2000. Ecosystem response of pasture soil communities to fumigation-induced microbial diversity reductions: an examination of the biodiversity -ecosystem function relationship. -Oikos 90: 279 -294.A technique based on progressive fumigation was used to reduce soil microbial biodiversity, and the effects of such reductions upon the stability of key soil processes were measured. Mineral soil samples from a grassland were fumigated with chloroform for up to 24 h and then incubated for 5 months to allow recolonisation by surviving organisms. The diversity of cultivable and non-cultivable bacteria, protozoa and nematodes was progressively reduced by increasing fumigation times, as was the number of trophic groups, phyla within trophic groups, and taxa within phyla. Total microbial biomass was similar within fumigated soils, but lower than for unfumigated soil. There was no direct relationship between biodiversity and function. Some broad-scale functional parameters increased as biodiversity decreased, e.g. thymidine incorporation, growth on added nutrients, and the decomposition rate of plant residues. Other more specific parameters decreased as biodiversity decreased, e.g. nitrification, denitrification and methane oxidation. Thus specific functional parameters may be a more sensitive indicator of environmental change than general parameters. Although fumigation reduced soil microbial biodiversity, there was evidence to suggest that it selected for organisms with particular physiological characteristics. The consequences of this for interpreting biodiversity -function relationships are discussed. The stability of the resulting communities to perturbation was further examined by imposing a transient (brief heating to 40°C) or a persistent (addition of CuSO 4 ) stress. Decomposition of grass residues was determined on three occasions after such perturbations. The soils clearly demonstrated resilience to the transient stress; decomposition rates were initially depressed by the stress and recovered over time. Resilience was reduced in the soils with decreasing biodiversity. Soils were not resilient to the persistent stress, there was no recovery in decomposition rate over time, but the soils with the highest biodiversity were more resistant to the stress than soils with impaired biodiversity. The study of functional stability under applied perturbation is a powerful means of examining the effects of biodiversity.

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It is frequently hypothesised that high soil fungal/bacterial ratios are indicative for more sustainable agricultural systems. Increased F =B ratios have been reported in extensively managed grasslands. To determine the shifts in fungal/bacterial biomass ratio as influenced by grassland management and to find relations with nitrogen leaching potential, we sampled a two-year-old field experiment at an organic experimental farm in the eastern part of The Netherlands. The effect of crop (grass and grass-clover), N application rate (0, 40, 80, 120 kg N ha À1 ) and manure type (no manure, farm yard manure and slurry) on the F=B ratio within three growing seasons was tested, as well as relations with soil and crop characteristics, nitrate leaching and partial N balance. Biomass of fungi and bacteria was calculated after direct counts using epifluorescence microscopy. Fungal and bacterial biomass and the F =B ratio were higher in grass than in grassclover. The F =B ratio decreased with increasing N application rate and multiple regression analysis revealed a negative relationship with pH. Bacterial activity (measured as incorporation of [ H]thymidine and [14 C]leucine into bacterial DNA and proteins) showed the exact opposite: an increase with N application rate and pH. Leaching increased with N application rate and was higher in grass-clover than in grass. Partial N balance was more positive at a higher N application rate and showed an inverse relationship with fungal biomass and F =B ratio. We conclude that the fungal/bacterial biomass ratio quickly responded to changes in management. Grasslands with higher N input showed lower F =B ratios. Grass-clover had a smaller fungal biomass and higher N leaching than grass. In general, a higher fungal biomass indicated a lower nitrogen leaching and a more negative partial N balance (or smaller N surplus), but more observations are needed to confirm the relationship between F =B ratio and sustainability. r

Understanding the impacts of agricultural intensification and extensification on soil biota communities is useful in order to preserve and restore biological diversity in agricultural soils and enhance the role of soil biota in agroecosystem functioning. Over four consecutive years, we investigated the effects of agricultural intensification and extensification (including conversion of grassland to arable land and vice versa, increased and decreased levels of mineral fertilization, and monoculture compared to crop rotation) on major soil biota group abundances and functional diversity. We integrated and compared effects across taxonomic levels to identify sensitive species groups. Conversion of grassland to arable land negatively affected both abundances and functional diversity of soil biota. Further intensification of the cropping system by increased fertilization and reduced crop diversity exerted smaller and differential effects on different soil biota groups. Agricultural intensification affected abundances of taxonomic groups with larger body size (earthworms, enchytraeids, microarthropods, and nematodes) more negatively than smaller-sized taxonomic groups (protozoans, bacteria, and fungi). Also functional group diversity and composition were more negatively affected in larger-sized soil biota (earthworms, predatory mites) than in smaller-sized soil biota (nematodes). Furthermore, larger soil biota appeared to be primarily affected by short-term consequences of conversion (disturbance, loss of habitat), whereas smaller soil biota were predominantly affected by long-term consequences (probably loss of organic matter). Reestablishment of grassland resulted in increased abundances of soil biota groups, but since not all groups increased in the same measure, the community structure was not completely restored. We concluded that larger-sized soil biota are more sensitive to agricultural intensification than smaller-sized soil biota. Furthermore, since larger-sized soil biota groups had lower taxonomic richness, we suggest that agricultural intensification exerts strongest effects on species-poor soil biota groups, thus supporting the hypothesis that biodiversity has an "insurance" function. As soil biota play an important role in agroecosystem functioning, altered soil biota abundances and functional group composition under agricultural intensification are likely to affect the functioning of the agroecosystem.

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