Water table depths impact methane transport pathways: Peatland-VU model simulation results

Peatlands are critical soil carbon reservoirs but rewetting and restoring these ecosystems can elevate methane (CH4) emissions. Water table depth (WTD) management has been used to reduce the CH4 cost of peatland rewetting. However, the mechanisms of CH4 emissions under different water table management scenarios, particularly the role of plant transport, ebullition and diffusion, are poorly understood. We combined observations from a mesocosm experiment with a process-based model to disentangle CH4 transport pathways in a temperate rice crop system grown on degraded fen peat. We calibrated Peatland-VU, a process based greenhouse gas emission model, against measured carbon dioxide (CO2) and CH4 fluxes, managed WTDs and soil temperatures from May 2021 to May 2022. Wilmott’s index of agreement between measured and simulated CO2 and CH4 fluxes was 0.83 and 0.46 respectively. The major simulated CH4 transport pathways during the calibration period were plant transport (95.4%), diffusion (4.6%) and no ebullition. The calibrated Peatland-VU simulated three different WTD scenarios (high: 0-10 cm, medium: 10-20 cm and low: 20-30 cm) to quantify its impact on individual CH4 transport pathways and trade-offs with respect to soil carbon sink. The dominant CH4 transport pathways under high WTDs were plant transport (53%) and ebullition (45%) while in medium and low WTDs it was plant transport (87 and 76%) and diffusion (13 and 24%), respectively. A sensitivity analysis revealed that parameters related to plant CH4 transport and CH4 oxidized during the plant-transport significantly impacted ebullition, plant transport and diffusion. Overall, we found that the peat profile was a carbon sink at high and medium WTDs and a carbon source at low WTDs. Our results highlight that CH4 emission pathways can vary significantly to management of WTDs and the plant CH4 transport is especially important in low WTD scenarios.