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

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.