Different responses of ash and beech on nitrate versus ammonium leaf labeling

The effects of tree species on the N cycle in forest systems are still under debate. However, contradicting results of different ¹⁵N labeling techniques of trees and N tracers in the individual studies hamper a generalized mechanistic view. Therefore, we compared Ca(¹⁵NO₃)₂ and ¹⁵NH₄Cl leaf‐labeling method to investigate: (1) N allocation patterns from aboveground to belowground, (2) the cycles of N in soil‐plant systems, and (3) to allow the production of highly ¹⁵N enriched litter for subsequent decomposition studies. 20 beeches (Fagus sylvatica ) and 20 ashes (Fraxinus excelsior ) were ¹⁵N pulse labeled from aboveground with Ca(¹⁵NO₃)₂ and 40 beeches and 40 ashes were ¹⁵N pulse labeled from aboveground with ¹⁵NH₄Cl. ¹⁵N was quantified in tree compartments (leaves, stem, roots) and in soil after 8 d. Beech and ash incorporated generally more ¹⁵N from the applied ¹⁵NH₄Cl compared to Ca(¹⁵NO₃)₂ in all measured compartments, except for ash leaves. Ash had highest ¹⁵N incorporation 45% of the applied with Ca(¹⁵NO₃)₂] in its leaves. Both tree species kept over 90% of all fixed ¹⁵N from Ca(¹⁵NO₃) in their leaves, whereas only 50% of the ¹⁵N from the ¹⁵NH₄Cl tracer remained in the leaves and 50% were allocated to stem, roots, and soil. There was no damage of the leaves by both salts, and thus both ¹⁵N tracers enable long‐term labeling in situ field studies on N rhizodeposition and allocation in soils. Nonetheless, the ¹⁵N incorporation by both salts was species specific: the leaf labeling with ¹⁵NH₄Cl results in a more homogenous distribution between the tree compartments in both tree species and, therefore, ¹⁵NH₄Cl is more appropriate for allocation studies. The leaf labeling with Ca(¹⁵NO₃)₂ is a suitable tool to produce highly enriched ¹⁵N leaf litter for further long term in situ decomposition and turnover studies.