Modelling the effects of rewetting on peatland CH4 dynamics

Undisturbed peatlands accumulate vast amounts of carbon in peat over long timescales. This is enabled by the presence of a high water table creating unfavourable conditions for organic matter decomposition, causing carbon uptake by vegetation to exceed carbon release. Drainage for agriculture and forestry purposes greatly influences the greenhouse gas (GHG) balance of peatlands by accelerating peat decomposition due to enhanced aeration. Although drained peatlands are associated with lower methane (CH4) emissions than their pristine counterparts, it is estimated that drained peatlands contribute 5% of global anthropogenic CO2 emissions.Rewetting by ditch blocking is currently being widely proposed as an instrument to revert the GHG balance of drained peatlands to pre-drained conditions. However, complex process interactions at the ecosystem scale, along with a trade-off between CO2 and CH4 emissions, make it difficult to determine the optimal management of the water table from a climate change mitigation perspective. Most importantly, we lack both observational data and understanding of rewetting trajectories at timescales relevant to climate forcing effects (decadal, centennial), to properly assess the climate impact and timeframe of rewetting as a management strategy.We address this lack of longer-term observational timeseries from rewetted sites by developing a new process-based peatland model, building on data from different sites at varying stages post-rewetting. Combining modelling with a space-for-time substitution we can investigate how the CO2 and CH4 dynamics of a drained peatland respond to rewetting on decadal timescales and improve our mechanistic understanding of the interactions between peatland hydrology, vegetation and biogeochemistry. Ultimately, we aim to provide an estimate of how long it takes for a rewetted peatland to become climate neutral.We build the model as a combination of empirical and mechanistic relations featuring the main plant and soil microbial processes necessary to simulate CO2 and CH4 exchange, while aiming for a simpler design than the complex ecosystem models that are often hard to constrain and parameterise.Here, we present the overall conceptual model design and the first modelling results where we have used in situ CH4 fluxes in high temporal and spatial resolution from drained and rewetted peatland sites in Trysil, Norway, to parameterise and evaluate our model. The CH4 module simulates the main production, oxidation and transport processes controlling net CH4 fluxes and serves as a first model iteration to be incorporated in the full model simulating both CO2 and CH4 dynamics in rewetted peatlands over time.