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Reactive nitrogen inputs to US lands and waterways: how certain are we about sources and fluxes?
Citation:
Sobota, D. J., J. E. COMPTON, AND J. A. Harrison. Reactive nitrogen inputs to US lands and waterways: how certain are we about sources and fluxes? FRONTIERS IN ECOLOGY AND THE ENVIRONMENT. Ecological Society of America, Ithaca, NY, 11(2):82-90, (2013).
Impact/Purpose:
Human activities have greatly altered the inputs and cycling of nitrogen, and this change has been referred to as non-sustainable and outside Earth’s boundary conditions. Changes in N inputs can alter air quality, land production and biodiversity, and water quality, and tracking these impacts to sources has become more important in today’s integrated management approaches to nutrient problems. While advances in GIS technology and land use mapping have allowed better tracking of nitrogen sources during the last few decades, few studies have examined the uncertainties associated with this source information. Descriptions of uncertainty for N fluxes are critical for N management, yet remain sparse for the nation. EPA scientists used existing data collected by a range of agencies and programs to examine ranges of current (1990s – 2000s) N inputs to the conterminous US. This synthesis of N source data shows that humans have clearly increased N input to the nation, but the magnitude of increase is highly uncertain and quite spatially variable. They found that 25.7 Tg N yr-1 is the best estimate of current, new N input associated with anthropogenic activities, but this may range from 14.1 to 30.4 Tg N yr-1. A teragram is 1012 grams. Synthetic fertilizer is the dominant N source for the nation, and the relative certainty is quite high for this source. Improvements in the monitoring and tracking of nitrogen sources and trends would be useful to managers and decision-makers attempting to reduce the impacts of reactive nitrogen at a variety of scales, for example in the Chesapeake Bay plan and the Upper Mississippi river basin, or in the Midwest and western states where nitrate contamination of drinking water is a major issue.
Description:
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.
URLs/Downloads:
. Reactive nitrogen in the United States: How certain are we about current input and sources? To be submitted to Frontiers in Ecology and the Environment (PDF, NA pp, 23 KB, about PDF)www.frontiersinecology.org
