Anaerobic methane oxidation by nitrate: kinetic isotope effect

The ratio of stable carbon isotopes (¹³C/¹²C) in different environments serves as a significant limitation in estimating the global balance of methane [Hornibrook et al., 2000]. In this case, the value of ¹³C/¹²C largely depends on the kinetic isotope effect associated with the metabolism of microorganisms that produce and consume CH₄. The article suggests a dynamic model of the processes of methane formation and its anaerobic oxidation with nitrate by methanotrophic denitrifying microorganisms (DAOM), which allowed estimating the fractionation factor of stable carbon isotopes. In the experiment with peat from the minerotrophic bog [Smemo, Yavitt, 2007], the dynamics of the amount of methane and was measured. The dynamic model showed that the introduction of nitrate leads to a slow decrease in the partial pressure of methane. Since methane in the DAOM process is a substrate, methane is enriched with heavier carbon ¹³C in the system under study. This leads to an increase in the value . The carbon isotope fractionation factor during methane oxidation with nitrate was equal to 1.018 and comparable with the fraction of carbon isotope fractionation in the process of acetoclastic methanogenesis (1.01). Model calculations have shown that during incubation the apparent fractionation factor of carbon isotopes with the simultaneous formation of methane and DAOM slowly decreases. The ratio of ¹³C/¹²C isotopes in dissolved and gaseous methane practically does not differ. The model showed that an increase in the initial concentration of nitrate increases the rate of DAOM, which leads to a decrease in the concentration of dissolved methane. In this case, the value of ¹³C/¹²C increases. In field studies, Shi et al. (2017) showed that the presence of DAOM in peat bogs in which fertilizers penetrate can be controlled by the amount of nitrate used and the depth of penetration into the anoxic layer. Two MATLAB files describing DAOM are attached to the article.