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

Abstract

Nitrogen concentration [N] and isotopic composition ( 15 N) 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 15 N-enriched urea solution at leaf level for three successive years (1993, 1994 and 1995), and were sampled in 2001. 15 N values in this experiment ranged from -4 to -7 for non-labelled tree. Labelling resulted in markedly increased 15 N of whole wood for years 1994, 1995 and 1996. Significant labelling-induced increases, compared to non-labelled rings, in 15 N 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 15 N , but did not reduce the 15 N 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.