SOIL NITROGEN MINERALIZATION UNDER NORTHERN HARDWOOD FORESTS ALONG A SULFATE AND NITRATE DEPOSITION GRADIENT IN THE GREAT LAKES REGION

Net N mineralization (ammonification and nitrification) in the 0-10 cm mineral soil zone of five northern hardwood forest sites along a gradient of SO and NO deposition from northeastem Minnesota to central lower Michigan was measured by an in situ buried bag technique at monthly intervals from September 1987 to April 1990. Soil nitrification rates (36.9 to 46.7 kg N·ha-1·yr-1) increased from north to south among the five study sites and were strongly associated with soil temperature (r=0.87, p<0.001). The rates of soil ammonification (66.8 to 84. 1 kg N·ha-1·yr-1) and amounts of total N mineralized (103.7 to 130.6 kg N·ha-1·yr-1)did not show a clear regional trend across the gradient sites. Significant correlations between SO(r=0.82, p<0.001), No(r=0.77, p=0.003) deposition and the adjusted means of ammonium-N after removing the effects of soil temperature indicated that SO and NO deposition had significantly impacts on ammonification process. Soil pH did not correspond to the gradient of H+deposition, which was not correlated with either ammonification or nitrification rates across the study sites.