Activation energy of soil organic matter decomposition

Activation energy (E_a) of (bio)chemical reactions – the fundamental parameter, defining the reaction rates – has never been critically evaluated and generalized for processes of organic matter transformations in soil. Based on the database of E_a for a broad range of i) oxidative and hydrolytic exoenzyme activities, ii) CO₂ production and iii) heat release during soil incubation, as well as iv) thermal decomposition of soil organic matter (SOM), we assessed the E_a for processes of SOM transformation. After a short description of the four approaches to assess E_a of SOM transformation – all based on the Arrhenius equation – we present the E_a of chemical (79 kJ mol^-1) and microbial (67 kJ mol^-1) mineralization, microbial decomposition (40 kJ mol^-1), and exoenzyme-catalyzed depolymerization (33 kJ mol^-1) of SOM. The catalyzing effects of exoenzymes reduce the energy barrier of SOM decomposition by more than twice that of its chemical oxidation (from 79 to 33 kJ mol^-1). The E_a of exoenzymatic hydrolysis of N-, P-, and S-containing organic compounds is about 9 kJ mol^-1 lower (40-fold faster reactions) than that of other (N-, P-, and S-free) organic substances. Under real soil conditions (not in suspension as in enzyme activity analysis), where organic substrates are physically protected and exoenzymes are partly deactivated, microbial mineralization of SOM is 140-fold faster compared to its chemical oxidation. The E_a of microbial mineralization of SOM increases from biochemically labile to stable pools. This is one of the reasons for the decrease in the CO₂ efflux from soil during long-term incubations.

Since processes with higher E_a are more sensitive to temperature increase, global warming will stimulate faster decomposition of stable organic compounds and accelerate the C cycle much stronger than the cycling of the nutrients N, P, and S. The consequence will be a shift in the stoichiometric ratios of microbially utilized substrates. Overall, E_a is an easily measurable crucial parameter of (bio)chemical transformations of organic matter in soil, enabling the assessment of process rates and the inherent stability of SOM pools, as well as their responses to global warming.