Addition of crop residues affects a detritus-based food chain depending on litter type and farming system

The addition of crop residues is a common farming practice to increase the organic carbon content of agricultural soils with particular importance in organically managed crops. Residues can be added either from the crop plant itself or from other plants and the type of litter may differentially affect decomposer populations. Effects of litter addition may cascade up to affect generalist predators via trophic cascades or modifications of structural microhabitat properties with unknown consequences for the role of these predators in below- and aboveground food webs. Wheat and maize litter were added to organically and conventionally managed wheat fields and effects on generalist predator and Collembola numbers, litter decomposition and carbon utilization as estimated by stable isotope analysis were studied. Significantly more predators were sampled in plots with maize litter compared to plots with wheat litter and under organic farming. Collembola numbers were not significantly affected by litter type or farming system. Litter mass loss was highest in plots that received wheat litter under organic management and was negatively related to predator, but not Collembola numbers. Individuals of Lepidocyrtus spp. (Collembola) incorporated high percentages of maize-borne carbon compared to predator species. Two linyphiid spider species were more closely linked to the maize-consuming prey in organically managed fields and one carabid species showed this pattern in conventionally managed fields. High litter decomposition levels and generalist predator numbers were only observed in wheat litter plots in organically managed fields. The addition of crop residues from the growing crop under organic management may therefore be a promising farming practice to simultaneously promote decomposition services and numbers of natural enemies. Future studies need to address this practice and potential effects on the litter–decomposer–predator food chain across sites that cover a range of different soil types and climatic conditions.