Microbial Regulation Mechanism Of Methane Oxidation In Paddy Soil Under Elevated Atmospheric CO₂

Atmospheric methane (CH4) is the second most important greenhouse gas. Flooded rice fields are a major source of atmospheric CH4. Methanotrophs inhabiting in rhizosphere and surface soil are able to oxidize up to90%of total CH4produced in anoxic soil and hence serve as a biobarrier of CH4emission into the atmosphere. The atmospheric CO2concentration is predicted to be450-550mmol mol-1by the middle of this century. Therefore, a better knowledge of the response of the active methanotrophs community to elevated atmospheric CO2in the rice field soil is important for the CH4emission mitigation measures making. China free air CO2enrichment (FACE) platform allow the study of the effects of elevated CO2on rice-wheat rotation ecosystem under natural conditions. For investigating the methanotrophic response of the elevated atmospheric CO2, the compare between the elevated and ambient treatments was carried out by using the stable isotope probing technique and other molecular microbial ecology methods.The laboratory study on methane oxidation under different initial CH4concentrations showed that the paddy soil in this area had a relatively high methane oxidation capability. The methane oxidation rates under each initial CH4concentrations reached the maximum during the30.5h-47h, the maximum rates of the8000μl l-1and18000μl l-1initial CH4concentrations treatments were higher than others. Combined with454pyrosequencing technique,13C-DNA based stable isotope probing of active methane oxidation microbes revealed that most typical methanotrophs(except for Methylothermus, Methylocella and Methylocapsa) had incorporated13C, Methylobacter and Methylosarcina are the most active methanotrophs in the soil. Elevated atmospheric CO2also changed the composition of the active methanotrophic community, the abundance of Methylobacter was increased while the relative abundance of Methylosarcina was decreased.The reliability of454 pyrosequencing results were tested and verified by traditional clone library analysis.Real-time quantitative PCR of pmoA gene was carried out to investigate the effect of elevated CO2on the population size of methanotrophs under two nitrogen fertilizer rates. Our results showed that pmoA genes were increased during the seedling and tillering stages in2009, and seedling, jointing and grain filling stages in2010despite the nitrogen fertilizer rate, indicating a stimulation of methanotrophic growth. The increasing range of high nitrogen fertilizer rate is higher than the normal nitrogen fertilizer rate. A significant decrease was also observed during the mature stage in2010which means the elevated CO2inhibiting the growth of methanotrophs. Furthermore, intensive fertilization appeared to stimulate the relative abundance of type I methanotrophs under the elevated CO2conditions.