Peatland porewater chemistry reveals field redox ranges for specific decomposition processes

Abstract Peat decomposition, driven by soil metabolic processes, was responsible for 2% of the anthropogenic greenhouse gas emissions in 2019. The peat soil redox potential (Eh) and pH reflect the chemical state of the soil and the (biogeo)chemical processes that occur and are therefore linked to the rate of peat decomposition and greenhouse gas production. We related porewater chemistry (pH, NO3-, SO42-, Mn(II), Fe(II), S2- and CH4, >2000 samples) to continuous Eh measurements (2020-2022) from five agricultural drained peatland sites in the Netherlands that consist of measuring plots with and without subsoil irrigation and drainage (SSI) in order to establish Eh ranges that identify specific metabolic processes. We found that methanogenesis, reduction of SO42-, Fe(III), NO3-/ Mn(IV) and O2 occurred at Eh (at pH 5.5) of < -49 mV, -49 – 104 mV, 104- 364 mV, 364- 700 mV and >700mV respectively. Eh over depth and time closely followed temporal and spatial fluctuations in groundwater level and porewater chemistry, and revealed the extent of O2 intrusion and the occurrence of methanogenesis. Higher and more constant groundwater levels year-round tended to deplete porewater Fe(II) and SO42-, which led to more reducing Eh and higher porewater CH4 concentrations. These mechanistic insights help to optimize GHG mitigation strategies for drained peat regions.