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

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.