COS 177-1 - Have we greatly overestimated nitrogen (N) inputs via biological N fixation in tropical forests?

Friday, August 10, 2012: 8:00 AM
F150, Oregon Convention Center
Cory C. Cleveland, Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT, Sasha C. Reed, Southwest Biological Science Center, U.S. Geological Survey, Moab, UT and Benjamin Z. Houlton, Land, Air and Water Resources, University of California, Davis, Davis, CA
Background/Question/Methods

Global estimates suggest that nearly ~25% of terrestrial biological nitrogen fixation (BNF) takes place in the tropical evergreen forest biome, with the majority of fixation occurring via symbiotic paths in the Fabaceae plant family.  While such large-scale estimates typically represent simple extrapolations of a handful of direct, highly variable plot-scale measurements, other indirect evidence also points to high rates of BNF in tropical forests.  However, several recent analyses have suggested that despite the presence of many putatively symbiotic N fixing species in the tropics, that in practice, the capacity to fix N is seldom realized, casting doubt on the validity of previous biome-scale estimates.  Here, we used indirect methods (analytical modeling and an N balance approach) to generate two independent estimates of BNF in a presumed hotspot of N fixation, a tropical rain forest site in central Rondônia in the Brazilian Amazon Basin.

Results/Conclusions

The two independent methods generated remarkably similar estimates of BNF.  However, our analysis indicated much lower N inputs via symbiotic N fixation (4-7 kg/ha/y) than has been suggested for the tropics as a whole (14-36 kg/ha/y).  This discrepancy may reflect errors associated with extrapolating bottom-up fluxes from plot-scale measures at only a handful of sites, estimation errors resulting from the indirect analyses, and/or the relatively low abundance (~5%) of putative symbiotic N-fixing legumes at the Rondônia site.  For example, the Fabaceae account for 10 –  49% of community biomass within Amazonian sites, yet it appears as if symbiotic N fixation in lowland tropical forests is lower than expected in some sites.  Much further work is needed to determine the importance of scale and scaling in understanding how contemporary N fixation fluxes compare to more integrative budgets of N throughputs in ecosystems.  However, multiple lines of evidence now suggest that that BNF in tropical forests – especially symbiotic N fixation – may be much lower than previously thought.