Rapid shift in nitrogen inputs and fluxes across the contiguous United States
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The effectiveness of 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. However, the most recent US-scale N inventory was conducted for the early 2000s, and many methodological improvements have been made since then. We compiled comprehensive 2002, 2007, and 2012 inventories of inputs and non-hydrologic 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 datasets. We found that at the national scale, inputs, outputs, and surpluses changed little between 2002 and 2012 (±8%), and remained dominated by agricultural processes, despite efforts to curb N losses. This constancy, however, belied counteracting fluxes and regional shifts. Throughout the eastern US, declines in deposition and fertilizer inputs combined with increased crop yields resulted in a decrease in terrestrial N surpluses there, which may explain recent water quality improvements. In the Midwest, crop harvest N increased despite yields being depressed by drought throughout much of the region. Fertilizer N inputs increased at a greater rate, however, leading to a larger terrestrial surplus N in the Midwest. A large relative increase (~320%) in N emissions due to an unusual wildfire season in 2012, one of the hottest and driest on record, offset fossil fuel emission reductions by ~20%. These changes coincided with regulations 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. Ultimately, inventory approaches allow managers to consider the broader context of both economic and domestic environmental goals when looking to minimize the loss of N to the atmosphere, soil, and water.