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 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 remains challenging. Developing effective N policies and programs requires understanding links between cropland N balances (i.e. N inputs minus N harvested in crops) and potential contributing factors. We present novel insights into these links using a national county-level assessment and propose a criteria-scoring method to inform US N policy and programs. First, we characterize cropland 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 balance, as well as counties that are underperforming based on these characteristics. Finally, we employ criteria scoring and hot spot analysis to identify 20 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, and/or increasing N use efficiency. These findings can guide future efforts to integrate N balance into regulatory and voluntary frameworks in US policy and programs.