Long-term base cation depletion from soils appears to be detrimental to the health and productivity of hardwood forests in the northeastern U.S. Understanding and predicting forest responses to acidic deposition and changes in soil chemistry would benefit from better knowledge of limitations to soil microorganisms, given their critical role mediating the supply of essential nutrients to plants. In a recent study at the Hubbard Brook Experimental Forest (HBEF), NH, soil microbial activity was not stimulated by calcium (Ca) additions to the base-poor forest ecosystem, despite an increase in forest floor pH of approximately 1 unit. We examined this result further by testing the hypothesis that Ca reduces the availability of labile carbon (C) to microorganisms. We conducted a plot-scale field experiment in which we added two levels of Ca (850 kg Ca/ha and 4250 kg Ca/ha) as the mineral wollastonite (CaSiO3), to four blocks of 2 x 2 m plots. Wollastonite addition rate at the lower level was intended to restore soil base status to pre-acid deposition levels. Two years after the Ca additions, we incubated soils in the laboratory to test treatment effects on C mineralization and net nitrogen (N) transformations. Mineralization of soil organic C (SOC) was reduced by the high level of Ca addition to field plots. To distinguish between mineralization of SOC and more recent C inputs, we added 13C-labeled ground leaf litter to laboratory incubations. Mineralization of this added C substrate was not sensitive to plot-level Ca addition. However, Ca added directly to laboratory incubations (as CaCl2) reduced C mineralization in the presence and absence of labeled leaf litter. Preliminary 13C data suggest that added Ca suppressed the mineralization of SOC as well as that of leaf litter. Net N transformations followed the same trends as C mineralization, except that there was virtually no nitrification in incubations amended with Ca in the laboratory. Our results suggest that base status of these soils influences mineralization of soil organic matter, but they do not clearly support our hypothesis that Ca binds with labile pools of C. Understanding the mechanisms by which Ca influences nutrient recycling will require further work distinguishing which organic C pools are most sensitive to interacting with soil Ca.