Analysis of methane production pathways in a riparian wetland of a temperate forest catchment, using δ_13 C of pore water CH_4 and CO_2

Abstract

To clarify how hydrological processes affect biogenic methane (CH_4) production and emission from soil surfaces, we analyzed the δ_13C 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_13C 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_13C-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.