s:935:"TI Intentional versus unintentional nitrogen use in the United States: trends, efficiency and implications 
AU Houlton, B.Z. 
AU Boyer, E. 
AU Finzi, A. 
AU Galloway, J. 
AU Leach, A. 
AU Liptzin, D. 
AU Melillo, J. 
AU Rosenstock, T.S. 
AU Sobota, D. 
AU Townsend, A.R. 
AB 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 N2 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  
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