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Abstract
Biological dinitrogen fixation in
Casuarina equisetifolia J .R. & G. Forst. and
Leucaena leucocephala (Lam.) de Wit was evaluated using the 15N-enrichment technique under field conditions in single-species and mixed-species plantings (with a nonfixing reference species,
Eucalyptus X
robusta J.E. Smith) between 1.0 and 3.5 years of age in Puerto Rico. Following periodic labelling of trenched and untrenched plantation quadrats with 15N-enriched ammonium sulfate, analyses of foliar and whole-tree (weighted average) N-isotopic ratios and tota! biomass N were used to estimate the proportion of nitrogen derived from biological dinitrogen fixation (PNDFA) and total nitrogen derived from fixation (TNDFA) in
C. equìsetifolia and
L. leucocephala. The 15Nenrichment technique yielded accurate estimates of dinitrogen fixation in maturing stands of these two tree species provided the reference species (
Eucalyptus) was grown in close proximity to the N-fixing species in trenched, mixed-species plots. Changes in the 15N/14N ratio of soil-available nitrogen in single-species plots of the N-fixing and reference were found to yield inaccurate estimates of dinitrogen fixation in the single-species plots of
C. equisetifolia and
L. leucocephala after 2 years of age. The results confirm earlier findings that foliar sampling is a useful nondestructive alternative to whole-tree biomass sampling for the 15N-enrichment protocol. Between 1.0 and 3.5 years after plantation establishment, PNDFA in
C. equisetifolia remained relatively constant between 50 and 60%, while PNDFA in
L. leucocephala declined from nearly 100% at 1 year to less than 40% at 3.5 years. The rate of dinitrogen fixation (kg·ha
-1·year
-1) did not decline as the stands matured. Cumulative dinitrogen fixation (TNDFA) estimates at 3.5 years were very similar between species: 73 in
C. equìsetifolìa and 74 kg N· ha
-1·year
-1 in
L. leucocephala.
Citation
Parrotta, John A.; Baker, Dwight D.; Fried, Maurice. 1996. Changes in dinitrogen fixation in maturing stands of Casuarina equisetifolia and Leucaena leucocephala. Canadian Journal of Forest Research 26: 1684-1691
