TEMPERATURE EFFECTS ON KINETICS OF MICROBIAL RESPIRATION AND NET NITROGEN AND SULFUR MINERALIZATION

Global climate change may impact the cycling of C, N, and S in forest ecosystems because increased soil temperatures could alter rates of microbially mediated processes. We studied the effects of temperature on microbial respiration and net N and S mineralization in surface soils from four northern hardwood forests in the Great Lakes region. Soil samples were incubated in the laboratory at five temperatures (5, 10, 15, 20, and 25 degrees C) for 32 wk. Headspace gas was analyzed for CO2-C at 2-wk intervals, and soils were extracted to determine inorganic N and S. Cumulative respired C and mineralized N and S increased with temperature at all sites and were strongly related (r(2) = 0.67 to 0.90, significant at P = 0.001) to an interaction between temperature and soil organic C. Production of respired C and mineralized N was closely fit by first-order kinetic models (r(2) greater than or equal to 0.94, P = 0.001), whereas mineralized S was best described by zero-order kinetics. Contrary to common assumptions, rate constants estimated from the first-order models were not consistently related to temperature, but apparent pool sizes of C and N were highly temperature dependent. Temperature effects on microbial respiration could not be accurately predicted using temperature-adjusted rate constants combined with a constant pool size of labile C. Results suggest that rates of microbial respiration and the mineralization of N and S may be related to a temperature-dependent constraint on microbial access to substrate pools. Simulation models should rely on a thorough understanding of the biological basis underlying microbially mediated C, N, and S transformations in soil.