Facultative nitrogen fixation by canopy legumes in a lowland tropical forest.
Symbiotic dinitrogen (N(2)) fixation is often invoked to explain the N richness of tropical forests as ostensibly N(2)-fixing trees can be a major component of the community. Such arguments assume N(2) fixers are fixing N when present. However, in laboratory experiments, legumes consistently reduce N(2) fixation in response to increased soil N availability. These contrasting views of N(2) fixation as either obligate or facultative have drastically different implications for the N cycle of tropical forests. We tested these models by directly measuring N(2)-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 N(2) fixation, leading to large variations in N inputs across the landscape, and low symbiotic fixation in mature forests despite abundant legumes.
Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA. alex.barron@alumni.carleton.edu
Alexander R
AR
Barron
Drew W
DW
Purves
Lars O
LO
Hedin
D017753:Ecosystem; D007887:Fabaceae / Q000254:growth & development / Q000378:metabolism*; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D009586:Nitrogen Fixation; D018517:Plant Roots / Q000254:growth & development / Q000378:metabolism; D013559:Symbiosis; D014197:Trees / Q000254:growth & development / Q000378:metabolism*; D014329:Tropical Climate
D009584:Nitrogen