Allocation and dynamics of CarbonenC and StickstoffdeN 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 13C and 15N 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 13C and 15N 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 13C and 15N 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 13CO2, but beech and ash incorporated similar 15N amounts (45%) into leaves. The photosynthates were transported belowground through the phloem more rapidly in beech than in ash. Ash preferentially accumulated 15N and 13C in the roots. In contrast, beech released more of this initially assimilated 13C (2.0% relative 13C allocation) and 15N (0.1% relative 15N allocation) via rhizodeposition into the soil than ash (0.2% relative 13C, 0.04% relative 15N 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.