Study of litter contribution to nitrogen nutrition of trees—temporal dynamics followed by isotopic marking

A multiple site and tree age set of in situ 15N labelled litter experiments were carried out in ten European beech forests to determine what becomes of litter N in the long term. The amount of litterderived 15N in the leaves of the labelled trees increased in the first years after the labelled litter was deposited at their foot, and then decreased. These dynamics reflect preferential mineralization of N from fresh litter compared to bulk soil N in the beginning, and then stabilization of litter-derived N in the humus and the mineral soil. Litter 15N uptake occurs earlier and over a longer period of time in mull sites compared to moder sites. This is in line with the slower activity of decomposers in the moder organic horizons and with the effects of earthworms (aggregation, vertical distribution) in mull sites. Jade SALLELES INRA Centre de Nancy-Lorraine UR Biogeochimie des ecosystemes forestiers F-54280 CHAMPENOUX (jade.salleles@nancy.inra.fr)