The flux of root-derived carbon via fungi and bacteria into soil microarthropods (Collembola) differs markedly between cropping systems

Recently fixed plant carbon (C) being released as rhizodeposits is a major resource fueling soil food webs. Soil microorganisms predominate in incorporating root-derived C and subsequently transfer it to higher trophic levels. However, variation in microbial community structure between cropping systems and its consequences for the incorporation of root-derived C into soil microbivores remain unclear. In the present study, we used 13CO2 to pulse label a crop monoculture (oilseed rape, Brassica napus L.), a mixed grass community (dominated by Lolium perenne L. mixed with clover Trifolium repens L.), and a young tree plantation (willow, Salix schwerinii E.L. Wolf and Salix viminalis L.). During 28 days, we traced the incorporation of root-derived 13C into phospholipid fatty acids (PLFAs) of soil microorganisms and neutral lipid fatty acids (NLFAs) of five Collembola species belonging to three functional groups: epedaphic (surface-dwelling), hemiedaphic (litter-dwelling), and euedaphic (soil-dwelling). The contribution of bacterial and fungal channels to the incorporation of root-derived C into Collembola varied considerably between cropping systems. Collembola incorporated more 13C from the bacterial channel in rape than in grass and willow, where fungi were the major C source. This corresponded to a similarly higher 13C incorporation into bacterial marker PLFAs in rape compared to grass and willow. By contrast, while the proportion of bacterial and fungal biomarkers in Collembola NLFAs was related to the 13C incorporation into microbial PLFAs, it did not correlate with the proportion of microbial PLFAs in the different cropping systems. This suggests that Collembola rely on specific microbial pools, presumably related to recent plant inputs. Within the same cropping system, hemiedaphic species incorporated more root-derived 13C from the bacterial channel compared to euedaphic and epedaphic species. The results demonstrate the remarkable importance of cropping system for the flux of root C into microorganisms and microbivore soil invertebrates. Changes in root C flux into bacterial and fungal resources among cropping systems resulted in differential utilization of these resources by soil microbivores, suggesting that in particular microorganisms fueled by rhizodeposits are vital resources for the nutrition of higher trophic levels in soil food webs.