Temperature Controls On Soil Microbial Processes: Including Heat Capacity Explains Temperature Optima and High Q10s At Low Temperatures.

Our current understanding of the temperature (T) response of biological processes in soil is largely based on the Arrhenius equation: k = Ae^-EA/RT where k is the rate constant, A is a pre-exponential factor, E_A is the activation energy and R is the universal gas constant. This predicts an exponential increase in rate as temperature rises whereas there is always a clearly identifiable temperature optimum for all microbial processes. This has in the past been explained by denaturation of enzymes at higher temperatures. While the denaturation temperature of enzymes is often close to the temperature optimum, the rate of denaturation is slow and it is almost certain that denaturation does not occur in organisms until much higher temperatures. Consequently, denaturation by itself cannot explain temperature optima of enzymatic or microbial processes in soil. We have developed further theory based on the Arrhenius equation recognising the large change in heat capacity for enzyme-driven processes. This new theoretical framework predicts a temperature optimum for enzymatic processes without the need for denaturation (Hobbs et al in press). Here, we apply this theory to a wide range of literature data on the response of microbial processes to temperature in soil, focussing on respiration rates and microbial growth but also including different enzymes, methane cycling, and denitrification. We find extremely good fits (R²>0.9) and prediction of temperature optimum with consistent heat capacity values. Lastly, the theory predicts high Q₁₀ values at low temperature optima and that Q₁₀ declines with increasing temperature as is frequently observed in soils but for which there has been little coherent explanation.