Agriculture, Ecosystems and Environment, 24 (1988) 57-67 57 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Soil Biotic Interactions in the Functioning of Agroecosystems

0. ANDRI~N 1, K. PAUSTIAN 1 and T. ROSSWALL 2

1Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, S-750 07 Uppsala (Sweden) eDepartment of Water in Environment and Society, University of LinkSping, S-581 83 Linh6ping (Sweden)

ABSTRACT

Andr~n, 0., Paustian, K. and Rosswall, T., 1988. Soil biotic interactions in the functioning of agroecosystems. Agric. Ecosystems Environ., 24: 57-67.

The Swedish integrated project "Ecology of Arable Land" was established in 1979 and will finish during 1988. Carbon (C) and nitrogen (N) flows in 4 cropping systems which had different inputs of C and N were investigated. Several process studies of, for example, nitrogen deposition, leach- ing, nitrification, denitrification and primary production were made. In the present paper only the results from field samplings of soil organisms, using soil corers and litter-bags, are discussed, in relation to decomposition studies from the field.

Total soil fauna biomass in the top soil (0-27-cm depth) was not affected by fertilization of barley with 120 kg N ha -1 year -1 (B0=B120). The biomass was higher in a N-fertilized (200 kg) grass ley than in barley, but highest in symbiotically N-fixing lucerne ley.

In spite of a numerous and diversified decomposer community, exhibiting clear successional trends in the litter-bags, a simple decomposition model with temperature and moisture as driving variables gave an excellent fit to measured total mass, nitrogen and water-solubles of the decom- posing litter over 2 years. The reasons for the good model fit, without considering organism dy- namics, are briefly discussed.

INTRODUCTION

Agroecosystems have certain characteristic, or even unique, features that distinguish them from natural ecosystems. Examples of such characteristic features include a high export of organic material and nutrients, a relatively insignificant litter layer and a low plant-species diversity. The agroecosystem is unique in its high degree of human manipulation, e.g. cultivation, fertiliza- tion and biocide t reatment (Lowrance et al., 1984).

There are several advantages with agroecosystems as an object for ecological studies, including: (1) the man-made homogeneity of both the plant commu-

0167-8809/88/$03.50 © 1988 Elsevier Science Publishers B.V.

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nity and the topsoil reduces the spatial variation of most measurable variables in comparison with natural ecosystems; (2) many agricultural practices, par- ticularly ploughing and harvesting, can be regarded as ecological field experi- ments, ready for investigation and interpretation in terms of interaction between soil organisms; also (3) there is a wealth of background information and many long-term field experiments available, but there have been few in- depth soil ecological studies, and the field is wide open for basic ecological research.

THE SWEDISH INTEGRATED PROJECT "ECOLOGY OF ARABLE LAND"

Background

Swedish agriculture is in a state of transition, and the environmental con- cerns and overproduction problems are similar to those of other industrialized countries. Over the past few decades, there has been a large increase in contin- uous cereal cropping and a corresponding reduction in perennial ley rotations for forage production. The amounts of nitrogenous fertilizers applied, espe- cially to cereal crops, has increased, and nitrogen losses to ground and surface waters are considered an important problem (Steen, 1985; Gustafson, 1987). In response to these problems there is considerable interest in changing pres- ent agricultural practices. With this background, an integrated ecological proj- ect was started in 1979 to study in detail carbon (C) and nitrogen (N) flows through arable land. The main objective of the project was to investigate the functions of plants, soil micro-organisms and soil fauna, with particular atten- tion to their importance in the circulation of nitrogen and carbon in 4 cropping systems with differing nitrogen input, above- and below-ground primary pro- duction, and organic-matter incorporation into the soil (Persson and Ross- wall, 1983; Andr~n, 1988).

The investigations directly concerning micro-organisms and soil fauna can be divided into 2 groups. One group was devoted to estimating the effects of the different cropping systems on the abundance and biomass of soil micro- organisms and soil fauna, and to calculate and integrate carbon and nitrogen flows through the organisms. The other group of investigations considered how the soil organisms in turn affect decomposition and nutrient cycling.

At its maximum in 1983/1984, the project involved 25 scientists, and the results to date have been presented in about 100 international publications. Several additional publications are in preparation, and a project synthesis vol- ume will be published during 1988/1989.

The experimental field

The experimental field of the project is situated in central Sweden, 45 km north of Uppsala (60 ° 10'N, 17°38'E), where the climate is cold temperate, with a mean annual temperature of + 5.4 ° C and c. 520-mm precipitation.

59

The field was used for about 1500 years as a hay meadow or pasture until about 100 years ago, when it was ploughed and converted to annual cropping, e.g. oats and barley. The soil can be described as a mixed, frigid, typic Hapla- quoll, consisting of a loam top soil (0-27 cm) followed by a sand layer overlay- ing a clay horizon. In 1979, the top soil contained about 2.2% C and 0.2% N and pH was slightly above 6.

In spring 1980, the following 4 treatments were established in 4 randomized blocks and maintained until 1984/1985: B0, barley with no N fertilizer, har- vested and ploughed every autumn and sown every spring; B120, barley fertil- ized with 120 kg ha-1 year-1 of N as calcium nitrate, harvested and ploughed every autumn and sown every spring; GL, meadow fescue perennial grass ley, receiving 200 kg N ha -~ year -~, cut twice per growing season; LL, lucerne (alfalfa) perennial ley, receiving no N fertilizer but symbiotically-fixing at- mospheric N, cut twice per growing season.

The experimental setup allowed us to investigate the effects of N-fertilizer additions (B0 vs. B120, GL vs. LL), and the effects of annual soil disturbance (barley vs. perennial leys ). For a comprehensive description of the experimen- tal field, see Steen et al. (1984).

This paper, dealing with interactions in the soil, summarizes results from the investigations of soil-organism biomass in the 4 cropping systems, as well as the results from a litter-bag experiment with barley straw under the unfer- tilized barley. Many other measurements, not discussed here, were made for estimates of, for example, nitrogen deposition, leaching, nitrification, denitri- fication, primary production above- and below-ground and symbiotic nitrogen fixation.

METHODS

The biomass estimates presented for the 4 cropping systems are based on the abundance of organisms in September 1982 and 1983. More frequent sam- plings of micro-organisms and soil fauna were made at various times, but the September samplings included simultaneous measurements of all major groups, and were intended to illustrate differences in the soil organism communities between the 4 treatments. Soil cores were taken to 10-cm depth for micro- organisms, to 20-cm depth for nematodes and enchytraeids, and to 14-cm depth for microarthropods. Micro-organism biomass estimates were based on direct counts as described by Clarholm and Rosswall (1980) and Schniirer et al. (1986b), nematodes were extracted using wet funnels according to Sohlenius ( 1979 ), enchytraeids were recovered with wet funnels according to O'Connor (1962) and microarthropods using a high gradient funnel extractor (Andrdn, 1985). Biomass estimates were made using length/mass regressions for ar- thropods, enchytraeids and nematodes, and for earthworms; individual ash- free dry biomass was determined by weighing and combustion. All animals

60

were assumed to have 50% C content of ash-free dry mass. The biomass esti- mates were adjusted to include the full depth of the top soil (0-27 cm).

One thousand litter-bags (1-mm mesh, 15 X 15 cm) were buried 10-15-cm deep under treatment B0 on 24 November 1980. Fourteen samplings were made, ending on 26 October 1982. The dynamics of total mass, water-soluble com- pounds, acid-insoluble compounds, nitrogen and organism abundances were monitored. Decomposition models of varying complexity were fitted to mea- surements of total mass, water-soluble compounds and nitrogen using non- linear regression and simulation modelling (Andrdn and Paustian, 1987). The organisms monitored included direct counts of bacteria, fungi (total and FDA- active), amoebae and flagellates according to Schniirer et al. (1986b). Nema- todes (reported in detail by Sohlenius and BostrSm ( 1984 ) ), microarthropods and enchytraeids (LagerlSf and Andrdn, 1985) were extracted using the tech- niques described above. Soil temperatures and soil moisture were monitored every 30 min using a computerized logger system (Alven~is et al., 1986 ).

RESULTS AND DISCUSSION

Crop influence on organism biomass

The effects of the 4 cropping systems on soil-organism biomass are sum- marized in Fig. 1. Peak root biomass was highest in the leys (Hansson, 1987), but did not differ much between treatments B0 and B120, although above- ground biomass differed considerably, and the grain yields (dry mass) were 1.5 t ha -1 for B0 and 2.9 t ha -1 for B120 (Hansson et al., 1987). Microbial biomass did not differ significantly (P> 0.05 ) between the crops, and the pro- portions between fungal and bacterial biomass, as estimated by direct counts, were similar under all crops (Schniirer et al., 1986a). Larger differences in numbers of protozoa and biomass were found, but the differences were not statistically significant (P > 0.05 ) (Schniirer et al., 1986a).

Populations of soil fauna in general seemed to respond positively to the leys, and the lucerne, especially, produced a high faunal biomass. The relative con- tributions of mesofauna (microarthropods + nematodes + enchytraeids ), ma- croarthropods and earthworms to the total faunal biomass were fairly similar under the 4 crops. However, earthworms contributed more under the leys, es- pecially lucerne, and macroarthropod biomass also contributed more under the leys. This may reflect the low litter input and the lack of a litter layer in the annually-ploughed barley (Curry, 1986).

Total mesofaunal biomass and the contributions of Acari, Nematoda, Col- lembola and Enchytraeidae to the total are shown in Fig. 2. Total Acari and total Enchytraeidae abundance were not significantly different between the crops. Collembola were significantly more abundant under LL than under the other crops ( P < 0.05 ), and for all mesofaunal groups significant differences in

61

BO Soil fauna 1 . 9 g C m - 2

E a r t h w . - -

Roots 50 Bact. 70 Fungi 150 Proto. 14

Mocroar. - - M e s o f .

GL Soil fauna 2.9 gC m - 2

E0rthw.

Roots 240 Boct. 90 Mesof. Fungi270 Proto. 5

- - M o c r o o r .

B120 Soil fauna 1 . 9 g C m - 2

E0rthw.

Roots 70 Boct. 90 Fungi 210 Proto. 3

Mocroor. - - Mesof

LL Soil fauna 6.5 g C m - 2

E o r t h ~ . ~

Roots 320 ~ ~\~\\ \~ Soct. 90 \ ~i~r"-"esof. Fungi 200 ~ l ~ ' Proto. 4 ~ ~ - ~ M o c r o o r .

Fig. 1. Biomass carbon (g m- 2) of soil organisms m the four cropping systems. Mean values 1982- 1983 for the top soil, 0-27-cm depth. Root biomass is expressed as annual peak biomass (data from Paustian, 1987). B0 = barley without N fertilizer, B120 = barley receiving 120 kg ha- 1 year- 1 fertilizer N, GL = meadow fescue ley receiving 120-t- 80 kg ha- 1 year- 1 fertilizer N and LL = lucerne (alfalfa) ley receiving no N fertilizer.)

abundance occurred at lower taxonomic levels and at specific sampling occa- sions (LagerlSf, 1987; Sohlenius et al., 1987). Total mesofaunal biomass did not differ drastically between treatments. However, nematodes had a relatively high biomass under leys and enchytraeids had a comparatively high biomass under barley, and a low biomass under the grass ley, which also had the lowest moisture in the topsoil during summer (see LagerlSf, 1987 ).

Although the differences in the soil organism communities between the 4 intensively-managed cropping systems were not very great, this should not be interpreted to mean that the soil organisms were unaffected by agricultural practices. In comparison with natural ecosystems, all 4 cropping systems were subjected to a high export and a reduced litter layer (cf. Introduction). Bio- mass estimates of the soil-organism community in abandoned farmland (Pers- son and Lohm, 1977) and coniferous forests (Persson et al., 1980) in Sweden, differ greatly from those reported in Swedish agroecosystems (Andr6n and

62

BO Mesofauna 0.57 g C m -2

GL

Ench. ~ Coll. Acori

Nema.

[nch.

Nemo.

B120 Mesofauna 0.51gCm -2

Ench. ~ Coll. Acori

Nema.

Mesofauna 0.34gCm -2 LL

Coll. Acari

Mesofauna 0.52gCm -2

£nch. Coll. Acari

- - Nema.

Fig. 2. Biomass carbon (g m- 2 ) of soil mesofauna in the four cropping systems. Mean values 1982 - 1983 for the top soil, 0-27-cm depth. Data from LagerlSf (1987) and Sohlenius et al. (1987). Abbreviations as in Fig. 1.

LagerlSf, 1983; LagerlSf, 1987) for example, cryptostigmatid mites, Diptera larvae and Coleoptera.

Barley straw decomposition

To investigate interactions between soil organisms in the decomposition of barley straw, a litter-bag experiment was performed. As a first step, decom- position models of varying complexity were fitted to mass loss data, without considering organism dynamics (Andr~n and Paustian, 1987). A simple model, assuming first-order kinetics and no decomposition at soil temperatures below 0 ° C, gave a good fit to total mass (Fig. 3a). When soil temperature and mois- ture were included as driving variables, the fit was improved further (Fig. 3b).

A more complex model was necessary to simulate changes in water-soluble and nitrogen fractions of the litter. The model included the decomposition of primary litter constituents, divided into "refractory" {water-insoluble, MR) and "labile" (water-soluble, M L) fractions, as well as the synthesis and decom- position of secondary materials, consisting of "active" (MA) and "stabilized"

63

10

v

s 0.5

I I 200 400

TIME ( DAYS>0°C )

1.0

,<{ } - -

0.5

B -L R 2 = 0.9952

100 200 300 ~ 0 500 TIME (DAYS)

l r . "123

01 I I I

, ' ' , . , . i . . . ' ? / . , I_o35 1981 19B2

Fig. 3. Measured remaining mass of barley straw decomposing in the field ( • ) and prediction line, obtained by fitting the model Mt=e-kt to mass data, where Mt is the fraction of original mass remaining at time t and k is the first-order decay constant. To the left is the result from using only days with a soil temperature >0°C (a), and to the right is the result from using measured soil temperature and water potential as driving variables (b) (redrawn from AndrSn and Paustian, 1987).

A B C

1 . 0 ¸

M R M L "B 0.B

~ 0.l, MS , o

_ _ TOTAL MASS

R2= 0.9605 i

zoo TIME {DAYS)

6 0 ~ ~ I.O ~ -- M R

~ . . . . . M L

~ - -HA ~o~ ~o.5 ~ ~-Ms

2o~ ~0.1

S - - , - 5 5-_--= ~--: t . J / . r - I

TIME (DAYS)

Fig. 4. (A) The decomposition model, which was simultaneously fitted to total mass, water-soluble compounds and nitrogen; ( B ) observed ( • ) and predicted ( - - ) values of the total mass and water- solubles; (C) dynamics of the model compartments. Redrawn from Andr~n and Paustian (1988) (see text for further explanations. )

(Ms) components (cf. Paul and van Veen, 1978). The active fraction was con- ceptualized as consisting of micro-organisms and labile metabolites, having an internal turnover (Fig. 4a). The climatic control parameters obtained by run- ning the model in Fig. 3b were used, and the model was fitted simultaneously to observed total mass, water-solubles and nitrogen. The parameters optimized

64

were the mass loss rate constants for ML, MR and MA, yield efficiency, the fraction of MA turnover that becomes Ms and the water-soluble as well as N fraction of MA.

In Fig. 4b the model fit is shown, and although the fit to total mass was reduced somewhat as compared with the simpler model above, all three mea- sured variables fitted well, including total N, which is not shown, but which closely followed water-solubles. The model's optimized values for yield effi- ciency (0.36), the fraction of MA turnover that becomes stabilized (0.47), and the concentrations of nitrogen (3.0%) and water-solubles (35%) in MA seemed reasonable. The dynamics of the conceptual compartments are shown in Fig. 4c, and the model predicted that after 2 years in the field about 20% of the remaining mass consisted of secondary components, i.e. MA and Ms (Andrdn and Paustian, 1987).

The litter-bags built up a numerous and diversified decomposer community rapidly, and about 30 species each of nematodes (Sohlenius and BostrSm, 1984) and arthropods (Lagerl5f and Andrdn, 1985) were recorded. However, the to- tal biomass of all organisms never exceeded 2% of the litter mass. When res- piration was calculated from biomass and soil temperature for nematodes, microarthropods and enchytraeids, their contributions to the total carbon loss during the 2-year period were 0.2, 0.2 and 0.8%, respectively (Sohlenius and BostrSm, 1984; LagerlSf and Andrdn, 1985 ).

Biomass dynamics of various organisms are shown in Fig. 5. Total biomass increased gradually during the 2 years, when total fungal mass was included. When only the FDA-active fungal fraction was included the increase was con- siderably smaller, indicating that inactive fungal hyphae were accumulating. The early peak in total biomass was caused by a temporary increase in amoebal biomass in May the first spring, reaching around 5 mg g- 1 initial straw. There was a continuous increase in bacterial biomass during most of the experimen- tal period. This increase, as well as the increase in total organism biomass,

lo f TOTAL 'OTA 10 f ALB,0T ( TOT FUNGI) ( FOA FUNGI)

0, ' l ' ' / ~ ' ' ' ' ' ' ' ' O.' m . . . . 1981 lg62 1981 1982

2. BACTERIA 0.05 ~ BACTERIV

1981 1982 1981 1962

Fig. 5. Biomass dynamics (rag) in litter-bags with 1 g initial ash-free dry mass of barley straw. Total biota (bacteria, fungi - total mass, protozoa, nematodes, microarthropods and enchytraei- dae), total biota with FDA-active fungal biomass only, bacteria and microbivorous nematodes together with predaceous mites.

65

indicates a negative correlation between microbial biomass and the absolute mass loss rate (i.e. g litter-bag-’ day-‘), which declined over time following a negative exponential function.

In other words, total organism biomass, bacterial and fungal biomass were poor indicators of biological activity. One reason for this may be the early high abundances of bacterivorous amoebae and nematodes (Fig. 5) and enchy- traeids (not shown), which are at least partly bacterivorous. During the initial period of rapid decomposition and presumably a high bacterial production, the bacterial population may have been kept at a low level by bacterivore grazing. As seen in Fig. 5, predatory microarthropods, including nematode-feeders then entered the litter-bags and could have reduced the grazing pressure on bacteria. Thus, the bacterivores were better indicators of bacterial activity than bacte- rial biomass in itself.

Discussing interactions based only on biomass dynamics data is highly spec- ulative, but in our final figure we would like to present a tentative food web (Fig. 6 ) . Well-organized food webs like this may well be misleading, since most soil animals show a lower food specificity than indicated in the figure. Oppor- tunistic feeding seems to be more general than conventional knowledge has stated, e.g. astigmatid mites that are considered fungivorous have been shown to be voracious nematode predators (Walter et al., 1986). However, even in the simplified Fig. 6 a high degree of parallelism can be seen. Thus, if a certain organism decreases in abundance, the pathways change, but the rate of mass loss may not change. This is consistent with results from laboratory experi-

THE FOOD WEB OF DECOMPOSING BARLEY STRAW

(SIMPLIFIED AND TENTATIVE)

Fig. 6. Hypothetical food web of barley-straw litter under decomposition.

66

ments with more than a few decomposer species present (Andr~n and Schniirer, 1985 ), although experiments with simpler systems indicate a more important role of, e.g. collembolan microbivores (Hanlon and Anderson, 1979).

A constant attack rate by decomposers, resulting from the multitude of de- composition pathways, may explain the apparent discrepancy between Figs. 3 and 5. That is, although there was a rich and dynamic decomposer community, total mass loss rate could be modelled accurately with a very simple exponen- tial decay model.

ACKNOWLEDGMENTS

M. Clarholm and J. Schniirer kindly allowed us to use unpublished data. The project has received financial support from the Swedish Council for

Planning and Coordination of Research (FRN), the Swedish Council for For- estry and Agricultural Research (SJFR), the Swedish Natural Science Re- search Council (NFR), the Swedish Environment Protection Board (SNV) and the Swedish University of Agricultural Sciences (SLU).

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