Beech trees fuel soil animal food webs via root-derived nitrogen

Root-derived resources are receiving increased attention as basal resources for soil animal food webs. They predominantly function as carbon and energy resources for microbial metabolism in the rhizosphere, however, root-derived nitrogen may also be important. We explored both the role of root-derived carbon (C) and nitrogen (N) for the nutrition of soil animal species. Using 13C and 15N pulse labeling we followed in situ the flux of shoot-derived C and N into the soil animal food web of young beech (Fagus sylvatica) and ash (Fraxinus excelsior) trees. For labeling with 13C, trees were exposed to increased atmospheric concentrations of 13CO2 and for labeling with 15N leaves were immersed in a solution of Ca15NO3. Twenty days after labeling root-derived N was detected in each of the studied soil animal species whereas incorporation of root-derived C was only detected in the ash rhizosphere. More root-derived N was incorporated into soil animals from the beech as compared to the ash rhizosphere, in spite of the higher 15N signatures in fine roots of ash as compared to beech. The results suggest that soil animal food webs not only rely on root C but also on root N with the contribution of root N to soil animal nutrition varying with tree species. This novel pathway of plant N highlights the importance of root-derived resources for soil animal food webs.