COS 177-3 - Regional and national significance of biological nitrogen fixation by crops in the United States

Friday, August 10, 2012: 8:40 AM
F150, Oregon Convention Center
Daniel J. Sobota, In residence at the Western Ecology Division, US EPA, National Research Council Postdoctoral Fellow, Corvallis, OR, Jana E. Compton, US EPA, NHEERL, Western Ecology Division, Corvallis, OR and John A. Harrison, School of Earth and Environmental Sciences, Washington State University Vancouver, Vancouver, WA
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.