Cropping systems and ecological groups of soil animals jointly affect the transfer of root-derived carbon and mineral nitrogen into the soil food web

Root-derived carbon (C) is a crucial resource fuelling soil food webs. However, the quantity of these resources varies with plant communities and may also influence the flux of mineral nitrogen (N) into belowground food webs. Yet, little is known about how different plant communities, especially in agricultural systems, influence the incorporation of plant C and mineral N into the soil macrofauna. Here, we combined pulse ¹³C-labelling of plants with ¹⁵N-labelling of soil in a crop monoculture (oilseed rape), a mixed grass community (grass and legume mixture) and a young tree plantation (willow) to trace the fluxes of root-derived C and mineral N into earthworms and centipedes as major soil decomposers and predators, respectively, over 28 days. Bulk stable isotope analysis and compound-specific stable isotope analysis of amino acids were used to quantify the uptake of ¹³C and ¹⁵N by soil macrofauna and to investigate the pathways by which these resources are channelled into soil macrofauna. Aligning with their use of plant-derived resources, epigeic and anecic earthworms incorporated more root-derived C than endogeic earthworms, with endogeic earthworms mainly relying on bacteria or bacterial necromass associated with soil organic matter. Generally, macrofauna incorporated both root-derived C and mineral N across cropping systems, but incorporation was more pronounced in rape and grass than in willow. Importantly, root-derived resources facilitated the incorporation of mineral N into soil animal food webs. Centipedes, as one of the most important predators in soil, mainly incorporated root-derived C and mineral N via preying on collembolans, whereas in willow epigeic earthworms likely also contributed to their diet. Overall, the fluxes of root-derived C and mineral N into the soil food web depended on plant communities and soil animal ecological groups, with higher fluxes in herbaceous crops than in tree plantations.