A process-based model for methane emission from flooded rice paddy systems

Methane is the second most important greenhouse gas after carbon dioxide. Rice paddy soils release approximately 15-20% of total methane emitted to the atmosphere. A process-based methane emission model was developed for rice paddy systems that highlights plant mediated methane transport. Sequential utilization of alternative electron acceptors such as oxygen, nitrate, Mn(IV), Fe(III) and sulfate in flooded soils is included and permits examination of the effects of fertilizer application and field drainage on methane emissions. Acetate and hydrogen, two representative electron donors produced from the biologically mediated decomposition of solid organic matter, are assumed to be the substrates driving the electron transfer processes. Effects of temperature on reaction kinetics and diffusion processes are based on empirical relationships observed in the laboratory and field. Other processes considered include the exudation of organic carbon and radial release of oxygen from roots, the infiltration flow induced by plant transpiration, the growth dynamics of rice plants, the vertical distribution of soil organic carbon and root biomass, dieback of roots, and loss of gaseous species through ebullition. The performance of the model is evaluated using methane flux data collected in Chongqing and Sichuan, China. Model simulations reveal that although hybrid rice cultivars are several times more efficient in mediating methane transport than traditional tall cultivars at seedling stage, the development of methane transport capacity over the growing season leads to a relatively small difference in total seasonal methane flux (∼15%) among fields planted with tall and hybrid cultivars. Application of nitrate fertilizer at a rate of 64 kg N/ha (about 50% of total nitrogen applied at the Chongqing site) could reduce methane emission by 7%. By converting both iron and manganese to oxidized forms, pre-season drainage is found to be able to reduce methane emissions by 8-10%. A 1-week drainage of a rice field during the growing season could further reduce the methane emission by 22-23% and might be a very promising methane-emission mitigation technique, since such drainage practices can also conserve water and improve rice yields. This model will be implemented on a national scale to establish national methane emission inventories and to evaluate the feasibility and cost-effectiveness of various mitigation options that could vary from site to site.