Biological Nitrogen Fixation

Biological nitrogen fixation supplies the largest quantity of new, natural nitrogen to terrestrial ecosystems, but little is known about its function and controls in tropical forests. Recent work at Barro Colorado Island and the facilities of the Smithsonian Tropical Research Institute has made substantial advances in our understanding of this process. Nitrogen fixation is performed by bacteria associated with leguminous trees and by free-living bacteria in soils, litter, and tree canopies. Research indicates that high nitrogen availability can suppress fixation, while other elements, such as phosphorus and molybdenum, and environmental factors, such as energy and carbon dioxide concentration, control rates of fixation. Tropical trees with symbiotic associations with nitrogen fixing bacteria use facultative fixation, adjusting fixation to their nitrogen demand versus soil supply. Furthermore, tree species differ in their rates and timing of symbiotic nitrogen fixation. These findings have inspired research on nitrogen fixation in other tropical forests and have contributed substantial new understanding to tropical forest biogeochemical cycling and function.