Chemical and microbial activation energies of soil organic matter decomposition

Abstract

Present concepts emphasize that substrate quality exerts an important control over substrate decomposability and temperature sensitivity of heterotrophic soil respiration (Rh). In this context, soil organic matter (SOM) quality is defined by its molecular and structural complexity and determines the ease by which substrate is oxidized. However, temperature not only affects SOM oxidation rates but also equally the physiology of soil microorganisms, making it difficult to use respiration rates as indicative for the quality inherent to a substrate. One way to distinguish these two would be to measure organic matter oxidation by controlled combustion and to compare the temperature sensitivity of this chemical process to that of enzyme-catalyzed microbial respiration. We analyzed reaction rates, thermal stability indices, and activation energies (Ea) during (i) microbial respiration (EaRh) and (ii) controlled combustion by differential scanning calorimetry (DSC) (EaDSC) of the same set of mineral and organic soils. A high thermal stability coincided with small heterotrophic respiration rates, indicating that thermal stability may be useful as a proxy for biological degradability. Under ambient conditions, enzymes greatly reduced Ea on average from 136 (EaDSC) to 87 (EaRh) kJ mol-1, thereby increasing CO2 release by a factor of 1.5 * 107 relative to the noncatalyzed chemical reaction. However, temperature dependency of chemical and microbial oxidation was not correlated, suggesting that they are determined by different sample properties. A high temperature sensitivity of microbial respiration is linked to parameters independent of chemical oxidizability, in our case, organic matter C/N ratio and soil pH. These factors are important controls for microbial, but not for chemical, oxidation. © 2013 The Author(s).