Allocation and dynamics of C and N within plant–soil system of ash and beech

Forest management requires a profound understanding of how tree species affect C and N cycles in ecosystems. The large C and N stocks in forest soils complicate research on the effects of tree species on C and N pools. In‐situ 13 C and 15 N labeling in undisturbed, natural forests enable not only tracing of C and N fluxes, but also reveal insight into the interactions at the plant‐soil‐atmosphere interface. In‐situ dual 13 C and 15 N pulse labeling of 20 beeches ( Fagus sylvatica L.) and 20 ashes ( Fraxinus excelsior L.) allowed tracing the fate of assimilated C and N in trees and soils in an unmanaged forest system in the Hainich National Park (Germany). Leaf, stem, root, and soil samples as well as microbial biomass were analyzed to quantify the allocation of 13 C and 15 N for 60 d after labeling and along spatial gradients in the soil with increasing distance from the stem. For trees of similar heights (≈ 4 m), beech (20%) assimilated twice as much as ash (9%) of the applied 13 CO 2 , but beech and ash incorporated similar 15 N amounts (45%) into leaves. The photosynthates were transported belowground through the phloem more rapidly in beech than in ash. Ash preferentially accumulated 15 N and 13 C in the roots. In contrast, beech released more of this initially assimilated 13 C (2.0% relative 13 C allocation) and 15 N (0.1% relative 15 N allocation) via rhizodeposition into the soil than ash (0.2% relative 13 C, 0.04% relative 15 N allocation), which was also subsequently recovered in microbial biomass. These results on C and N partitioning contribute to an improved understanding of the effects of European beech and ash on the C and N cycles in deciduous broad‐leaved forest. Differences in C and N allocation patterns between ash and beech are one mechanism of niche differentiation in forests containing both species.