Biological nitrogen fixation by crops (C-BNF) represents one of the largest anthropogenic inputs of reactive nitrogen (N) to land surfaces around the world. In the United States (US), existing estimates of C-BNF are uncertain because of incomplete or inadequate representations of N fixing crops, C-BNF rates, and locations where C-BNF takes place. Here we seek to improve current and historical estimates of C-BNF at regional and national scales by developing, testing, and applying a new model that includes the full range of N fixing crops cultivated in the US. With data acquired from the USDA Census of Agriculture, we modeled C-BNF as a function of crop yield and the fraction of N acquired from BNF relative to N assimilated from the soil. We compared a subset of model estimates with independent field-level data to assess model accuracy. Following model assessment, we estimated C-BNF at the national scale for the period of 1930 - 2010 and for specific cropland types described in the 2007 USDA Crop Data Layer. Lastly, we compared our new C-BNF estimates with other national N fluxes.
Results/Conclusions
A subset (n=184) of our modeled C-BNF estimates closely matched independent field-level estimates (Nash-Sutcliffe R2=0.88). According to our model, C-BNF in the US increased from 1.2 Tg N yr-1 in 1930 to 9.9 Tg N yr-1 in 2010. Our new yield-based C-BNF estimate for 2010 is 40% greater than recent national estimates based on constant per area C-BNF rates. C-BNF contributed the single largest share of N input to agricultural lands until the mid-1960s, when synthetic N fertilizer application surpassed C-BNF. Currently, C-BNF is the second largest source of N to all land surfaces in the US. C-BNF now contributes one- to two-thirds of annual N input to agricultural lands in the Mid-Atlantic, Ohio River Valley, Upper Mississippi River Valley, the Missouri River Basin, and the Pacific Northwest. Our model estimates provide a basis to investigate fates of C-BNF-derived N in the US, which would improve national efforts to quantify N loading to the atmosphere and aquatic ecosystems.