Decadal Shift in Nitrogen Inputs and Fluxes Across the Contiguous United States: 2002-2012

Abstract

The effectiveness of policies and management actions in reducing the release of excess nitrogen (N) to the environment is best assessed if N fluxes across air, land, and water are regularly quantified at relevant scales. Here we compiled 2002, 2007, and 2012 inventories of inputs and nonhydrologic N outputs along with fossil fuel emissions, food demand, and terrestrial N surpluses for all subbasins of the contiguous United States using peer-reviewed, publicly available data sets. We found that at the national scale, total inputs, outputs, and surpluses changed little (±6%) between 2002 and 2012 and remained dominated by agricultural processes, despite efforts to curb N losses. This consistency at the national scale, however, obscured large counteracting shifts at regional levels driven by variable fluxes across regions. Throughout the eastern United States, declines in deposition and fertilizer inputs combined with increased crop yields resulted in a decrease in terrestrial N surpluses, which may explain recent water quality improvements in the region. On the other hand, fertilizer N inputs in the Midwest increased at a greater rate than crop harvest N increased, leading to a larger terrestrial surplus N. A large relative increase (~320%) in N emissions in the West due to an unusual wildfire season in 2012 was also observed. These changes coincided with national policies that decreased N emissions and increased demand for domestic biofuels, potentially highlighting the capacity to change the source and magnitude of N inputs and fluxes across the landscape through market and regulatory actions.

Figure 1.

Conceptual diagram describing fluxes captured in the national N inventory for HUC-8 subbasins (n = 2,115, ~1,800 km²) across the contiguous United States. Estimates for emissions from fossil fuel combustion (stationary and mobile) along with recovered and applied livestock manure inputs were also generated.

Figure 2.

Flow diagram (a) of N fluxes (Tg N/year) as inputs, transfers to crop products and nonhydrologic outputs for 2012; height of the boxes are proportional to the flux (Tg N/year); and (b) changes in key N fluxes between 2002 and 2012. Fluxes that are not shown changed <0.02 Tg. Please note the difference between crop harvest and food/feed demand was labeled as “U.S. export”.

Figure 3.

Largest source of N inputs (a, c) and nonhydrologic outputs (b, d) for all HUC−8 subbasins of the CONUS in 2002 and 2012. Note that livestock N content and livestock N recovered are not included in the terrestrial surplus calculation because they are accounted for in other fluxes of the inventory; their illustration in the figure is to better demonstrate the distribution of intensive livestock production in HUC−8s scattered throughout the CONUS. CONUS = contiguous United States.

Figure 4.

Total N inputs and nonhydrologic N outputs along with calculated terrestrial N surpluses (i.e., total N inputs−nonhydrologic N outputs) for all HUC−8 subbasins of the contiguous United States in 2012 (a, c, and e). Panels (b), (d), and (f) display the absolute difference of total N inputs, nonhydrologic N outputs, and terrestrial N surpluses between the 2002 and 2012 inventories (i.e., 2012–2002).

Figure 5.

Select N inputs (agricultural fertilizer, N−fixing crop cultivation, and total deposition) and a nonhydrologic N output (harvested crop N content) for all HUC−8 subbasins of the contiguous United States in 2012 (a, c, e, and g). Panels (b), (d), (f), and (h) display the absolute difference of the select N fluxes between the 2002 and 2012 inventories (i.e., 2012–2002). Maps of all inputs and nonhydrologic N outputs characterized are available in the supporting information.