Activation Energy of Organic Matter Decomposition in Soil and Consequences of Global Warming

Abstract

The activation energy (E_a) is the minimum energy necessary for (bio)chemical reactions acting as an energy barrier and defining reaction rates, for example, organic matter transformations in soil. Based on the E_a database of (i) oxidative and hydrolytic enzyme activities, (ii) organic matter mineralization and CO₂ production, (iii) heat release during soil incubation, as well as (iv) thermal oxidation of soil organic matter (SOM), we assess the E_a of SOM transformation processes. After a short description of the four approaches to assess these E_a values—all based on the Arrhenius equation—we present the E_a of chemical oxidation (79 kJ mol⁻¹, based on thermal oxidation), microbial mineralization (67 kJ mol⁻¹, CO₂ production), microbial decomposition (40 kJ mol⁻¹, heat release), and enzyme-catalyzed hydrolysis of polymers and cleavage of mineral ions of nutrients (33 kJ mol⁻¹, enzyme driven reactions) from SOM. The catalyzing effects of hydrolytic and oxidative enzymes reduce E_a of SOM decomposition by more than twice that of its chemical oxidation. The E_a of enzymatic cleavage of mineral ions of N, P, and S from their organic compounds is 9 kJ mol⁻¹ lower (corresponding to 40-fold faster reactions) than the hydrolysis of N-, P-, and S-free organic polymers. In soil, where organic compounds are physically protected and enzymes are partly deactivated, microbial mineralization is ~140-fold faster compared to its pure chemical oxidation. Because processes with higher E_a are more sensitive to temperature increase, global warming will accelerate the decomposition of stable organic compounds and boost the C cycle much stronger than the cycling of nutrients: N, P, and S. Consequently, the stoichiometry of microbially utilized compounds in warmer conditions will shift toward organic pools with higher C/N ratios. This will decouple the cycling of C and nutrients: N, P, and S. Overall, the E_a of (bio)chemical transformations of organic matter in soil enables to assess process rates and the inherent stability of SOM pools, as well as their responses to global warming.