Microbial decomposition processes are typically described using first-order kinetics, and the effect of elevated temperature is modeled as an increase in the rate constant. However, there is experimental data to suggest that temperature increases the pool size of substrate C available for microbial respiration with little effect on first-order rate constants. We reasoned that changes in soil temperature alter the composition of microbial communities, wherein dominant populations at higher temperatures have the ability to metabolize substrates that are not used by members of the microbial community at lower temperatures. To gain insight into changes in microbial community composition and function following soil wanning, we used molecular techniques of phospholipid fatty acid (PLFA) and lipopolysaccharide fatty acid (LPS-OHFA) analysis and compared the kinetics of microbial respiration for soils incubated from 5 to 25°C. Substrate pools for microbial respiration and the abundance of PLFA and LPS-OHFA biomarkers for Gram-positive and Gram-negative bacteria differed significantly among temperature treatments, providing evidence for a shift in the function and composition of microbial communities related to soil warming. We suggest that shifts in microbial community composition following either large seasonal variation in soil temperature or smaller annual increases associated with global climate change have the potential to alter patterns of soil organic matter decomposition by a mechanism that is not considered by current simulation models.

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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°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, and inorganic N. First-order kinetic models described the accumulation of CO ; 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~' at 5°C, 0.123 wk ' at 10°C, and 0.101 \vk ' at 25°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°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 ., 5°C). P REDICTING THE EXTENT to which climate change might alter ecosystem C balance and rates of organic matter decomposition is, in part, contingent on a better understanding of the physiological response of soil microorganisms to altered temperature and precipitation regimes. On a global basis, soil organic matter contains twice as much C as the earth's atmosphere (Schlesinger, 1977; Post et al., 1982; Jenkinson et al, 1991), and the release of CO 2 from this globally important pool is mediated by the physiological activities of soil microorganisms. Organic substrates entering the soil serve as a source of energy for heterotrophic biosynthesis, during which microbial respiration returns a portion of substrate C to the atmosphere as CO 2. Some have hypothesized that warmer global temperatures will enhance microbial activity, rates of organic matter decomposition, and hence, the global flux of CO 2 from soil (

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 CO 2-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 > 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.

Achieving more uniform grazing at landscape and paddock scales is seen as an important management objective by pastoralists in northern Australia, but it is difficult to attain in practice. This paper presents a brief review of some key factors to be considered in attempts to modify grazing distribution in extensive rangelands by drawing on the preliminary results of a project that is investigating several options for achieving more uniform grazing. Subdividing the landscape into smaller paddocks and, to a lesser extent, installing additional water points in large paddocks are effective in distributing grazing more widely across the landscape. However, these approaches are less effective in achieving uniform grazing within paddocks, and areas of concentrated use still occur even in small paddocks. To achieve spatial grazing objectives, it is necessary to use management tools that operate at the appropriate scale. Attaining more even use within paddocks should therefore be viewed as a separate management objective, requiring different techniques, to obtaining more effective use of the landscape as a whole. Integrating the use of several spatial management tools that act at different scales is likely to be most effective in improving grazing distribution. Our findings also highlight the importance of understanding paddocks in terms of the spatial arrangement of forage resources and their acceptability and quality in relation to water points and other landscape features. Differences between individual cattle in the way they use the landscape are important in producing more uniform use in larger paddocks and may also offer other opportunities for improving the use of landscape resources overall. Finally, the implications of more uniform use for livestock production and other land use values should be considered, with the protection of biodiversity values potentially requiring special management arrangements where more even use is achieved.