Slope position controls on methane uptake and emissions in a temperate mountainous forest in Japan

Abstract

Abstract Upland forest soils represent the second-largest global sink for atmospheric methane (CH 4 ), but accurate estimation of this sink is hindered by the high spatial heterogeneity of CH 4 fluxes, particularly in complex mountainous terrain. This study investigated the mechanisms underlying CH 4 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 CH 4 concentration gradient analysis, and CH 4 oxidation potential assays to examine surface fluxes and subsurface oxidation processes across ridges, mid-slopes, valley bottoms, and riparian positions. CH 4 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 CH 4 diffusion and methanotrophic activity. Although deep soil layers retained CH 4 oxidation potentials comparable to those of surface soils, CH 4 consumption per unit soil volume was greatest in the uppermost 5 cm. These findings indicate that the surface layer acts as an efficient CH 4 biofilter, creating a substrate-limited regime in which atmospheric CH 4 is largely consumed before it reaches deeper horizons. In contrast, the saturated riparian zone acted as a strong CH 4 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 CH 4 fluxes and should be considered in watershed-scale CH 4 budget assessments.