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°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 = 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 > 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. N.W. MacDonald, Dep. of Biology, Grand Valley State Univ., Allendale, MI 49401-9403; D.R. Zak, School of Natural Resources and Environment, Univ. of Michigan, Ann Arbor, MI 48109-1115; and K.S. Pregitzer, School of Forestry and Wood Products, Michigan Technological Univ., Houghton, MI 49931-1295. Research was performed at and supported by the School of Natural Resources and Environment, Univ. of Michigan, Ann Arbor. Received 14 Jan. 1994. *Corresponding author. Published in Soil Sci. Soc. Am. J. 59:233-240 (1995). G CLIMATIC CHANGE could have major impacts on C, N, and S cycling in forest ecosystems by increasing soil temperature (Jenkinson et al., 1991; Raich and Schlesinger, 1992). Because soil temperature exerts strong control over microbial activity (Nadelhoffer et al., 1991; Ellert and Bettany, 1992; Tate et al., 1993), accurate prediction of climatic effects on C, N, and S cycles depends on a clear understanding of the effects of temperature on the microbially mediated release of these constituents from soil organic matter. Although a few studies have examined the effects of temperature on in situ microbial respiration and N and S mineralization (Schlentner and Van Cleve, 1985; Foster, 1989), most studies have utilized laboratory incubations to examine the influence of temperature on these processes (Cassman and Munns, 1980; Addiscott, 1983; Marion and Black, 1987; Howard and Howard, 1993). Kinetics of microbial respiration and net mineralization of N and S commonly have been described using a first-order rate equation [y = a(l — &~)', Stanford and Smith, 1972; Paustian and Bonde, 1987; Ellert and Bettany, 1988; Zaketal., 1993]. The parameter a represents the pool size of labile substrate, and k is the rate constant for a particular process (Deans et al., 1986). Many studies have assumed that pool sizes are unaffected by incubation temperature and that rate constants predictably increase with rising temperature (Stanford et al., 1973; Campbell et al., 1981, 1984). Based on these Abbreviations: HSD, honestly significant difference; Qm = temperature coefficient; USGS, United States Geological Survey; resp, C respiration; Nmin, net N mineralization; Smin, net S mineralization. *, **, ***, Significant at the 0.05, 0.01, and 0.001 probability levels, respectively. 234 SOIL SCI. SOC. AM. J., VOL. 59, JANUARY-FEBRUARY 1995 assumptions, the pool size is typically determined at high temperatures (35-40 °C), and the rate constant is adjusted to other temperatures using gio or a similar factor (Marion et al., 1981; Parton et al., 1987; Paustian and Bonde, 1987; Cabrera and Kissel, 1988; Zak et al., 1993). Although this reasoning is intuitive, we are unaware of any study that has explicitly tested the assumptions of constant pool size and temperature-dependent rate constants in concert. Moreover, questions about the appropriate use of rate constants determined under different incubation conditions (Clark and Gilmour, 1983) and suggestions that temperature and pool size are related (Marion and Black, 1987; Ellert and Bettany, 1988, 1992) indicate that examination of the assumptions underlying the first-order kinetic model is warranted. We investigated the effects of temperature on microbial respiration and on net mineralization of N and S in surface soils from northern hardwood forests in the Great Lakes region. The study sites span a geographic region where forests are expected to change dramatically during the next century as a direct result of global climate change (Pastor and Post, 1988). Objectives of our study were to: (i) determine if the temperature response and kinetics of microbial respiration and mineralization of N and S differ among sites, and (ii) examine the relationships among temperature, kinetic rate constants, and labile C, N, and S pools as estimated from the first-order