A Diffusion-Driven CH4-O2 Boundary Structures Methane Oxidation and Carbon Transformation in Upland Soils

Although extensive work has characterized high-affinity atmospheric methane oxidation in upland soils and sustained oxidation in chronically methane-rich environments, shallow point-source inputs introduce transient methane pulses into otherwise aerated heterotrophic soils. Whether these pulses migrate rapidly toward the atmosphere or instead create localized redox boundaries that restructure soil carbon pools and microbial metabolism remains unclear. Here, we conducted a controlled natural gas release experiment to quantify coupled geochemical and microbial responses in near-surface soils across a methane gradient. The release produced a spatial interval where measured CH4 and reconstructed O2 availability overlapped, identifying a redox transition associated with shifts in carbon geochemistry and methane-oxidation-related functional potential. Spatially resolved δ13C and C:N measurements revealed strong 13C enrichment of soil organic carbon (SOC) at the plume center, while elevated carbonate abundance and isotope composition distinguished a geochemical transition between the methane-rich plume center and distal reference soils. Within the intermediate CH4-O2 overlap zone, methane-associated monooxygenases (MMOs) and C1 assimilation genes were coordinately enriched, supporting structured C1 metabolic potential across the redox boundary. Metagenomic assembly and reconstruction linked this methane-responsive interval to Actinomycetota-affiliated genomes encoding expanded monooxygenase repertoires, including sMMO-like systems supported by operon architecture and catalytic-subunit phylogeny. This association provides a mechanistic link between transient methane exposure, redox-boundary formation, and microbial carbon transformation in aerated soils. Together, these findings show that shallow methane inputs can generate spatially constrained biogeochemical hotspots where gas transport, carbonate accumulation, and monooxygenase-associated C1 assimilation converge, and define conditions under which soil processes may influence methane transport toward the atmosphere.