Soil Temperature

Many physical, chemical, and biological processes that occur in soil are influenced by temperature. Increasing temperature enhances mineralization of SOM or decomposition of plant residues by increasing rates of physiological reactions and by accelerating diffusion of soluble substrates in soil. An increase in temperature can also induce a shift in the composition of the microbial community. Whereas rates of molecular diffusion always increase with increasing temperature, solubility of gases in soil solution do not, and can even decrease, thereby slowing microbial activity (Table 2.4).

The relation between a chemical reaction rate and temperature was first proposed by Arrhenius:

The constant A is called the frequency factor and is related to the frequency of molecular collisions, Ea is the activation energy or energy required to initiate the reaction, R is the gas constant R and has a value of 8.314 X 10-3 kJ mol1 T-1, e is the base of the natural logarithm, T is the temperature in degrees Kelvin, and k is the specific reaction rate constant (time-1).

Converting Eq. (2.5) to natural logarithmic form gives ln k = (-EJRT) + ln A. (2.6)

By determining the value of k over a moderate range of soil temperatures, the plot of ln k versus 1/T yields the activation energy from the slope of a line and the frequency factor from the intercept. Soil chemical reaction rates increase, often very sharply, at low temperatures, with increases in temperature due to increased molecular interactions. It is generally accepted that a temperature coefficient, Q10, of approximately 2 over the range 15 to 35°C can be used to describe the relationship between temperature and soil chemical and biochemical processes. That is, a twofold increase or decrease in reaction rate is associated with a shift of 10°C.

But the relationship between temperature and biologically mediated processes is more complicated. While it is expected that the rate of enzyme-catalyzed reactions will increase as temperature increases, at least until some high temperature is reached that causes enzyme inactivation, the increase is not always a factor of 2. Typically enzyme-catalyzed reactions tend to be less affected by temperature changes. Studies have found that a 10°C temperature increase, from 15 to 25°C, can increase soil C and N mineralization rates by up to threefold. And in addition to increasing the specific reaction rate constant, the sizes of the organic matter pools undergoing mineralization are affected by temperature. Thus, the increase in biological activity at higher temperatures is likely due to shifts in microbial community structure.

Though microbial activity at temperatures <5°C is slower than at warmer temperatures, it is not negligible and is significantly higher than the Q10 relationship developed over the mesophilic range 10 to 20°C would predict. Microbial activity at soil temperatures lower than 0°C has been recorded. Psychrophilic organisms are capable of growth at these low temperatures by adjusting upward the osmotic concentration of their cytoplasmic constituents to permit cell interiors to remain unfrozen. Mineralization activity during cool periods when plants are dormant or soils are barren could play a significant role in overwinter losses of soil nutrients, N in particular.

A generalized temperature response curve for soil microbial activity, assuming soil moisture and aeration are not limiting, is shown in Fig. 2.8.

Rates of reaction increase quite sharply over the mesophilic temperature range from 10 to 25°C, suggesting a selection and/or adaptation of soil microorganisms. Nevertheless different microbial communities are likely active as temperatures change, and while individual species differ in their optimal temperature response, this general activity response to temperature is similar for many organisms.

Very few soils maintain a uniform temperature in their upper layers. Variations may be either seasonal or diurnal. Because of the high specific heat of water, wet

Temperature (°C)

FIGURE 2.8 Generalized soil microbial activity response curve to soil temperature, assuming soil moisture and aeration are not limiting.

Temperature (°C)

FIGURE 2.8 Generalized soil microbial activity response curve to soil temperature, assuming soil moisture and aeration are not limiting.

soils are less subject to large diurnal temperature fluctuations than are dry soils. Among factors affecting the rate of soil warming, the intensity and reflectance of solar irradiation are critical. The soil's aspect (south- versus north-facing slopes), steepness of slope, degree of shading, and surface cover (vegetation, litter, mulches) determine effective solar irradiation. Given the importance of soil temperature in controlling soil processes, models of energy movement into the surface soil profile have been developed. They are based on physical laws of soil heat transport and thermal diffusivity and include empirical parameters related to the temporal (seasonal) and sinusoidal variations in the diurnal pattern of near-surface air temperatures. The amplitude of the diurnal soil temperature variation is greatly dampened with profile depth.

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