Modeling methane emissions from rice soils

Abstract

Rice paddies are a major source of anthropogenic methane emitted to the atmosphere. Methane is an important greenhouse gas since it is 20 times more absorptive than carbon dioxide and methane emissions have been increasing at a rate of about 1% per year. Modeling the methane production and transport processes in rice soils can yield insight into methods for reducing emissions from rice paddies. A mechanistic model is developed to describe the production of methane in rice soils and its subsequent transport to the atmosphere. The Monod kinetic model is used for methane production. Two independent sources of carbon are incorporated: the carbon initially in the soil and that given off by the rice plant roots. The model assumes that methane is emitted to the atmosphere only by transport through the plant, which is consistent with field observations, and that methane transport to the plant is by simple mass transfer. Kinetic parameters are determined by independent experiments involving the decay rate of acetic acid (carbon substrate) in a laboratory culture of a sample of the actual rice soil. Mass transfer coefficients and root carbon exudate function parameters are adjusted so that data from methane flux measurements in an experimental rice field agree approximately with the model simulation. Results from this initial modeling effort are encouraging. The agreement between model output and field data are well within the precision of the measurements.