Exploring the role of soil pore structure in methane emissions from rice paddy fields: A combination of x-ray micro-computed tomography and three-dimensional lattice Boltzmann modeling

Methane emissions during the rice-growing season primarily originate from the rhizosphere. Understanding the impact of different soil pore structures on methane emissions can deepen our insight into the pore-scale mechanisms of greenhouse gas (GHG) emissions in paddy soils and assist in developing management practices to mitigate these emissions. In this study, we investigated the differences in pore characteristics between rhizosphere and non-rhizosphere soils using x-ray micro-computed tomography. Subsequently, we employed the three-dimensional (3D) lattice Boltzmann method (LBM) to simulate tortuosity (using single-phase LBM) and methane flow and retention (using multi-phase LBM) in both rhizosphere and non-rhizosphere soils. Our findings indicate that the pore structure and hydraulic properties of rhizosphere soil were different from those of non-rhizosphere soil due to the role of plant roots. Due to increased connectivity and decreased tortuosity, the methane saturation in rhizosphere soil (0.2–0.25) is lower than that of non-rhizosphere soil (∼0.35), reflecting a reduced methane retention capacity of rhizosphere soil. From the perspective of pore size distribution, we observed a reduction in the proportion of small pores (80–200 μm) and increase in the critical diameter in rhizosphere soil, contributing to its lower methane retention capacity. Our study highlights the structural differences and methane retention capacities between rhizosphere and non-rhizosphere soils, establishing a preliminary relationship between them. This provides a new perspective for studying GHG release at the pore scale.