Fertilizer addition lessens the flux of microbial carbon to higher trophic levels in soil food webs of grassland.

Roots and root-derived C compounds are increasingly recognised as important resources for soil animal food webs. We used (13)C-labelled glucose as a model C compound representing root exudates to follow the incorporation of root-derived C into the soil animal food web of a temperate grassland over a period of 52 weeks. We investigated variations in glucose C incorporation with fertilizer addition and sward composition, i.e. variations in plant functional groups. The approach allowed the differentiation of trophic chains based on primary decomposers feeding on litter and phytophagous species feeding on roots (i.e. not incorporating glucose C) from those based on secondary decomposers feeding on microorganisms (thereby assimilating glucose C). Each of the studied soil animal species incorporated glucose C, indicating that the majority of grassland soil animal species rely on microorganisms as food resources with microorganisms being fuelled by root exudates. However, incorporation of glucose C into soil animal species varied markedly with species identity, suggesting that detritivorous microarthropods complement each other in channelling microbial C through soil food webs. Fertilizer addition markedly reduced the concentration of glucose C in most soil animal species as well as the absolute transfer of glucose C into oribatid mites as major secondary decomposers. The results suggest that fertilizer addition shifts the basis of the decomposer food web towards the use of unlabelled resources, presumably roots, i.e. towards a herbivore system, thereby lessening the link between microorganisms and microbial grazers and hampering the propagation of microbial C to higher trophic levels.