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 mi crobially 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 s amples were incubated in the laboratory at five temperatures (5, 10, 1 5, 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 0.9 0, significant at P = 0.001) to an interaction between temperature and soil organic C. Production of respired C and mineralized N was closel y fit by first-order kinetic models (r(2) greater than or equal to 0.9 4, P = 0.001), whereas mineralized S was best described by zero-order kinetics. Contrary to common assumptions, rate constants estimated fro m 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 co nstant pool size of labile C. Results suggest that rates of microbial respiration and the mineralization of N and S may be related to a temp erature-dependent constraint on microbial access to substrate pools. S imulation models should rely on a thorough understanding of the biolog ical basis underlying microbially mediated C, N, and S transformations in soil.