Soil processes are not influenced by the functional complexity of soil decomposer food webs under disturbance

A 3 yr experiment, using field lysimeters with seedlings of Scots pine (Pinus sylvestris) growing in raw humus, was established to study how functional complexity of the soil decomposer food web affects ecosystem functioning. The functional complexity of decomposer system was manipulated by (1) allowing either microfauna (fine mesh) or microfauna+mesofauna (coarse mesh) to enter the initially defaunated systems, and (2) treating half of the lysimeters with wood ash. To test whether altering functional complexity of the decomposer community is related to the systemʹs ability to resist disturbance, the lysimeters were later on disturbed with drought. Ecosystem function, measured as leaching of mineral N and dissolved organic C (DOC) and primary production, and the structure and biomass of soil fauna and soil microbial communities were analysed. We hypothesised that reduced functional complexity of soil decomposer communities retards system functioning which results in impaired ability of soil systems to resist disturbance. Fine mesh was effective in reducing biomass of mesofauna, especially that of microarthropods, in the lysimeters. Leaching of N from the lysimeters and microbial communities were unaffected by the mesh size. Before drought, more DOC was leached from the lysimeters with fine mesh. Pine seedling development was poorer in the lysimeters with coarse mesh than in those with fine mesh. Wood ash, independently of the mesh size, increased number of nematodes and leaching of NO3−–N (only after drought), decreased leaching of NH4+–N and changed microbial community structure. The conditions for the pine seedlings were less favourable in the ash-treated than in the ash-free soils as indicated by the higher root-to-shoot ratio in the former lysimeters. The systemʹs ability to resist disturbance was not affected by the treatments. Our results suggest that the abiotic factors (e.g. moisture, pH) overrode the biotic factors in the lysimeters with respect to ecosystem functioning, and that the changes in the structure and biomass of soil fauna were not reflected as changes in system functioning.