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Stable soil aggregates and high microbial diversity are essential for efficient nutrient recycling in soil. This study evaluated effects of different arable land-use systems on soil aggregation and associated composition and total diversity of bacteria and fungi. For this purpose, 5- and 3-year-old field trials were selected, two in sub-humid (orthic Acrisols and oxidic Ferralsols) and one in semi-arid (chromic Cambisol) region in Kenya. The treatments included combinations of tillage (conventional tillage [CT] and reduced tillage [RT]), crop residue management (0 and 2 t ha?1 maize stover) and cropping systems (continuous maize, maize-soybean intercropping and rotation) in the sub-humid sites, and of different tillage (CT, no-till and tied-ridging) and organic fertilization management (manure and crop residue) in semi-arid Kenya. The innovativeness of this study is the research in tropical soils where the concept of aggregate hierarchy is not generally accepted yet. The results showed that aggregate mean weight diameter (MWD) at 0-15 cm depths had increased from 19 to 34% under RT relative to CT management (P < 0.05). Crop residue additions also significantly increased MWD by 22% at the top 5 cm (P < 0.01) compared to treatments without residue in oxidic Ferralsols. At all the three sites, tillage resulted in greatest shifts of bacterial and fungal community composition, while cropping system had largest positive effect on the total microbial diversity indices. Total microbial diversity indices as well as MWD were higher in soybean-maize intercropping systems than under rotation or continuous maize cropping. Changes in total bacterial diversity related inversely to changes in the amount of free silt + clay, while fungal diversity related inversely to the contents of soil macro-aggregates >2 mm (P < 0.05). We conclude that RT plus supply of organic resources and intercropping are best strategies to increase total diversity of soil bacteria and fungi and improve soil structure in these tropical soils.

The design of sustainable agroecosystems requires knowledge of plant species impacts on soil functioning. To address this need, we manipulated plant species diversity in pastures of eastern Amazonia. Four plant species (Arachis pintoi, Brachiaria brizantha, Leucaena leucocephala and Solanum rugosum) were grown alone and in every possible combination on experimental plots within three replicate farms. After 28 months, soils were sampled to determine impacts on 5 categories of variables: soil macrofauna, aggregate morphology, chemical fertility, water storage and compaction. No clear effects of plant species richness were observed on any of the soil properties measured. However, individual plant species had significant impacts on variables in all 5 categories. Most notably, the herbaceous legume, A. pintoi, promoted both earthworm and ant densities and a corresponding 87% increase in biogenic aggregates in plots with vs. without A. pintoi. Meanwhile, B. brizantha increased the proportion of root-derived aggregates, while negatively impacting ant densities. Significant covariation was observed among many of the 5 data sets (categories), namely soil aggregate morphology and soil macrofauna, as well as aggregate morphology and soil compaction. This research demonstrates that plant species composition can impact soil properties through faunal-mediated effects, and stresses the necessity of considering soil macrofauna in agroecosystem management.

Soil nematode communities in the 0‐15 cm soil layer are used as indicators for describing the processes of fallow succession in the semi-arid zone of West Africa (Senegal). Abundance of plant feeding nematodes, non-plant feeding nematodes, plant parasite index (PPI), species richness and Shannon evenness of plant parasitic nematodes were measured at five stages of succession: fields, early (1‐3 years), intermediate (8‐10 years), old (18‐20 years) fallows, and forest stage. These nematological indexes were analyzed simultaneously by multivariate analysis to show the indicative properties of nematode communities. Overall, changes in abundance of nematode groups, PPI and diversity, show continuous trends from early fallow to mature stages of the succession; these trends parallel theoretical trends in secondary succession. In addition, soil nematode parameters were meaningful and expressed interactions of various uncontrolled factors with successional processes, such as environmental conditions or cropping history. During the 3 years of the survey, the structure of nematode communities, described by index analysis, showed little temporal change and supported the use of nematodes as stable indicators. ©2000 Elsevier Science B.V. All rights reserved.