Limited ¹⁵N transfer from stem-labeled leguminous trees to associated grass in an agroforestry system
2010
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 ¹⁵N abundance may be unreliable because ¹⁵N discrimination occurs during several soil and plant processes. The aim of this study was to test the use of the stem-¹⁵N labeling method to assess in situ N transfer from a legume tree to associated grass. Grass ¹⁵N content was measured during a 203-day field experiment from 0 to 5m from the labeled trees. The results indicated that transfer of the added ¹⁵N was limited in space (i.e., up to 1m from the trees) and delayed in time (i.e., ¹⁵N 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 ¹⁵N abundance data suggested that on average 33% of grass N was derived from N₂ 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.
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