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 15 N labeling techniques of trees and N tracers in the individual studies hamper a generalized mechanistic view. Therefore, we compared Ca( 15 NO 3 ) 2 and 15 NH 4 Cl 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 15 N enriched litter for subsequent decomposition studies. 20 beeches ( Fagus sylvatica ) and 20 ashes ( Fraxinus excelsior ) were 15 N pulse labeled from aboveground with Ca( 15 NO 3 ) 2 and 40 beeches and 40 ashes were 15 N pulse labeled from aboveground with 15 NH 4 Cl. 15 N was quantified in tree compartments (leaves, stem, roots) and in soil after 8 d. Beech and ash incorporated generally more 15 N from the applied 15 NH 4 Cl compared to Ca( 15 NO 3 ) 2 in all measured compartments, except for ash leaves. Ash had highest 15 N incorporation [45% of the applied with Ca( 15 NO 3 ) 2 ] in its leaves. Both tree species kept over 90% of all fixed 15 N from Ca( 15 NO 3 ) in their leaves, whereas only 50% of the 15 N from the 15 NH 4 Cl 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 15 N tracers enable long‐term labeling in situ field studies on N rhizodeposition and allocation in soils. Nonetheless, the 15 N incorporation by both salts was species specific: the leaf labeling with 15 NH 4 Cl results in a more homogenous distribution between the tree compartments in both tree species and, therefore, 15 NH 4 Cl is more appropriate for allocation studies. The leaf labeling with Ca( 15 NO 3 ) 2 is a suitable tool to produce highly enriched 15 N leaf litter for further long term in situ decomposition and turnover studies.