Soil pore structure mediates the effects of soil oxygen on the dynamics of greenhouse gases during wetting-drying phases.

The timing and magnitude of greenhouse gas (GHG) production depend strongly on soil oxygen (O) availability, and the soil pore geometry characteristics largely regulate O and moisture conditions relating to GHG biochemical processes. However, the interactions between O dynamics and the concentration and flux of GHGs during the soil moisture transitions under various soil pore conditions have not yet been clarified. In this study, a soil-column experiment was conducted under wetting-drying phases using three pore-structure treatments, FINE, MEDIUM, and COARSE, with 0 %, 30 %, and 50 % coarse quartz sand applied to soil, respectively. The concentrations of soil gases (O, nitrous oxide (NO), carbon dioxide (CO), and methane (CH)) were monitored at a depth of 15 cm hourly, and their surface fluxes were measured daily. Soil porosity, pore size distribution, and pore connectivity were quantified using X-ray computed microtomography. The soil O concentrations were found to decline sharply as soil moisture increased to the water holding capacities of 0.46, 0.41, and 0.32 cm cm in the FINE, MEDIUM, and COARSE, respectively. The dynamic patterns of the O concentrations varied across the soil pore structures, decreasing to anaerobic in FINE (<0.01 %) and MEDIUM (0.02 %), and to hypoxic (4.42 %) in COARSE. Correspondingly, the soil NO concentration was the highest in FINE (101 μL L) and the lowest in COARSE (10 μL L), whereas the highest surface NO flux was observed in MEDIUM (131 μg N m h). As soil CO concentrations declined, CO fluxes increased from FINE to MEDIUM to COARSE. Most pores of FINE, MEDIUM, and COARSE were 15-80 μm, 85-100 μm, and 105-125 μm, respectively, in terms of diameter. The X-ray CT visible (>15 μm) porosity in FINE, MEDIUM and COARSE were 0.09, 0.17, and 0.28 mm mm, respectively. The corresponding Euler-Poincaré numbers were 180,280, 76,705, and -10,604, respectively, indicating higher connectivity in COARSE than in MEDIUM or FINE. In soil dominated by small air-filled porosity which limits gas diffusion and result in low soil O concentration, NO concentration was increased and CO flux was inhibited as the moisture content increased. The turning point in the sharp decrease in O concentration was found to correspond with a moisture content, and a pore diameter of 95-110 μm was associated with the critical turning point between holding water and O depletion in soil. These findings suggest that O-regulated biochemical processes are key to the production and flux of GHGs, which in turn are dependent on the soil pore structure and a coupling relationship between NO and CO. Improved understanding of the intense effect of soil physical properties provided an empirical foundation for the future development of mechanistic prediction models for how pore-space scale processes with high temporal (hourly) resolution up to GHGs fluxes at larger spatial and temporal scales.