Process-based modeling of methane emissions from rice fields

Rice paddies are known to be a major anthropogenic emission source of atmospheric methane (CH4), the second most important greenhouse gas (GHG) following carbon dioxide (CO2). In order to estimate CH4 emissions from rice paddies under various environmental and management conditions, this study developed a process-based biogeochemistry model, DNDC-Rice, which explicitly simulates relevant processes in rice plant (e.g. water and nitrogen uptake, photosynthesis, respiration, carbon and nitrogen allocation, CH4 transport) and in paddy soils (e.g. water, heat and gas transport, organic matters decomposition, Fe reduction/ oxidation, CH4 production/oxidation). The performance of DNDC-Rice model was evaluated using experimental data of 6 rice paddy sites with varied treatments of residue incorporation, water managements, sulfate application, or atmospheric CO2 concentration ([CO2]). DNDC-Rice consistently estimated the variations in CH4 emission rate as a function of residue incorporation, water managements and sulfate application, showing its potential to estimate CH4 emissions under a wide range of conditions. For predicting CH4 emissions under elevated [CO2], on the other hand, it was suggested that DNDC-Rice needs further improvements concerning plant processes such as photosynthesis and nitrogen uptake under elevated [CO2]. In a regional application, DNDC-Rice was combined with a GIS database on climate, soil and managements of rice paddies in Hokkaido, Japan, to assess the CH4 mitigation potentials of alternative water managements (AWM). This assessment showed that AWM can reduce seasonal CH4 emissions from rice paddies in Hokkaido by up to 41% as compared to the conventional water management in the region. By constructing a national-scale database, DNDC-Rice will be likewise applicable for computing the national GHG inventory and mitigation potential of CH4 emissions from rice paddies in Japan. At present, DNDC-Rice has been validated and applied mostly in Japan. Therefore, it needs to be calibrated and validated under the conditions of rice cultivars, climate and soils in other rice-growing countries, in order to contribute to their mitigation/adaptation strategies under climate change.