Regional and national significance of biological nitrogen fixation by crops in the United States

Background/Question/Methods

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.