Analysis of methane production pathways in a riparian wetland of a temperate forest catchment, using δ¹³C of pore water CH₄ and CO₂

To clarify how hydrological processes affect biogenic methane (CH 4 ) production and emission from soil surfaces, we analyzed the δ 13 C of CH 4 and CO 2 and chemical constituents dissolved in groundwater at a wetland in the headwater catchment of a temperate forest in Japan. We estimated the contribution of acetate fermentation using the δ 13 C isotope mass balance of dissolved CH 4 and CO 2 . CH 4 production pathways (e.g., acetate fermentation and carbonate reduction) changed temporally and spatially with hydrologically controlled redox conditions. The proportion of methanogenesis attributable to acetate fermentation usually decreased with temperature, suggesting that carbonate reduction dominated under conditions of high CO 2 concentration. In particular, the groundwater table and summer temperatures were key controlling factors in the interannual and intra‐annual changes in CH 4 production pathways, controlling oxygen supply and consumption and, therefore, redox conditions in the soil. Under high temperature and high water table conditions during summer, the soil was strongly reduced and the proportion of carbonate reduction increased. Acetate fermentation also increased episodically, resulting in sporadic increases in δ 13 C‐CH 4 . The calculated acetate contribution obviously decreased in periods of low water table and high temperature when the soil surface was relatively oxic, implying deactivation of acetoclastic methanogenesis under oxic conditions. Thus, hydrological processes control the supply of these electron donors and acceptors and therefore play an important role in determining the relative proportions of CH 4 ‐producing pathways. Our results also indicate that an increase in acetate contribution under highly reducing conditions stimulates CH 4 production and emission from the soil surface.