Some tropical N 2 -fixing trees exhibit specific characteristics for phosphorus (P) acquisition and utilisation that contrast with the large nitrogen (N) fluxes in their litterfall. To investigate differences in N and P cycling in N 2 -fixing plantations, litterfall and fresh leaf quality of a N 2 -fixing Acacia mangium plantation were compared with that of a non-N 2 -fixing Swietenia macrophylla plantation and a coniferous Araucaria cunninghamii plantation. The N concentration in the A. mangium litterfall was higher than that in the litterfall of the two other species, whereas the P concentration in the A. mangium leaf litterfall was 0.16 mg g -1 , which was only 12-22% of that of the other species. The P concentration in the reproductive parts of A. mangium was markedly higher (16.1 mg g -1 ) than those in the other fractions. The N:P ratio was higher in the leaf fall (81) compared to the fresh leaves (29) of A. mangium, in contrast to the N:P ratios in the leaf samples of the other two species. An analysis of a global litterfall dataset of tropical plantations indicated that N:P ratios in litterfall were significantly higher in N 2 -fixers than in non-N 2 -fixers, and those of A. mangium were high among species in the N 2 -fixer group. These results indicated that A. mangium efficiently retranslocated P in contrast to very large N cycling, under field conditions. These differences may be related to other physiological characteristics of A. mangium.

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To investigate soil amelioration effects by older tropical fast-wood plantations, we examined the fine litterfall and accompanying nutrient flux of a 20-year-old Acacia mangium site over 3 years under a wet tropical climate in Sabah, Malaysia. The litterfall of a Swietenia macrophylla site and an Araucaria cunninghamii site was also examined for comparison. Annual nitrogen (N) flux through litterfall (kg N ha -1 ) was larger in A. mangium (207-223) than in S. macrophylla (126-153) or A. cunninghamii (72-94), whereas annual phosphorus (P) flux through litterfall (kg P ha -1 ) was considerably smaller in A. mangium (2.7-3.4) than in S. macrophylla (7.5-15.6) or A. cunninghamii (7.8-9.2). N flux through litterfall, forest floor N, and N concentration in topsoil (0-5 cm) were in the order of A. mangium [ S. macrophylla [ A. cunninghamii, but other element fluxes were not related to concentrations in soils. Our findings suggest that topsoil N increased because of a large N flux from litterfall. We conclude that these plantation trees, including A. mangium have the potential to produce a N flux in litterfall for the rapid return of organic N to soils larger than or equivalent to that in adjacent primary forests. However, the litterfall of a single species may lead to deficits of a particular element and cause nutrient imbalances. Using a mixture of fertilizer tree species or applying mixed litter might be a better solution.

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