Nitrogen mineralization and nitrification in upland and peatland forest soils in two Canadian Shield catchments

Net mineralization and nitrification in surface forest soils were measured in upland forest stands and valley peatlands using in situ soil incubations at two headwater catchments of Harp Lake, Ontario from July 1995 to October 1996. No difference in either net N mineralization or nitrification was observed between the two adjacent catchments despite differences in catchment N export. Annual rates of net N mineralization in surface 10 cm were higher in forest soils of the deciduous (11.5 +/- 3.1 g/m(2); mean +/- SE) and conifer-mixed (conifer-hardwoods) (13.9 +/- 2.3 g/m(2)) stands than in peatland soils (1.6 +/- 0.6 g/m(2)). Mean annual nitrification rates were higher in deciduous soils (6.6 +/- 0.7 g N/m(2)) than in mixed stands (1.9 +/- 0.6 g N/m(2)) and peatland soils (0.1 +/- 0.2 g N/m(2)). Annual net N mineralization and nitrification were correlated with variations in soil C and N content and moisture associated with different forest stands. Frequent winter incubations indicate that net mineralization and nitrification under snow cover in upland surface soils can contribute as much as 49 and 23% of the annual net production, respectively. The importance of forest vegetation patterns, winter N transformations, and dissolved organic N pools to total N and NO3- cycling and leaching in these catchments is discussed.