Limited N-15 transfer from stem-labeled leguminous trees to associated grass in an agroforestry system

Belowground nitrogen (N) transfer may be an important process for the N economy in legume-based intercropping systems. In situ estimates of N transfer based on the natural N-15 abundance may be unreliable because N-15 discrimination occurs during several soil and plant processes. The aim of this study was to test the use of the stem-N-15 labeling method to assess in situ N transfer from a legume tree to associated grass. Grass N-15 content was measured during a 203-day field experiment from 0 to 5 m from the labeled trees. The results indicated that transfer of the added 15N was limited in space (i.e., up to I m from the trees) and delayed in time (i.e., N-15 reached the tree roots more than 3 months after labeling), which prevented estimation based on the stem-15N labeling method. Calculations made using the observed natural N-15 abundance data suggested that on average 33% of grass N was derived from N-2 fixation by the tree. New tree labeling methods have to be developed to verify these values and to obtain reliable estimates of in situ N transfer in agroforestry systems. (C) 2009 Elsevier B.V. All rights reserved.