Non-microbial methane emissions from soils

Traditionally, methane (CH₄) is anaerobically formed by methanogenic archaea. However, non-microbial CH₄ can also be produced from geologic processes, biomass burning, animals, plants, and recently identified soils. Recognition of non-microbial CH₄ emissions from soils remains inadequate. To better understand this phenomenon, a series of laboratory incubations were conducted to examine effects of temperature, water, and hydrogen peroxide (H₂O₂) on CH₄ emissions under both aerobic and anaerobic conditions using autoclaved (30min, 121°C) soils and aggregates (>2000μm, A1; 2000-250μm, A2; 250-53μm, M1; and <53μm, M2). Results show that applying autoclaving to pre-treat soils is effective to inhibit methanogenic activity, ensuring the CH₄ emitted being non-microbial. Responses of non-microbial CH₄ emissions to temperature, water, and H₂O₂ were almost identical between aerobic and anaerobic conditions. Increasing temperature, water of proper amount, and H₂O₂ could significantly enhance CH₄ emissions. However, the emission rates were inhibited and enhanced by anaerobic conditions without and with the existence of H₂O₂, respectively. As regards the aggregates, aggregate-based emission presented an order of M1>A2>A1>M2 and C-based emission an order of M2>M1>A1>A2, demonstrating that both organic carbon quantity and property areresponsible for CH₄ emissions from soils at the scale of aggregate. Whole soil-based order of A2>A1>M1>M2 suggests that non-microbial CH₄ release from forest soils is majorly contributed by macro-aggregates (i.e., >250μm). The underlying mechanism is that organic matter through thermal treatment, photolysis, or reactions with free radicals produce CH₄, which, in essence, is identical with mechanisms of other non-microbial sources, indicating that non-microbial CH₄ production may be a widespread phenomenon in nature. This work further elucidates the importance of non-microbial CH₄ formation which should be distinguished from the well-known microbial CH₄ formation in order to define both roles in the atmospheric CH₄ global budget.