Book excerpt: Annual values of atmospheric deposition of nitrogen to the Mississippi River System drainage basin were computed for 15 years (1979? 1993) using National Atmospheric Deposition Program wet deposition data and 3 years (1990? 1992) of National Dry Deposition Network dry deposition data. Wet deposition of nitrogen was measured as nitrate (NO3) and ammonium (NH4), dry deposition was determined as nitrate, the sum of gaseous nitric acid (HNO3) and particulate nitrate. Fifteen year average wet deposition of nitrate and ammonium were 44 and 42.