W.A.M. Didden

Simulation of nitrogen mineralization in the belowground food webs of two winter wheat fields.

Food webs in conventional (high-input) and integrated (reduced-input) farming systems were simulated to estimate the contribution of soil microbes and soil fauna to nitrogen mineralization during the growing season. Microbes accounted for approximately 95% of the biomass and 70% of total nitrogen mineralization in both management practices. Among the soil fauna, amoebae and bacterivorous nematodes were the most important contributors to nitrogen mineralization. The contribution of nematodes showed more temporal and spatial variability than the contribution of amoebae. The model calculated nitrogen mineralization rates close to the observed rates for both fields and depth layers. In the integrated plot there were relatively high rates of mineralization in the 0-10 cm layer compared with the 10-25 cm layer, whereas in the conventional plot no differences were observed between depth layers (...)

Soil meso- and macrofauna in two agricultural systems: factors affecting population dynamics and evaluation of their role in carbon and nitrogen dynamics.

Abstract Data on soil meso- and macrofauna from winter wheat fields in the Netherlands under conventional and integrated management were analysed to explain their population dynamics and to evaluate their contribution to carbon and nitrogen dynamics. To compare the functioning of the soil fauna in both systems, respiration and mineralisation were calculated on a daily basis, and canonical correspondence analysis was used to analyse the response of soil fauna to abiotic factors and to biotic factors of the previous sample date. Mainly because of the occurrence of earthworms, which were absent in the conventional system, soil fauna was clearly more important in the integrated system: integrated/conventional ratios of biomass and respiration were 58.43 and 6.49, respectively for soil meso- and macrofauna. An indication of a shift of bacterivorous to fungivorous fauna was found in the integrated system under the assumption that earthworms were 80% microbivorous and 20% saprophagous, but bot when it was assumed that they were 100% saprophagous. It was calculated that meso- and macrofauna mineralised 3.3 kg N ha−1 year−1 in the conventional and 39.8 kg N ha−1 year−1 in the integrated system, or 4.3% and 39.4% of the total in situ mineralisation, respectively. Total faunal mineralisation was 49.1% and 86.6% of the total in situ mineralisation in conventional and integrated systems, respectively. It was found that in both systems, seasonal factors played an important role in explaining biomass dynamics and that in the course of time, the upper layer in the integrated system increased in importance. It appeared from the analyses that food relations may differ in the two systems, with probably more complicated and diverse relations occurring in the integrated system. Analysis of the results of this study in comparison with the results of a food web approach showed general agreement; the largest deviations occurred on lower trophic levels, which may be associated with the greater sensitivity of the food web approach to the accuracy of parameter estimates.

Population dynamics in the belowground food webs in two different agricultural systems

Abstract The biomass of 17 different groups of organisms was established every 6 weeks during 1 year in two arable fields cropped to winter wheat; one field was under conventional management (CONV) and the other under integrated management (INT). Bacteria showed the highest average biomass, followed by earthworms (INT only) and amoebae. Most of the groups of organisms had higher biomasses in INT than in CONV. The difference was statistically significant for protozoans, bacterivorous, fungivorous, and phytophagous nematodes and earthworms. Predatory Collembola, cryptostigmatic and bacterivorous mites, and enchytraeids showed a smaller biomass in INT than in CONV. The annual biomass production for each group was estimated using simulation model calculations. Bacteria showed the highest production followed by amoebae and earthworms (INT only). Most of the groups showed a higher biomass production in INT than in CONV. Exceptions were predatory and nematophagous mites, predatory and omnivorous Collembola, and enchytraeids. The total annual production was approximately 32 kg C ha−1 cm−1 depth in CONV and approximately 57 kg C in INT. The population dynamics were analysed by hierarchical cluster analysis. Four different clusters were found in CONV and INT. Bacteria, fungi, protozoans, bacterivorous nematodes and predatory mites showed the same trend in population dynamics in CONV and INT. All other groups showed different population dynamics in CONV and INT. This observation and the composition of these clusters suggested different conditions in CONV and INT.

Earthworm communities in grasslands and horticultural soils

Abstract In a survey of 42 farm sites, comprising grassland and two types of horticultural farms (growing vegetables or flower bulbs), earthworm communities were sampled by hand-sorting and a number of soil physico-chemical characteristics recorded. For heavy metals the availability in the soil solution was estimated based on the measured absolute content. Abundance, biomass and species richness were significantly higher in grassland soils than in horticultural soils, and within the horticultural farms significantly higher in vegetable than in flower-bulb farms. No epigeic species were found in horticultural soils. The differences between the various farm types were probably related to the intensity of management practices, such as soil tillage, harvesting and crop protection measures, that result in less soil organic matter of lower quality. The data from the grassland farms were analyzed in conjunction with the physico-chemical soil characteristics, applying canonical correspondence analysis. Community parameters such as species richness and diversity apparently were not related to soil characteristics, but an appreciable part of the variation in abundance and biomass of several individual species could be explained by factors such as lutum content (particles <16 μm), pH, bio-availability of metal ions and geographical position of the sample site. It is concluded that repeated surveys of this type may become a valuable tool for monitoring soil quality.

Simulation of dynamics in nitrogen mineralisation in the belowground food webs of two arable farming systems

Food web dynamics in a conventional (high-input) and an integrated (reduced-input) arable farming system were modelled to simulate the dynamics in N mineralisation during 1 year under winter wheat. The simulated N mineralisation rates were compared with the observed in situ N mineralisation rates. In the lower depth layers (10–25 cm) the simulated rates matched the observed rates better than in the upper depth layers (0–10 cm). Declines in N mineralisation were better matched than peaks in N mineralisation. The food web model simulated net N immobilisation in the conventional practice and net N mineralisation in the integrated practice for the period following harvest, which was combined with the addition of crop residues and tillage, and in the conventional practice also with soil fumigation. These simulated rates were in agreement with the observed rates. The results indicate that in the investigated arable soils, N mineralisation depended strongly on bacteria decomposing soil organic matter and microbivores, especially protozoans, releasing N from the bacterial biomass.

Dynamics and stratification of Enchytraeidae in the organic layer of a Scots pine forest.

Abstract In a Dutch Scots pine forest an experiment was conducted to quantify the role of soil biota in the functioning of the soil ecosystem, and the effects of enhanced nitrogen deposition. For this, the site was sampled at 8-week intervals during 2.5 years. This paper reports on the population dynamics of enchytraeids in the field and in stratified litterbags. Mean yearly abundance of the enchytraeid community in the field was 47 600 m–2, or 0.70 g (dry weight) m–2. The community consisted mainly of three species: Cognettia sphagnetorum, Marionina clavata and Achaeta eiseni, of which C. sphagnetorum was dominant. The enchytraeid populations showed a marked stratification in the same sequence. Freshly fallen pine needles were colonized by C. sphagnetorum, while other species followed much later. It was found that data from the litterbags were reasonably comparable with field data, when expressed per gram of dry substrate, but less so when expressed per square metre. Multiple regression analysis of the data showed that the population dynamics in the litter layer could largely be explained by temperature and moisture fluctuations; in deeper layers other factors, such as the stage of decomposition, were probably more important.