Net nitrogen mineralization and nitrification in trembling aspen forest soils on the Boreal Plain

In situ net N mineralization and net nitrification rates were measured in organic forest floor (LFH) and mineral horizons of mature and logged trembling aspen (Populus tremuloides Michx.) stands on the Boreal Plain in western Canada. Cumulative May to September mineralization for mature and logged plots was 1354 +/- 534 and 1631 +/- 1584 mg N.m(-2), respectively, in the LFH horizon and 810 +/- 394 and -305 +/- 3957 mg N.m(-2), respectively, in the mineral horizon. Net nitrification in mature and logged plots was 86 +/- 142 and 658 +/- 435 mg NO3-N.m(-2), respectively, in the LFH horizon and 67 +/- 50 and 409 +/- 325 mg NO3-N.m(-2), respectively, in the mineral horizon. Monthly mean NH4-N concentrations in the LFH tended to be higher in logged plots than in mature plots. Winter net N mineralization and nitrification rates in the LFH of mature plots were up to 7% and 11% of growing season net rates, respectively. In comparison, these rates in logged plots were up to 127% and 59% of the growing season net rates, respectively, indicating that winter activity may make a large contribution to annual net mineralization and nitrification after logging.