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Facultative nitrogen fixation by canopy legumes in a lowland tropical forest
Alexander R. Barron, Drew W. Purves and Lars O. Hedin
Vol. 165, No. 2 (February 2011), pp. 511-520
Stable URL: http://www.jstor.org/stable/41500654
Page Count: 10
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Symbiotic dinitrogen (N₂) fixation is often invoked to explain the N richness of tropical forests as ostensibly N₂-fixing trees can be a major component of the community. Such arguments assume N₂ fixers are fixing N when present. However, in laboratory experiments, legumes consistently reduce N₂ fixation in response to increased soil N availability. These contrasting views of N₂ fixation as either obligate or facultative have drastically different implications for the N cycle of tropical forests. We tested these models by directly measuring N₂-fixing root nodules and nitrogenase activity of individual canopydominant legume trees (Inga sp.) across several lowland forest types. Fixation was substantial in disturbed forests and some gaps but near zero in the high N soils of mature forest. Our findings suggest that canopy legumes closely regulate N₂ fixation, leading to large variations in N inputs across the landscape, and low symbiotic fixation in mature forests despite abundant legumes.
Oecologia © 2011 Springer