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In a northwestern Connecticut forest, we quantified the carbon (C) and nitrogen (N) content of the forest floor and the top 15 cm of mineral soil and the rate of midsummer net N mineralization beneath six different tree species. There were large interspecific differences in forest floor depth and mass, in the size and distribution of C and N pools at varying soil depths, and in rates of midsummer net N mineralization and nitrification. Forest floor mass ranged from 3.2 kg/m2 to 11.0 kg/m2 and was smallest beneath sugar maple and largest beneath hemlock. The pool size of C in the forest floor ranged from 1.1 kg/m2 to 4.4 kg/m2 while the N content of the forest floor ranged from 83 g/m2 to 229 g/m2. Forest floor C and N pools were smallest beneath sugar maple and highest beneath hemlock. Soil C:N ratios (range: 14.8–19.5) were lower beneath sugar maple, red maple, and white ash than beneath beech, red oak, and hemlock, whereas the opposite was true of the midsummer rate of net N mineralization (range: 0.91–2.02 g·m−2·28 d−1). The rate of net nitrification was positively correlated with the rate of net N mineralization. Interspecific differences in litter production and quality explain the large differences among species in the size of the forest floor C and N pools and in net N mineralization rates. The differences in the size and distribution of C and N pools beneath the different species suggest that the mechanisms regulating the process of species replacement in these forests will mediate the effects of anthropogenic, environmental changes in soil C and N dynamics.

Part of the missing sink in the global CO 2 budget has been attributed to the positive effects of CO 2 fertilization and N deposition on carbon sequestration in Northern Hemisphere terrestrial ecosystems. The genus Sphagnum is one of the most important groups of plant species sequestrating carbon in temperate and northern bog ecosystems, because of the low decomposability of the dead material it produces. The effects of raised CO 2 and increased atmospheric N deposition on growth of Sphagnum and other plants were studied in bogs at four sites across Western Europe. Contrary to expectations, elevated CO 2 did not signi®cantly affect Sphagnum biomass growth. Increased N deposition reduced Sphagnum mass growth, because it increased the cover of vascular plants and the tall moss Polytrichum strictum. Such changes in plant species composition may decrease carbon sequestration in Sphagnum-dominated bog ecosystems.

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