『Abstract
　Human activities have profoundly altered the global nitrogen (N) cycle. Increases in anthropogenic N have had multiple effects on the atmosphere, on terrestrial, freshwater and marine ecosystems, and even on human health. Unfortunately, methodological limitations challenge our ability to directly measure natural N inputs via biological N fixation (BNF) - the largest natural source of new N to ecosystems. This confounds efforts to quantify the extent of anthropogenic perturbation to the N cycle. To address this gap, we used a pair of indirect methods - analytical modeling and N balance - to generate independent estimates of BNF in a presumed hotspot of N fixation, a tropical rain forest site in central Rondonia（後のoの頭に＾） in the Brazilian Amazon Basin. Our objectives were to attempt to constrain symbiotic N fixation rate in this site using indirect methods, and to assess strengths and weaknesses of this approach by looking for areas of convergence and disagreement between the estimates. This approach yielded two remarkably similar estimates of n fixation. However, when compared to a previously published bottom-up estimate, our analysis indicated much lower n inputs via symbiotic BNF in the Rondonia（後のoの頭に＾） site than has been suggested for the tropics as a whole. This discrepancy may reflect errors associated with extrapolating bottom-up fluxes from plot-scale measures, those resulting from the indirect analyses, and/or the relatively low abundance of legumes at the Rondonia（後のoの頭に＾） site. While indirect methods have some limitations, we suggest that until the technological challenges of directly measuring N fixation are overcome, integrated approaches that employ a combination of model-generated and empirically-derived data offer a promising way of constraining N inputs via BNF in natural ecosystems.

Keywords: Amazon Basin; Ecosystem modeling; mass balance; Nitrogen fixation; Nutrient cycling; Rondonia（後のoの頭に＾）; Tropical forest』

Introduction
Methods
　Study site
　Modeled estimates of BNF
　N balance estimates of BNF
　N inputs
　N losses
Results
　Modeled estimates of BNF
　N balance estimates of BNF
　　N inputs
　　N losses
Discussion
Acknowledgments
Open Access
References