Abstract
Symbiotic nitrogen fixation is one of the major pathways of N input to forest ecosystems, enriching N availability, particularly in lowland tropics. Recently there is growing concern regarding the wide areas of fast-growing leguminous plantations that could alter global N2O emissions. Here, we highlight substantially different N and phosphorus utilization and cycling at a plantation of Acacia mangium, which is N2-fixing and one of the major plantation species in tropical/subtropical Asia. The litterfall, fresh leaf quality and fine-root ingrowth of A. mangium were compared to those of non-N2-fixing Swietenia macrophylla and coniferous Araucaria cunninghamii in wet tropical climates in Borneo, Malaysia. The N and P concentrations of the A. mangium fresh leaves were higher than those of the other two species, whereas the P concentration in the leaf-litterfall of A. mangium was less than half that of the others; in contrast the N concentration was higher. The N:P ratio in the A. mangium leaf was markedly increased from fresh-leaf (29) to leaf-litterfall (81). Although the N flux in the total litterfall at the A. mangium plantation was large, the fine-root ingrowth of A. mangium significantly increased by applying both N and P. In conclusion, large quantities of N were accumulated and returned to the forest floor in A. mangium plantation, while its P resorption capacity was efficient. Such large N cycling and restricted P cycling in wide areas of monoculture A. mangium plantations may alter N and P cycling and their balance in the organic layer and soil on a stand level.
Highlights
The amount of tropical plantations is increasing due to the demand for woody biomass
Arai et al [6] reported that A. mangium plantations produced 8-fold larger N2O emissions compared to adjacent secondary forests during wet seasons and these results were supported by Konda et al [7]
We reviewed the difference in N and P retranslocation and in N and P demand in the case of the A. mangium plantation compared with two other plantations of Swetenia macrophylla and Araucaria cunninghamii; as well as litterfall data from adjacent primary forests [19]
Summary
The amount of tropical plantations is increasing due to the demand for woody biomass. Arai et al [6] reported that A. mangium plantations produced 8-fold larger N2O emissions compared to adjacent secondary forests during wet seasons and these results were supported by Konda et al [7]. They attributed the reasons for the larger emissions to dinitrification [6] and N mineralization [7], due to higher N availability under. In addition to standing plantations, logging of tropical forests can increase N2O emissions by promoting soil N mineralization [14] Such large areas of short-rotation A. mangium plantations can be a crucial source of N2O, which may reduce CO2 mitigation effects by accumulating. We discussed how A. mangium plantations can influence the regional and global N and P cycling and nutrient conditions for other plants
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