RESPONSE OF CARBON MINERALIZATION TO COMBINED CHANGES IN SOIL-MOISTURE AND CARBON-PHOSPHORUS RATIO IN A LOW PHOSPHORUS HISTOSOL

We conducted a laboratory study to evaluate the hypothesis that the ef fects of soil moisture on aerobic and anaerobic carbon mineralization in a histosol from a phosphorus-limited ecosystem depend on both C:P r atio 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 con tent theta(rel) = theta(v)/epsilon, where theta(v) is volumetric water content and epsilon is the saturated water content) varied from 0.14 to 1.00, corresponding to soil water potentials (Psi) of -188 to 0 bar s, 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 (a). As the C:P ratio decreased, respiration became increasingly sensitive to changes in relative water content, with maximum respiration rates (a,,) observ ed at theta(rel) between 0.60 and 1.00 and C:P values between 20 and 2 00. There was a negative, statistically significant linear relationshi p between relative respiration rate (alpha/alpha(max)) and soil water potential at C:P < 2000, described by the equation: alpha/alpha(max) = -0.077 log Psi + 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 theta(rel) at which maximum respiration rate was observed to betwee n 0.2 and 0.4, depending on the C:P ratio of the soil. Methane evoluti on from glucose-amended soil was sensitive to changes in C:P and soil moisture, with CH4 production observed at increasingly lower values of theta(rel) with decreasing C:P. These results suggest that the effect s of soil moisture on carbon metabolism in these soils are dependent o n both C:P ratio of the soil and carbon availability.