Nitrogen mineralization associated with birch and fir under different soil moisture regimes

Abstract

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 moisture was significantly greater in 1996 than in 1995 (season averages were 2.1 times and 1.4 times dry soil mass, respectively). Net mineralization was significantly greater in both plot types in 1996 than in 1995. The magnitude of difference, however, was much greater in fir plots. Nitrification increased in birch plots and significantly decreased in fir plots from 1995 to 1996. Results of a three-way ANOVA showed significant year and species main effects for net mineralization and a significant species × year interaction for nitrification. There were no significant correlations between net N mineralization and measured soil chemistry variables in 1995. In 1996 there were significant positive correlations between total N, and net mineralization and nitrification in birch plots and between soil moisture and net mineralization 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 conditions depending on the local canopy species in a mixed-species forest.