Temperature sensitivity of anaerobic methane oxidation versus methanogenesis in paddy soil: Implications for the CH₄...

The global methane (CH₄ ) budget is based on a sensitive balance between methanogenesis and CH₄ oxidation (aerobic and anaerobic). The response of these processes to climate warming, however, is not quantified. This largely reflects our lack of knowledge about the temperature sensitivity (Q₁₀ ) of the anaerobic oxidation of CH₄ (AOM)-a ubiquitous process in soils. Based on a ¹³ CH₄ labeling experiment, we determined the rate, Q₁₀ and activation energy of AOM and of methanogenesis in a paddy soil at three temperatures (5, 20, 35°C). The rates of AOM and of methanogenesis increased exponentially with temperature, whereby the AOM rate was significantly lower than methanogenesis. Both the activation energy and Q₁₀ of AOM dropped significantly from 5-20 to 20-35°C, indicating that AOM is a highly temperature-dependent microbial process. Nonetheless, the Q₁₀ of AOM and of methanogenesis were similar at 5-35°C, implying a comparable temperature dependence of AOM and methanogenesis in paddy soil. The continuous increase of AOM Q₁₀ over the 28-day experiment reflects the successive utilization of electron acceptors according to their thermodynamic efficiency. The basic constant for Q₁₀ of AOM was calculated to be 0.1 units for each 3.2 kJ mol^-1 increase of activation energy. We estimate the AOM in paddy soils to consume 2.2~5.5 Tg CH₄ per year on a global scale. Considering these results in conjunction with literature data, the terrestrial AOM in total consumes ~30% of overall CH₄ production. Our data corroborate a similar Q₁₀ of AOM and methanogenesis. As the rate of AOM in paddy soils is lower than methanogenesis, however, it will not fully compensate for an increased methane production under climate warming.