The effects of peat thickness and water table depth on CO$_2$ and N$_2$O emissions from agricultural peatlands – a process-based modelling approach

Peatlands are critical carbon (C) reservoirs, storing over a fifth of the global soil organic C stock. However, some peatlands are drained and cultivated for agricultural use, which makes them a significant source of greenhouse gas (GHG) emissions. Managing water table depth (WTD) is considered a key operation for mitigating GHG emissions in cultivated peatlands. Modelling the impacts of water management would be a cost-efficient way of studying its large-scale effects, both in the present and in the future. Here, we used the process-based model LandscapeDNDC (LDNDC) to assess the relationships between WTD, peat layer thickness and the GHG exchange. We simulated a boreal agricultural peatland (NorPeat, Finland), which was cultivated with silage grass and barley during the study years 2019–2022. The site was monitored with an eddy covariance (EC) tower, and divided into six drainage blocks with distinct peat profiles, each equipped with sensors for continuous water table measurements. The model performance was evaluated on a daily and seasonal level using EC measurements of carbon dioxide (CO$_2$), nitrous oxide (N$_2$O) and water fluxes for the study years, alongside with satellite retrievals of the leaf area index and three-year data from block-specific dark chamber flux measurements of CO$_2$ and N$_2$O. ... mehrThe LDNDC model was found to be suitable for drained peatland simulations, although the performance was the highest when verified against measurements from shallow peat soils. Although the simulated N$_2$O fluxes were generally comparable with observations, their accuracy was not as high as it was for CO$_2$. To study the impact of WTD on GHG fluxes, we had three different scenarios in addition to the baseline runs with measured conditions; these scenarios had an average WTD of 50, 30 and 15 cm below the soil surface. The study results showed a clear relationship between CO$_2$ emissions and WTD (r=0.86 between exposed organic matter and net ecosystem carbon balance). GHG mitigation was achieved in all scenarios with increased water table; even in the most modest scenario, the annual reduction from the baseline was 5.8 t CO$_{2 e}$. ha$^{−1}$ in deep peat blocks and 2.5 t CO$_{2 e}$. ha$^{−1}$ in shallow peat blocks. CO$_2$ emissions were found to be more strongly affected than N$_2$O emissions. In the highest water table scenario, which resembled conditions close to paludiculture, the net ecosystem exchange of CO$_2$ was close to zero in most of the years, when fields were cultivated with forage grasses. The implications of raising the WTD were found to be insensitive to model parameters that control evapotranspiration or organic matter decomposition. These findings highlight that even moderate water management practices are valuable in order to mitigate GHG emissions in cultivated peatlands.