C. Vankessel et al., RECYCLING OF THE NATURALLY-OCCURRING N-15 IN AN ESTABLISHED STAND OF LEUCAENA-LEUCOCEPHALA, Soil biology & biochemistry, 26(6), 1994, pp. 757-762
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.