Nitrogen mineralization associated with birch and fir under different soilmoisture regimes

We examined net nitrogen mineralization rates in forest floor beneath birch -dominated or fir-dominated canopy plots in a high-elevation mixed-species forest in. New Hampshire during the 1995 and 1996 growing seasons. Soil moi sture was significantly greater in 1996 than in 1995 (season averages were 2.1 times and 1.4 times dry soil mass, respectively). Net mineralization wa s significantly greater in both plot types in 1996 than in 1995. The magnit ude of difference, however, was much greater in fir plots. Nitrification in creased in birch plots and significantly decreased in fir plots from 1995 t o 1996. Results of a three-way ANOVA showed significant year and species ma in effects for net mineralization and a significant species x year interact ion for nitrification. There were no significant correlations between net N mineralization and measured soil chemistry variables in 1995. In 1996 ther e were significant positive correlations between total N, and net mineraliz ation and nitrification in birch plots and between soil moisture and net mi neralization in fir plots. These results support a growing body of research suggesting that species feedbacks influence rates of net N mineralization in mixed species forests. Further, this study provides novel evidence that rates of net N mineralization may respond differently to changing abiotic c onditions depending on the local canopy species in a mixed-species forest.