Inter-annual mobility of nitrogen between beech rings: A labelling experiment

Abstract

Nitrogen concentration [N] and isotopic composition (δ15N) in annual growth rings of 16-year-old beech trees (Fagus sylvatica L.) were measured before and after treatment of wood using organic solvents. The trees, grown under field conditions in Northeastern France, were labelled with 15N-enriched urea solution at leaf level for three successive years (1993, 1994 and 1995), and were sampled in 2001. δ15N values in this experiment ranged from -4 to -7‰ for non-labelled tree. Labelling resulted in markedly increased δ15N of whole wood for years 1994, 1995 and 1996. Significant labelling-induced increases, compared to non-labelled rings, in δ15N were also found in the rings formed before and after the labelling period, pointing to an important mobility of N across the rings. The extraction removed 36% of total nitrogen and 14% of total carbon. The treatment improved the inter-annual resolution of [N] and δ15N, but did not reduce the 15N signal in rings prior to labelling to the same level as in control tree as was expected. After extraction, significant correlation between August precipitation and [N] variations at the inter-annual level was found for the 1990-2000 period.