Global patterns of terrestrial biological nitrogen (N-2) fixation in natural ecosystems

Human activities have clearly caused dramatic alterations of the terrestria l nitrogen cycle, and analyses of the extent and effects of such changes ar e now common in the scientific literature. However, any attempt to evaluate N cycling processes within ecosystems, as well as anthropogenic influences on the N cycle, requires an understanding of the magnitude of inputs via b iological nitrogen fixation (BNF). Although there have been many studies ad dressing the microbiology, physiology, and magnitude of N fixation at local scales, there are very few estimates of BNF over large scales. We utilized >100 preexisting published estimates of BNF to generate biome- and global- level estimates of biological N fixation. We also used net primary producti vity (NPP) and evapotranspiration (ET) estimates from the Century terrestri al ecosystem model to examine global relationships between these variables and BNF as well as to compare observed and Century-modeled BNF. Our data-ba sed estimates showed a strong positive relationship between ecosystem ET an d BNF, and our analyses suggest that while the model's simple relationships for BNF predict broad scale patterns, they do not capture much of the vari ability or magnitude of published rates. Patterns of BNF were also similar to patterns of ecosystem NPP. Our "best estimate" of potential nitrogen fix ation by natural ecosystems is similar to 195 Tg N yr(-1), with a range of 100-290 Tg N yr(-1). Although these estimates do not account for the decrea se in natural N fixation due to cultivation, this would not dramatically al ter our estimate, as the greatest reductions in area have occurred in syste ms characterized by relatively low rates of N fixation (e.g., grasslands). Although our estimate of BNF in natural ecosystems is similar to previously published estimates of terrestrial BNF, we believe that this study provide s a more documented, constrained estimate of this important flux.