63

Presidential Address delivered at the Eleventh Annual General Meeting.

SOME ASPECTS OF SOIL ECOLOGY

K. E. LEE

Soil Bureau, US.1.H., LOwer Hilli.

It is well known that in most soils vastnumbers and great variety of living organismsmay be found. They range in complexityfrom the simplest of single-celled animalsplants and bacteria, through to burrowingmammals and higher plants, and includerepresentatives of almost every major groupof organisms. Photosynthesis by the greenplants that grow on the soil surface is thesource of energy upon which the whole popula-tion ultimately depends, and most of theorganisms are decomposers of plant tissue.

Macfadyen (in press) has recently con-structed a balance sheet for the total annualflow of energy through a meadow exploitedby means of grazing cattle. Of the total energycaptured by plant photosynthesis, less thanone seventh is respired by the plants, about twosevenths are consumed by herbivores, andabout four sevenths are exploited by organismsengaged in the decomposition of plant materialafter it is dead. Macfadyen has furtherdeveloped this line of investigation and hasana lysed the proportion of the total energyhandled bv the soil animals engaged in whathe calls the "decomposer industry" into fourcomponents~ due respectively to the herbivores~carnivores (predators)~ large decomposers(those which have a mechanical effect on thesoil) and small decomposers.

Some of the material exploited by the decom-posers is returned to the soil in a form thatcan be used again by higher plants, and thisis the basis of organic cycJes~ which are afundamental concern of soil science. However,from the point of view of the farmer andagriculturist~ it is most important that, dis-counting the cne seventh of energy respiredby pasture plants~ two thirds of the energyavailable from grass growth is lost to the"decomposer industry" and never becomesavailable to the grazing animal. Study of theecology of individual species and groups of soilorganisms~ especially of their distribution~numbers, population dynamics, food prefer-ences and energy consumption, acquires a

special importance for the effective use of soilsand economic plants. Earlier ideas of a trophicpyramid~ with herbivores feeding on grass andthemselves being fed on by carnivores areoversimplified. Most of the energy availablein the system is actually falling to the unseendecomposers beneath the soil surface.

Apart from a few large animals, such asearthworms, which are able to make burrows,soil organisms live in the naturally occurringspaces between soil aggregates~ and to a largeextent on the surfaces of the aggregates. Thefeeding roots of higher plants share theseecological niches with the microfauna andmicroflora, and the environment that prevailsin these minute spaces is of fundamentalimportance not only to plant growth, but asa reflection of the general physical, chemicaland biological conditions in the soil that arethe basic concern of soil scientists. If we~ forinstance, plot the numbers of dominant groupsof soil animals in a series of soils~ we findpeak populations of one group or another atdifferent stages in the series. If we continuethis process further, say to individual speciesof mites, or Coli em bola, we find peak popula-tions under particular plant species, in soilswith certain textures or structure, moisturecharacteristics, and so on. If we knew enoughabout the ecological preferences of some com-mon soil animals, their food requirements,limiting temperatures and moisture, reactionto acidity, pattern of nitrogen excretion, wecould predict much of importance to the soilscientist, and also to the growth of plants, froma study of population figures. This mav soundrather impracticable, but Ghilarov (t956a) hassuccessfully used what he calls "soil inverte-brate complexes" as diagnostic criteria forRussian soils whose affinities were in dispute.In New Zealand~ profile characteristics due tosoil animals, especially earthworms~ may beused to distinguish yellow-grey carths frombrown-grey earths, and may prove to be themost useful character for field recognition ofsome yellow-grey earths.

64 PRESIDENTIAL ADDRESS

I should like now to discuss some trends

in the study of soil organisms that havedeveloped during the last few years and theirimplications both in the furtherance of thestudy of ecology and the better understandingof the properties of soils and soil forming

processes.

In New Zealand, soil biology as a continuingstudy began with the formation of a biologysection in the Soil Bureau, in 1946. It aroseinitially as an offshoot of soil surveys.Pedologists in Ihe field felt the need to getsome better understanding of the influence ofliving organisms on profile development andthe processes of soil formation. The firststudies were largely qualitative, and wereaimed at finding out what kinds of organismswere present,

-how the species composition

varied from one soil to another, and whetherdifferences could be correlated in some logicalmanner with soil maps, vegetation, climate andvarious soil properties such as moisture content,texture, structure and chemical information.The lack of basic information concerningthe constitution of soil populations is so greatthat this qualitative approach has continuedright through 10 the present, and will need tocontinue for many years to come. In NewZealand soils there are many groups, particu~larly of animals, of which we know virtuallynothing, and there is an open field for taxo~nomic and ecological work. So far, we haveaccumulated information on some bacteria,yeasts, fungi, algae, protozoa, and earthworms~and on a number of minor groups in the fauna,and have had some success in relating theirdistribution to pedological infonnation.

Comparative studies have been made of soilorganisms under particular vegetation types invarious parts of New Zealand, e.g., a compre-hensive study of soils under grazed pasturesand native tussock, and of changes induced bythe destruction of native vegetation and itsreplacement by pasture.

Qualitative studies have given soil biologistsa background against which detailed quantita"tive and physiological studies can be plannedand interpreted, and in the last few years anincreasing proportion of the Soil Bureau'sbiological work has been quantitative andphysiological.

Dr Stout has worked on the ecology of theanimal and microbial populations of the litterlayer of a soil under beech forest, relatingnumbers to physical features of the environ-

men!. Using measurements of the respiratoryrate of litter decomposing organisms, made at

monthly intervals for a year, he has attemptedto work out a balance sheet of the carboncycle, with encouraging results. He intendsnow to study the colonisation of dead leaves

by soil organisms, and relate populationchanges to the rate and process of decay, andmake a study of the contribution of individualgroups of organisms to the processes of decom-position. This is an example of the kind ofwork that is occupying an increasingly impor-tant place in soil biology work in New Zea-land. We are trying to get down to quantitativeassessment of the contribution of individualspecies and groups of organisms to the turn-over of organic material in the soil, and henceto their contribution to the processes thatcontrol soil formation and fertility.

Mycological work has recently been concen-trated on the fungi that live in close proximityto grass roots. Among these fungi are some,known as endophytic mycorrhizae, which growpartly within plant roots and also have hyphalsystems ramifying through the surroundingsoil. The pathways by which minerals becomeavailable to plants are not well understood.This is particularly true, for instance, of thetransference of nitrogen, originally fixed bynodule bacteria of legumes, through the soilto become available to grasses. It may wellbe that endophytic mycorrhizae provide thepathway, and a study of these fungi may provemost rewarding.

On the other hand, it is known that abouthalf the nitrogen excreted by earthworms isexcreted in the fonn of ammonia. It has beencalculated by Satchell (pers. comm.) in Eng-land that a population of eartbwonns,Lumbricus terrestris, in a woodland soil, turnsover approximately 100 kg/ha. of nitrogenper year. This figure is approximately equalto the amount of nitrogen available from thelitter fall. Since earthworms in New Zealandpastures feed mainly on dead roots of pastureplants (Waters t955), it is possible that theyare major contributors to the mineralisationof nitrogen and its cycling through the soil.An attempt to assess the amount of nitrogencycled by earthworms is being planned.

A great deal of work in soil ecology is atpresent in progress in Britain and Europe. AtLiverpool, Prof. Burges and his school havemade a detailed study of microbial ecology inan iron-humus podzol under pines. They have

LEE: SOIL ECOLOGY 65

used direct microscopic examination of plasticimpregnated sections of soil and litter horizonsto measure the quantity of fungal hyphaeand the species involved in successive stagesof decomposition of plant debris; and haveextended their studies to measurements ofoxygen uptake by soil and litter samples, andthe use of this data as a measure of microbialactivity. Their work has contributed much toan understanding of the processes and rate ofbreakdown of soil organic matter, and at thesame time it is important for its illustration ofsuccession in microbial communities andrelationships between the fauna and microfloraof the soil.

Direct examination of soil sections as amethod of studying the ecology of soil organ-isms is a relatively new technique, and onecapable of providing much information,especially concerning micro-organisms. Thepreparation of satisfactory sections requiresthe spaces in the soil to be impregnated witha matrix that will bind the soil particlestogether without causing significant shrinkageof organic material, and without excessivedamage to soil organisms in the soil spaces.Impregnation techniques have been developedlargely in England and Europe. Haarlov inDenmark used agar and Minderman inHolland used gelatine, but these materials aresuitable mainly for peats and soils with highorganic matter content. Kubiena, in Germany,has developed a technique using plastic resinsfor impregnation, and this technique, combinedwith freeze drying of samples to minimiseshrinkage, has been developed in Europe andEngland to become a most useful tool in soilecology and also in the study of soil structure.

Much of the work on soil animals done inEngland and Europe has consisted of detailedstudies of populations at specific sites, ratherthan the broad coverage of a range of soilsthat has been characteristic of New Zealandsoil biology. Soil animals are very numerous.Up 10 2.3 million arthropods per squaremetre have been found in forests in Sweden(Forsslund t948), up to 20 million nematodesper square metre in the top few centimetresof Danish pasture soils (Overgaard Nielsent949), up to about 200,000 Enchytraeidae(Oligochaeta) per square metre (OvergaardNielsen 1955) and about t250 lumbricid earth-worms per square metre (Waters 1955). Mostsoil animals exhibit violent fluctuations innumbers, both seasonally and in response to

small changes in physical and chemicalcharacteristics of the soil evironment. A widevariety of techniques for the extraction ofanimals from soil samples has been developed.Many have evolved from the classical Berlesefunnel, which has been refined to a high degreeof efficiency. Notable in the development ofextraction methods have been Murphy andMacfadyen in Britain. A recently publishedvolume on research methods in soil zoology(Murphy 1963) discusses extraction methodsin detail. Now that efficient methods of extrac-tion are available, soil animals are a fruitfuland readily accessible field for students ofanimal populations, and for establishing basicprinciples in the study of mechanisms ofcompetition between species, reaction toenvironmental change, control of animal popu-lations, and many other fundamental aspects ofanimal ecology.

For measuring oxygen uptake and carbondioxide production, much use has been madeof the Warburg apparatus. Respiratory mea-surements done in the Soil Bureau have beencarried out with a Warburg apparatus. Thisrecords reduction in pressure in a sealedchamber as oxygen is used up and carbondioxide is produced and absorbed. Its useful-ness is limited by the small volume of air con-tained in the system, and consequently therelatively short period for which experimentscan be continued. Macfadyen has developedan ingenious and very accurate respirometerwhich can be run continuously for long periods,and which automatically records the amountof oxygen used and the rate at which it is used.The apparatus was designed originally for usewith small soil arthropods, but undoubtedlyhas a much wider application in physiologicalund ecological research.

Ghilarov 0949, 1956 a & b, t958) bas putforward the hypothesis that terrestrial inverte-brates evolved from aquatic ancestors by wayof the soil as an intermediate habitat. Hepoints out that in the spaces between soilparticles there is an environment in whichmany invertebrate groups of aquatic origincould live and obtain oxygen without dangerof desiccation. The fact that the soil faunadoes include a wide variety of invertebrates,ranging from most delicate aquatic animals,such as protozoa, ostracods and copepods,through intermediate forms, to thoroughlywaterproofed and highly successful terrestrialanimals such as spiders and some insects,. giye~

66 PRESIDENTIAL ADDRESS

some credibility to his hypothesis. Whether histheory is accepted or not, a close study of thephysiology, ecology and morphology of soil-inha biting animals could provide muchanimals might have adapted themselves to lifein a terrestrial environment.

I have not mentioned a wide range of workthat is in progress in the fields of physiology,feeding behaviour, pest control, biochemicalstudies of humus formation and decomposition,and many other aspects of soil biology. I hopeI have left with you the impression that thestudy of the ecology of soil organisms is anexpanding and vigorous branch of biology. Itis only a short time since soil science was theexclusive province of geologists, chemists andphysicists. Biologists have now been acceptedas essential contributors to soil studies and Ibelieve that especially those with an ecologicaloutlook and a willingness to use a variety oftechniques and to collaborate with physicists,chemists and pedologists, can make a majorcontribution to soil science, and at the sametime add much to the understanding of funda-mental biological problems.

REFERENCES

FORSSLUND, K. H., 1958. Nagot om insamJingsmetodikenvid marksfaunaundersokninger. Medd. Skogsforskn.1nst. Stockh., 37 (7), 1-22.

GHILAROV, M. S., 1949. The peculiarities of the soilas an environment and its significance in theevolution of insects. Moscow & Leningrad, (inRussian).

GHII..AROV,M. S., 1956(a). Significance of the soilfauna studies for the soil diagnostics. Proc. 6thCongo Int. Soc. Soil Sci., Paris, 3: 23.

GHILAROV,M. S., 1956(b). Soil as the environment ofthe invertebrate transition from the aquatic to theterrestrial life. Proc. 6th Congo Int. Soc. Soil Sci.,Paris, 3: 307-13.

GHILAROV,M. S., 1958. Proc. 15th Int. Congo Zool.,London, 354--7.

MACFADYEN,A. (in press). Energy flow in ecosystemsand its exploitation by grazing in Symposium onGrazing ed. Crisp, D. J. British Ecological Sec.Oxford.

MURPHY, P. W. (Ed.), 1963. Progress in soil zoology.Butterworths Scientific Publications, London.

OVERGAARD, NIELSON, C., 1949. Studies on the soilmicrofauna. II. Nat. Jutland. 2: 1-131.

OVERGAARD, NIELSON, C., 1955. In Soil Zoology ed. D.K. McE. Kevan, Butterworths Scientific Publica.tions, London.

WATERS,R. A. S., 1955. Numbers and weights of earth-wonns under a highly productive pasture. N.Z. J.ScL Tech. A36: 516-25.