

of ley-farming systems. Australian Journal of Agricultural Research 49,

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Unkovich, M., Pate, J.S., 2000. An appraisal of recent field measurements

of symbiotic N2fixation by annual legumes. Field Crops Research 65,

211228.

Zebarth, B.J., Alder, V., Sheard, R.W., 1991. In situ labelling of

legume residues with a foliar application of a 15N-enriched urea

solution. Communications in Soil Science and Plant Analysis 90,

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J.A. Baldock*

CSIRO Land and Water,

PMB 2, Glen Osmond, Australia SA 5064

E-mail address: jeff.baldock@csiro.au

R.A. Ballard

South Australian Research and Development Institute,

P.O. Box 397, Adelaide, Australia SA 5001

* Corresponding author. Tel.: 618-8303-8537; fax: 618-8303-8550.

J.A. Baldock, R.A. Ballard / Soil Biology & Biochemistry 36 (2004) 11911193 1193