Abstract

Upland forest soils represent the second-largest global sink for atmospheric methane (CH₄), but accurate estimation of this sink is hindered by the high spatial heterogeneity of CH₄ fluxes, particularly in complex mountainous terrain. This study investigated the mechanisms underlying CH₄ uptake and emissions across slope positions in a temperate secondary forest in Kyoto, Japan, over three years (2023–2025). We integrated closed-chamber measurements, soil CH₄ concentration gradient analysis, and CH₄ oxidation potential assays to examine surface fluxes and subsurface oxidation processes across ridges, mid-slopes, valley bottoms, and riparian positions. CH₄ uptake showed pronounced spatial variation, with the uptake rates in the drier ridge and mid-slope areas approximately twice those at the valley bottom. This pattern was primarily associated with lower soil moisture at upslope positions, which facilitated CH₄ diffusion and methanotrophic activity. Although deep soil layers retained CH₄ oxidation potentials comparable to those of surface soils, CH₄ consumption per unit soil volume was greatest in the uppermost 5 cm. These findings indicate that the surface layer acts as an efficient CH₄ biofilter, creating a substrate-limited regime in which atmospheric CH₄ is largely consumed before it reaches deeper horizons. In contrast, the saturated riparian zone acted as a strong CH₄ source, with summer emissions reaching magnitudes up to approximately 100 times greater than upslope uptake. These findings demonstrate that topographically controlled soil moisture regimes govern the spatial heterogeneity of CH₄ fluxes and should be considered in watershed-scale CH₄ budget assessments.