We conducted a laboratory study to evaluate the hypothesis that the effects of soil moisture on aerobic and anaerobic carbon mineralization in a histosol from a phosphorus-limited ecosystem depend on both C:P ratio of the soil and carbon availability. The effects of varying soil moisture and soil C:P ratio on carbon mineralization were studied in a pristine histosol with low P content (C:P = 2000). Relative water content (Θrel = ΘV/∊, where ΘV is volumetric water content and ∊ is the saturated water content) varied from 0.14 to 1.00, corresponding to soil water potentials (Ψ) of −188 to 0 bars, respectively. The C:P ratio was varied from 2000 to 20 by addition of PO4 to the soil. At the highest C:P value (2000), changes in water content had no significant effect on respiration rate (α). As the C:P ratio decreased, respiration became increasingly sensitive to changes in relative water content, with maximum respiration rates (αmax) observed at Θrel between 0.60 and 1.00 and C:P values between 20 and 200. There was a negative, statistically significant linear relationship between relative respiration rate (α/αmax) and soil water potential at C:P > 2000, described by the equation: α/αmax = −0.077 log Ψ + 0.898. Respiration in glucose-amended pristine soil showed little response to water content; however, combined addition of glucose and phosphorus to the soil resulted in a shift in the values of Θrel at which maximum respiration rate was observed to between 0.2 and 0.4, depending on the C:P ratio of the soil. Methane evolution from glucose-amended soil was sensitive to changes in C:P and soil moisture, with CH4 production observed at increasingly lower values of Θrel with decreasing C:P. These results suggest that the effects of soil moisture on carbon metabolism in these soils are dependent on both C:P ratio of the soil and carbon availability.