Soil temperature, matric potential, and the kinetics of microbial respiration and nitrogen mineralization

Soil temperature and matric potential influence the physiological activity of soil microorganisms. Changes in precipitation and temperature can alter microbial activity in soil, rates of organic matter decomposition, and ecosystem C storage. Our objective was to determine the combined influence of soil temperature and matric potential on the kinetics of microbial respiration and net N mineralization. To accomplish this, we collected surface soil (0-10 cm) from two northern hardwood forests in Michigan and incubated samples at a range of temperatures (5, 10, and 25 degrees C) and matric potentials (-0.01, -0.15, -0.30, -0.90 and -1.85 MPa) that encompass field conditions. Soils were maintained at each temperature-matric potential combination over a 16-wk laboratory incubation, during which we periodically measured the production of CO(2) and inorganic N. First-order kinetic models described the accumulation of CO(2) and inorganic N and accounted for 96 to 99% of the variation in these processes. First-order rate constants (k) for net N mineralization significantly increased with temperature, but the k for microbial respiration did not increase in a consistent manner; it was 0.107 wk(-1) at 5 degrees C, 0.123 wk(-1) at 10 degrees C, and 0.101 wk(-1) at 25 degrees C. Matric potential did not significantly influence k for either process. Substrate pools for microbial respiration and net N mineralization declined between -0.01 and -0.30 MPa, and the decline was greatest at the highest soil temperature; this response produced a significant temperature-matric potential interaction. We conclude that high rates of microbial activity at warm soil temperatures (e.g., 25 degrees C) are limited by the diffusion of substrate to metabolically active cells. This limitation apparently lessens as physiological activity and substrate demand decline at relatively cooler soil temperature (e.g., 5 degrees C).