Intentional versus unintentional nitrogen use in the United States: trends, efficiency and implications

Abstract

Human actions have both intentionally and unintentionally altered the global economy of nitrogen (N), with both positive and negative consequences for human health and welfare, the environment and climate change. Here we examine long-term trends in reactive N (Nr) creation and efficiencies of Nr use within the continental US. We estimate that human actions in the US have increased Nr inputs by at least *5 times compared to pre-industrial conditions. Whereas N 2 fixation as a by-product of fossil fuel combustion accounted for *1/4 of Nr inputs from the 1970s to 2000 (or *7 Tg N year -1 ), this value has dropped substantially since then (to \5 Tg N year -1 ), owing to Clean Air Act amendments. As of 2007, national N use efficiency (NUE) of all combined N inputs was equal to *40 %. This value increases to 55 % when considering intentional N inputs alone, with food, industrial goods, fuel and fiber production accounting for the largest Nr sinks, respectively. We estimate that 66 % of the N lost during the production of goods and services enters the air (as NO x , NH 3 , N 2 O and N 2 ), with the remaining 34 % lost to various waterways. These Nr losses contribute to smog formation, acid rain, eutrophication, biodiversity declines and climate change. Hence we argue that an improved national NUE would: (i) benefit the US economy on the production side; (ii) reduce social damage costs; and (iii) help avoid some major climate change risks in the future.

Losses of Ammonia and Nitrate from Agriculture and Their Effect on Nitrogen Recovery in the European Union and the United States between 1900 and 2050

“…For the United States, about 3 to 5 Tg cBNF is from soybean, and 2 to 4 Tg cBNF is from alfalfa (Houlton et al, 2012;USEPA, 2011). The IMAGE value for N fixation by soybean in 2005 is 100 kg N ha -1 in the United States, using a dry mass N content in soybean grains of 6.2%, and is consistent with empirical data by Salvagiotti et al (2008), ranging between 60 and 150 kg N ha …”

The interactive effects of excess reactive nitrogen and climate change on aquatic ecosystems and water resources of the United States

“…Inorganic fertilizer and animal manure are the dominant anthropogenic sources of N in many large US river basins (Puckett 1995;Howarth et al 2011b;Hong et al 2011). Although Nr removal from individual fields can vary widely, approximately 50 % of the N used in agriculture is unintentionally lost to the environment, with a significant fraction flowing to freshwaters (Schaefer et al 2009;Sutton et al 2011;Howarth et al 2011b;Houlton et al 2012). In addition atmospheric N deposition contributes to most US watersheds, and is the dominant Nr source to all mountain ecosystems (Hong et al 2011;Baron et al 2011).…”

Nitrogen–climate interactions in US agriculture

“…About 60 % of this is emitted from agricultural soils, 30 % from animal waste treatment, and 10 % from burning crop residues and vegetation cleared for new agricultural activities (Robertson 2004;Houghton et al 2001). Row crop agriculture is thus responsible for about 50 % of the global anthropogenic N 2 O flux (Robertson 2004).…”

“…Inevitably, this huge advance in global N use has been accompanied by considerable growth in Nr loss to the environment exacerbating atmospheric greenhouse gas (GHG) forcing. For example, atmospheric concentrations of nitrous oxide (N 2 O), the most long-lived form of gaseous Nr, have risen 18 % since 1750 (Houghton et al 2001).…”