Temporal and vertical variation of in-situ methane turnover from stable isotope studies at a boreal peatland

Boreal peatlands emit a substantial amount of CH₄, a potent greenhouse gas, to the atmosphere. Despite decades of effort made on studying CH₄ efflux to the atmosphere, understanding the dynamics of the different co-occurring processes underlying CH₄ emission remains a challenge in peatland CH₄ modeling, especially during the non-growing seasons. Stable isotope signatures of the soil pore water and emitted CH₄ can provide information on these processes. To this end, we conducted a first systematic study using stable isotope methods on in-situ major CH₄ turnover processes along a peat profile in a typical boreal peatland (Siikaneva fen) in Southern Finland.

We run a cavity ring-down spectrometer continuously in 2022 to capture the dynamics of belowground dissolved CH₄ and CO₂ concentrations and their δ¹³C natural abundance signatures at 10, 30 and 50cm. Same variables were measured at 40cm above peatland surface to estimate ecosystem-scale average δ¹³C value of the emitted CH4 using nocturnal boundary-layer accumulation approach. These data were used to indicate CH₄ production pathways, in-situ CH₄ oxidation and transport pathways on an annual basis. Additionally, ¹³C pulse labelling experiments targeting acetoclastic methanogenesis, hydrogenotrophic methanogenesis and methanotrophy were performed both in-situ and in the lab condition to trace all these processes which cannot be separated by the isotope natural abundance approach alone.

Preliminary results indicated a successful implementation of these novel methods. Continuous measurement of soil gas showed systematic differences in the vertical profile of soil pore water isotopes between winter and summer. δ¹³C-CH₄ values were highest in the deepest layer during winter but they were the lowest in summer. As expected, CH₄ production pathway moved towards acetoclastic methanogenesis through winter-summer transition. In winter, the δ¹³C-CH₄ values from emitted CH₄ was higher than those from the soil which indicating CH₄ oxidation, while in summer the opposite was found due to CH₄ diffusive plant transport. CH₄ concentrations were higher in summer than in winter from all the depths, while at -30cm had the highest concentration. In-situ labelling experiments showed a higher rate of acetoclastic methanogenesis at -30cm than at -50cm, while hydrogenotrophic methanogenesis was similar at both depths. Experiments also demostrated that there was substantial methanotrophy in the soil as deep as 50cm belowground.