Linking Ecosystem CH4 Fluxes to Soil Profile CH4 Concentrations and Oxidation Rates: Year-Round Measurements and Drought Effects in a Danish Farmland

Abstract

Methane (CH₄) oxidation in well-drained soils is a key process contributing to the global CH₄ sink. Yet, temporal and depth-specific CH₄ oxidation is rarely described despite being critical for the surface net CH₄ uptake. Here, we linked year-round field observations of CH₄ fluxes in well-drained cultivated soils with subsurface CH₄ concentrations, laboratory incubations, and process-based modeling to uncover these mechanisms. Field observed CH₄ fluxes ranged from −0.43 to 0.19 mg CH₄ m⁻² day⁻¹ with an average of −0.15 ± 0.01 mg CH₄ m⁻² day⁻¹ over the year-round study period. Much higher CH₄ uptakes were observed in summer than in winter, indicating marked seasonal variations. Modeling using the CoupModel to simulate soil temperatures and water content as drivers, along with an analytic reaction-based model to simulate CH₄ fluxes, shows that the depth infiltration of atmospheric CH₄ is a critical parameter for defining a CH₄ oxidation reaction zone below the surface. The thickness of the reaction zone varied seasonally. Sensitivity tests of CH₄ concentrations and oxidation profiles in response to contrasting precipitation scenarios reveal that CH₄ oxidation during drought scenarios is increased at deeper depths due to higher CH₄ availability. However, CH₄ oxidation in near-surface layers decreased due to low soil water content, resulting in a significantly lower net surface CH₄ uptake. Our findings suggest that both the depth-specific CH₄ oxidation profile and net surface CH₄ fluxes will likely change under future warmer and drier periods.