Modeling methane emission from rice paddies: model and modification
稻田甲烷排放模型研究——模型及其修正

Precise estimates of CH_4 emissions from rice fields have been difficult to determine due to regional differences in spatial and temporal variability in climate, soils and agricultural practices. To obtain estimates of methane emissions from regional or global rice paddies, attention must be focused on an examination of methodologies by which the current high uncertainties in the estimates might be reduced. One possible way to do this is the development of predictive models. With an understanding of the processes of methane production, oxidation and emission, Huang et al. developed a model to predict methane emission from rice paddy soils. Validated against independent field measurements of methane emission from rice paddies in USA, China and Italy, Huang's model provides a realistic estimate of the observed results. However, Huang's model was primarily developed for continuous flooding and not for intermittent irrigation rice paddies. Moreover, the methane emission via ebullition was not taken into account in their model. The objective of this paper is to modify Huang's model so as to make it capable of simulating methane emissions from irrigated rice fields under various agricultural practices. We accept the hypothesis from the original model that methanogenic substrates are primarily derived from rice plants and added organic matter. Rates of methane production in flooded rice soils are determined by the availability of methanogenic substrates and the influence of environmental factors. The fraction of methane emitted through plants is controlled by rice growth and development. Rates of methane transported from soil to the atmosphere via plants are determined by the rates of production and the emitted fraction. Modification to the original model focuses on the effect of water regime on methane production/emission and the methane transport via bubbles. The effect of water regime was quantified by soil Eh. Changes of the soil Eh during drainage course and re-flooding course were simulated by two differential equations, respectively. Simulation of the soil Eh during the re-flooding course associated the decomposition of added organic carbon. Methane emission via bubbles was modeled by soil temperature, rice root growth and the difference between methane production rate and a critical rate when the bubbles occur. New features of the present model, named as CH_4MOD, are the incorporation of bubble flux process and the drainage events into the original model. Overall, the advantages of this model make it particularly applicable to the simulation of methane emissions from irrigated rice fields with few input parameters of irrigation patterns, type and amount of organic matter amendment, rice grain yield, air temperature, and soil sand percentage.