Carbon flux through fungi and bacteria into the forest soil animal food web as indicated by compound-specific 13C fatty acid analysis

1.Soil food webs are compartmentalized and comprise major energy channels, such as the plant litter, fungal and bacterial channel. However, the relative contribution of basal resources to these channels for nutrition of higher trophic levels is largely unknown. 2.The study took advantage of the Swiss Canopy Crane Project, where a mature temperate forest was labelled with 13C-depleted CO2. By exchanging leaf litter between the 13CO2-enriched area and an unlabelled control area, we were able to separate carbon fluxes originating from leaf litter and roots, respectively. 3.Fatty acid (FA) composition and d13C signatures of individual FAs of basal resources and soil animals, including decomposers and predators, were analysed. Marker FAs allowed differentiation of the contribution of plant litter, fungi and bacteria to the nutrition of higher trophic levels. 4.Our findings suggest that root-derived carbon is of major importance for the soil animal food web and that it enters the soil animal food web mainly via feeding on ectomycorrhizal fungi. 5.In contrast to previous studies that emphasized the dominance of the fungal energy channel, significant amounts of bacterial FAs in each of the predator species studied indicate that the bacterial energy channel is considerably more important for the nutrition of higher trophic levels than previously assumed. The flux of energy along the two channels presumably contributes to food web stability. 6.Compound-specific 13(CO2) FA analysis of basal resources and soil animals allowed the separation of energy fluxes originating from ectomycorrhizal vs. saprotrophic fungi, from bacteria feeding on leaf litter vs. bacteria feeding on root exudates, and from leaf litter itself, providing information on the partitioning of these different energy channels in unprecedented detail.