Substrate heterogeneity and microfauna in soil organic 'hotspots' as determinants of nitrogen capture and growth of ryegrass

In this study we simultaneously manipulated the patchiness of complex organic resources and the composition of microfaunal populations (protozoa and nematodes) in soil, to influence microbial mineralization processes and to elucidate the underlying mechanisms of nutrient acquisition from decomposing plant residues by ryegrass plants. Hotspot treatments of decreasing patchiness were established by filling laboratory microcosms with defaunated soil and adding labelled (C-13, N-15) grass residues as 1-layer, 4-layer or completely mixed within the soil. Microfaunal treatments were set up by inoculation of the soil with either protozoa or bacterivorous nematodes, a combination of both or neither (control). The microcosms were planted with surface sterile ryegrass seedlings. Growth of ryegrass plants was enhanced by both, increasing patchiness of the organic matter in soil (1-layer > 4-layer > mixed) and microfloral-microfaunal interactions (protozoa + nematodes = protozoa, nematodes > control). The presence of microfauna enhanced the decomposition of hotspot material. Protozoan grazing in particular increased the availability of N in soil and leaching water and led to a concomitant increase in plant growth. While root foraging in organic hotspots enhanced the spatial coupling of mineralization and plant uptake, microfaunal grazing increased the temporal coupling of nutrient release and plant uptake. Consequently the greatest plant biomass was found in treatments combining aggregation of organic material in patches and the presence of microfauna. (C) 2000 Elsevier Science B.V, All rights reserved.