Soil temperature may affect microbial activity both directly and indirectly, through its impact on other factors such as soil moisture and litter quantity. Higher temperatures are associated with higher rates of microbial activity. Moreover, changes in soil temperature also affect the microbial community composition. The effect of temperature is also modulated by other factors. For example, in water limited environments higher temperatures do not necessarily lead to higher decomposition rates if the soil is dry. In cold climates, temperature fluctuations determine freeze-thaw cycles, which kill soil microbes, release pulses of available nutrients, and lead to higher rates of decomposition and N mineralization in the subsequent growing season.
Conditions of limited soil water availability reduce the rate of microbial activity due to the emergence of conditions of microbial water stress due to dehydration, and to the reduction in the size of the water films coating the soil grains. Low moisture contents limit the mobility and the supply of substrate to the soil microbes by diffusion through the soil solution. In wet soils, microbial activity is limited by the low soil aeration and the consequent limitations in the amounts of oxygen available for the decomposition process. Thus, under water-logging conditions typical of wetland soils, decomposition is for most part inhibited due to the limited supply of oxygen to the soil microbes and to the limited transport of the CO2 produced in the decomposition process (soil respiration). The optimal environment for microbial activity is provided by a warm soil that is both moist and aerated. These conditions are met with intermediate moisture contents between those of dry and completely saturated soils. The effect of soil moisture is also modulated by other factors -in energy-limited environment, for example, boreal forest, higher moisture does not necessarily lead to higher decomposition rates. Soil moisture fluctuations, typical of arid and semiarid environments, are associated with pulses in the rates of decomposition and mineralization, with consequent pulses in the availability of soil mineral nutrients. Other abiotic factors such as solar irradiance, soil pH, and soil physical properties (e.g., clay content) also affect the decomposition rates. For example, it has been found that in arid environments litter decomposition can undergo a process of photodegradation, which provides a shortcut in the cycling of soil carbon.
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