Responses of microbial-feeding nematodes to organic matter distribution and predation in experimental soil habitat

Abstract

Soils are spatially heterogeneous environments, a condition which is likely to affect the structure and function of soil food webs. To study the influence of soil organic matter distribution on decomposer food webs, we established microcosms that contained either: (1) large patches of initially sterile humus-litter mixture placed within sterile sand matrix (creating a large-patch habitat; LPH); or (2) blend of sand and humus-litter mixture (creating a small-patch habitat; SPH). Ten species of bacteria and ten species of fungi, and two bacterial-feeding (Acrobeloides tricornis, Caenorhabditis elegans) and two fungal-feeding (Aphelenchus avenae, Aphelenchoides sp.) nematodes were added to both habitats to form the two lowest trophic levels of the food web. Microbivore predation was manipulated by adding a predatory nematode (Prionchulus punctatus) to half of the replicates of both habitats. The microcosms were destructively sampled three times during the 15-week experiment to estimate nematode biomass. The biomasses of A. tricornis and C. elegans were on average 35% and 21% lower, respectively, in LPH than in SPH, whereas the biomass of the two fungivores was not affected by organic matter distribution. Top predator biomass was higher in LPH than in SPH at the first and second samplings. The top predator had both negative and positive effects on its prey species, and the effects varied with sampling. The biomass ratio between fungivores and bacterivores was marginally higher (P=0.080) in LPH (average 1:14) than in SPH (1:20), and significantly higher in the presence (1:13) than in the absence (1:25) of the top predator. In LPH, 69% of A. tricornis and 84% of top predator biomass was found in sand, while the majority (83%) of C. elegans lived in patches. At the end of the experiment a higher proportion of A. tricornis and A. avenae lived in patches in the presence than in the absence of the top predator. The results infer that organic matter distribution may affect the biomass of microbial-feeding and predatory nematodes in soil and produce a shift between the bacterial and fungal energy channels. Mechanisms mediating the effects of organic matter distribution on microbivores and on the top predator seem to differ, however, and therefore a change in organic matter distribution may increase biomass at one trophic level while simultaneously decreasing biomass at another.