Hot spots of opportunity for improved cropland nitrogen management across the United States

Abstract:

Nitrogen (N) is a vital input to crop production, but its excess use is a leading cause of environmental and human health problems in many parts of the world. In the United States (US), as in other nations, reducing N pollution (e.g., N₂O emissions to the atmosphere and nitrate loading to aquatic ecosystems) largely remains elusive. Cropland partial N balance (i.e., N inputs minus N harvested in crops) is a useful proxy for N losses to the environment. Developing effective N policies requires understanding links between cropland N balances and the contributing factors driving N use. We present novel insights into these links using a national county-level assessment and propose a criteria-scoring method to guide targeting of interventions. First, we characterize cropland partial N balances across the US in 2011-2013 and identify counties (~25%) where N input reductions are less likely to result in crop yield declines. Second, we identify agronomic, environmental, social, demographic, and economic factors correlated with N balances, as well as counties that are under-performing based on these factors. Finally, we employ criteria scoring and hot spot analysis to identify twenty spatial clusters of opportunity for improved cropland nitrogen management. These hot spots collectively account for ~63% of total surplus N balance for croplands but only ~24% of cropland area in the US. N flows for these hot spots indicate variable opportunities across the US landscape to improve cropland N balances by reducing N fertilizer use, better managing manure N, or increasing crop yields. These findings can guide future efforts to integrate N balances into regulatory and voluntary frameworks in US policy.