**7. Oxidation process of methane in the paddy field soil**

In paddy fields, CH4 production and oxidation happen simultaneously, so it is difficult to directly determine CH4 production and oxidation separately. CH4 oxidation may occur in aerobic and anaerobic ways. Previously CH4 oxidation was usually determined by comparing CH4 fluxes from flooded soils under aerobic and anaerobic incubation conditions [68]. By using this approach, CH4 oxidation accounted for up to >90% of CH4 production [52]. With the development of CH4 oxidation inhibitors and isotopic methods, CH4 oxidation is now widely measured using these approaches and shows relatively low ratio (less than 70%) to CH4 production [69]. Generally, CH4 emission is extremely influenced by the balance between CH4 production and CH4 oxidation in paddy fields [70]. Oxidation of CH4 reduces the emission of CH4 in the soil of rice field to the atmosphere. CH4 is relatively inert in anoxic environments, but is oxidized by methanotrophic bacteria as soon as oxygen becomes available [71]. Aerobic methanotrophic bacteria are present in the oxic surface layer of the submerged paddy soil and in the rhizosphere where oxygen is available in a shallow layer around the rice roots. The most distinctly possible sites for CH4 oxidation in rice fields are the water–soil interface and the rhizosphere. It has been shown that CH4 oxidation is taking place within these zones of the paddy soil so that part of the produced CH4, does not reach the atmosphere [72]. The increase of CH4 production may, in turn, stimulate methanotrophic growth and CH4 oxidation [73], but methanotrophic growth can be limited by low oxygen concentrations [74, 75]. Because larger rice

plants stimulate oxygen transport into the rhizosphere, water management practices can also affect CH4 oxidation rates through their effect on plant growth. The anaerobic CH4 oxidation in the subsoil of a rice paddy was below 5% of the CH4 emission during 38 whole growth periods [76]. The microbial aerobic oxidation activity of CH4, the population of aerobic CH4 oxidizers and the factors influencing the activity of CH4 oxidation were investigated in three types of paddy rice soil in Zhejiang Province, China. The experiment results concluded that the population of CH4 oxidizing bacteria was at maximum within the peak-tillering, heading and flowering stages, during which the largest population of methanogenic bacteria also appeared. Temperatures from 25 to 35°C and pH from 6 to 8 were the optimum conditions for aerobic oxidation of CH4 in paddy rice soil [77]. Soil particle sizes also significantly affect the activity of CH4 oxidation. Approximately 95% of the CH4 produced in flooded soils is oxidized to carbon dioxide before it is released to the environment. Therefore, the oxidation is important for the biogeochemical cycling of CH4 [24].
