Changes in wet nitrogen deposition in the United States between 1985 and 2012

The United States (US) is among the global hotspots of nitrogen (N) deposition and assessing the temporal trends of wet N deposition is relevant to quantify the effectiveness of existing N regulation policies and its consequent environmental effects. This study analyzed changes in observed wet deposition of dissolved inorganic N (DIN?=?ammonium?+?nitrate) in the US between 1985 and 2012 by applying a Mann?Kendall test and Regional Kendall test. Current wet DIN deposition (2011?2012) data were used to gain insight in the current pattern of N deposition. Wet DIN deposition generally decreased going from Midwest?>?Northeast?>?South?>?West region with a national mean rate of 3.5 kg N ha?1 yr?1. Ammonium dominated wet DIN deposition in the Midwest, South and West regions, whereas nitrate and ammonium both contributed a half in the Northeast region. Wet DIN deposition showed no significant change at the national scale between 1985 and 2012, but profound changes occurred in its components. Wet ammonium deposition showed a significant increasing trend at national scale (0.013 kg N ha?1 yr?2), with the highest increase in the Midwest and eastern part of the South region. Inversely, wet nitrate deposition decreased significantly at national scale (?0.014 kg N ha?1 yr?2), with the largest reduction in the Northeast region. Overall, ratios of ammonium versus nitrate in wet deposition showed a significant increase in all the four regions, resulting in a transition of the dominant N species from nitrate to ammonium. Distinct magnitudes, trends and patterns of wet ammonium and nitrate deposition suggest the needs to control N emissions by species and regions to avoid negative effects of N deposition on ecosystem health and function in the US.