TRENDS IN WET DEPOSITION OF NITROGEN IN THE UNITED STATES,
1985-2001
Mark Nilles, U.S. Geological Survey
Nearly 27 teragrams of ammonia and nitrogen oxides are emitted annually to the
atmosphere in the United States. Current assessments identify livestock agriculture and
fertilizer application as the primary sources of ammonia emissions. Deposition of
atmospheric nitrogen compounds to aquatic and terrestrial ecosystems can account for a
significant portion of nitrogen inputs. Trends in wet deposition of oxidized and reduced
forms of nitrogen have not followed the well-documented declines in sulfate deposition
in the U.S. Monthly data from 149 sites in the National Atmospheric Deposition
Program/National Trends Network were evaluated for trends over the period 1985-2001
using a parametric model to remove the influence of inter-annual variations in
precipitation amount, followed by a non-parametric test for detection of monotonic
trends. For ammonium deposition, increases were detected at 58 sites while only 2 sites
exhibited declining trends. Sites with increasing ammonium deposition were distributed
nationally with the exception of the northeastern U.S. where few sites exhibited
increasing trends. On a network-wide basis, ammonium deposition increased by 19
percent over the 17-year period of analysis. While the majority of sites did not exhibit
any trend in nitrate deposition, a greater number of sites (23) had significant increasing
trends versus sites with declining trends (12). Sites with increasing trends in nitrate were
predominantly located in the western and southeastern U.S. while sites with declining
trends were mostly located in the northeastern U.S.
Mark A. Nilles, U.S. Geological Survey, Box 25046, MS-401, Denver, CO 80225
Ph. 303-236-1878, Fax 303-236-1880, manilles@usgs.gov