The Temperature Optima of Soil Biogeochemical Processes and Temperature Sensitivity Can be Explained By Macromolecular Rate Theory.

The temperature-dependence of soil biological processes is a key determinant of how soil ecosystems contribute to the global carbon and nitrogen cycles and how they will respond to a warming climate. Temperature dependence as described by the Arrhenius function provides an excellent description of chemical reaction rates but does not predict the temperature optimum of biological rates that is clearly evident in laboratory and field measurements. We have developed a theory of biological rates building on the Arrhenius function and incorporating a temperature-dependent activation energy as required by first principles from thermodynamics. We have called this “Macro-Molecular Rate Theory (MMRT)” which accounts for large changes in the flexibility of enzymes (macromolecules) during catalysis. This property is measured as a change in heat capacity (DC^‡_p) for the reaction. MMRT predicts Arrhenius-like behaviour at lower temperatures but then predicts a temperature optimum at higher temperatures (Hobbs et al., 2013. ACS Chemical Biology. 8: 2388-2393). We have shown that this theory fits biogeochemical data collected from laboratory and field studies with important implications for changes in absolute temperature sensitivity as temperature rises (Schipper et al., 2014. Global Change Biology).  By definition, a temperature optimum requires a decreasing absolute temperature sensitivity of biological processes to zero when the optimum is reached. Consequently, changes in rates of soil biological processes to increases in temperature depend on both the change in ecosystem temperature and the temperature optimum of the process. Temperature optima of many soil biological processes are very poorly documented but would lead a better understanding of how soil systems will respond to increasing global temperatures.