RECYCLING OF THE NATURALLY-OCCURRING N-15 IN AN ESTABLISHED STAND OF LEUCAENA-LEUCOCEPHALA

The natural N-15 abundance method was used to trace the sources of N i n a stand of Leucaena leucocephala and its understorey species. Change s in the deltaN-15 of various above-ground parts of the L. leucocephal a trees and understorey vegetation were monitored for 6 yr. The result ing data provide the first direct evidence of internal N cycling betwe en a N2-fixing tree and non-N2-fixing understorey species within an ag ro-ecosystem. The deltaN-15 of all tree parts and understorey vegetati on decreased with time. One year after the plantation was established, the non-N2-fixing understorey species exhibited a significant enrichm ent in N-15 relative to the L. leucocephala. At 4 and 6 yr after plant ing, however, the deltaN-15 N value of the understorey vegetation had decreased significantly and was nearly identical to that of the L. leu cocephala. The decline in deltaN-15 values of the understorey species was attributed to net N-mineralization of plant parts shed by L. leuco cephala, and to root exudation and decay. That is, a portion of the N2 fixed during the early stages of tree growth was made available to th e understorey species through the decomposition and subsequent incorpo ration into the available soil-N pool of the abscissed L. leucocephala parts. This indicates that there was a tight coupling between N2 fixa tion and the availability of net mineralized N. The natural N-15 abund ance method appears to provide a suitable means by which the fate of f ixed-N in a forest ecosystem can be monitored, provided the difference between the initial deltaN-15 value of the N2-fixing and non-N2-fixin g (reference) species is of sufficient magnitude. The rate of change i n the deltaN-15 value of the understorey species can provide valuable insights into the intensity of net N-mineralization and N-cycling.