Role of water flow in modeling methane emissions from flooded paddy soils

Methane (CH4) is a potent greenhouse gas that is emitted from paddy fields, and the large CH4 fluxes represent a worldwide issue for the rice production eco-compatibility. In this work a model is proposed to investigate the role of water flows on CH4 emissions from flooded paddy soils. The model is based on a system of partial differential mass balance equations of the chemical species affecting CH4 fate, and water flows are modeled by the Darcy equation. Moreover, in order to properly model the dynamics of CH4, a number of physico-chemical processes and features not included in currently available CH4 emission models are considered: paddy soil stratigraphy; nutrient adsorption and root water uptake; gas transport and respiration within root aerenchyma compartment. The proposed model allows to simulate the spatio-temporal dynamics of chemical compounds within paddy soil as well as to quantify the influence of different processes on nutrient input/output budgets. Simulations without water flow have shown a considerable overestimation of CH4 emissions due to a different spatio-temporal dynamics of dissolved organic matter (DOC – source of energy for CH4 production). In particular, when water fluxes have not been modeled the overestimation can reach 54%, 41% and 67% of daily minimum, daily maximum, and total over the whole growing season CH4 emission, respectively. Moreover, the model results suggest that roots influence CH4 dynamics principally due to their nutrient uptake, while root effect on advective flow plays a minor role. Finally, the analysis of CH4 transport fluxes has shown the limiting effect of upward dispersive transport fluxes on the downward CH4 percolation.