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Facultative nitrogen fixation by canopy legumes in a lowland tropical forest

Alexander A Barron, Drew W Purves, and Lars O Hedin


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 canopy-dominant 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.


Publication typeArticle
Published inOecologia
PublisherSpringer Verlag

Newer versions

Johannes Meyerholt, Soenke Zaehle, and Matthew J. Smith. Different representations of biological nitrogen fixation cause major variation in projected terrestrial biosphere responses to elevated levels of atmospheric CO2 , AGU, 6 October 2015.

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