Substantial Mitigation Potential for Greenhouse Gases Under High Water Levels in a Cultivated Peatland in the Arctic

Drained cultivated peatlands are recognized as substantial global carbon emission sources, prompting the exploration of water level elevation as a mitigation strategy. However, the efficacy of raised water table level (WTL) in Arctic/subarctic regions, characterized by continuous summer daylight, low temperatures and short growing seasons, remains poorly understood. This study presents a two-year field experiment conducted at a northernmost cultivated peatland site in Norway. We used sub-daily CO₂, CH₄, and N₂O fluxes measured by automatic chambers to assess the impact of WTL, fertilization, and biomass harvesting on greenhouse gas (GHG) budgets and carbon balance. Well-drained plots acted as GHG sources as substantial as those in temperate regions. Maintaining a WTL between -0.5 and -0.25 m effectively reduces CO₂ emissions, without significant CH₄ and N₂O emissions, and can even result in a net GHG sink. Elevated temperatures, however, were found to increase CO₂ emissions, potentially attenuating the benefits of water level elevation. Notably, high WTL resulted in a greater suppression of maximum photosynthetic CO₂ uptake compared to respiration, and, yet caused lower net CO₂ emissions due to a low light compensation point that lengthens the net CO₂ uptake periods. Furthermore, the long summer photoperiod in the Arctic also enhanced net CO₂ uptake and, thus, the efficacy of CO₂ mitigation. Fertilization primarily enhanced biomass production without substantially affecting CO₂ or CH₄ emissions. Conversely, biomass harvesting led to a significant carbon depletion, even at a high WTL, indicating a risk of land degradation. These results suggest that while elevated WTL can effectively mitigate GHG emissions from cultivated peatlands, careful management of WTL, fertilization, and harvesting is crucial to balance GHG reduction with sustained agricultural productivity and long-term carbon storage. The observed compatibility of GHG reduction and sustained grass productivity highlights the potential for future paludiculture implementation in the Arctic.