Reactive nitrogen inputs to US lands and waterways: how certain are we about sources and fluxes?

An overabundance of reactive nitrogen (N) as a result of anthropogenic activities has led to multiple human health and environmental concerns. Efforts to address these concerns require an accurate accounting of N inputs. Here, we present a novel synthesis of data describing N inputs to the US, including the range of estimates, spatial patterns, and uncertainties. This analysis shows that human‐mediated N inputs are ubiquitous across the country but are spatially heterogeneous, ranging from < 0.1 to 34.6 times the background N input for individual water‐resource units (8‐digit Hydrologic Unit Codes). The Midwest, Mid‐Atlantic, central California, and portions of the Columbia River valley currently receive the highest N loads. Major opportunities to advance our understanding of N sources can be achieved by: (1) enhancing the spatial and temporal resolution of agricultural N input data, (2) improving livestock and human waste monitoring, and (3) better quantifying biological N fixation in non‐cultivated ecosystems.