Carbon balance and emissions of methane and nitrous oxide during four years of moderate rewetting of a cultivated peat soil site

Abstract. We experimented a gradual water table rise at a highly degraded agricultural peat soil site with plots of willow, forage and mixed vegetation (set-aside) in southern Finland. We measured the emissions of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) for four years. The mean annual ground water table depth was about 80, 40, 40 and 30 cm in 2019–2022, respectively. The results indicated that a 10 cm raise in the water table depth was able to slow down annual CO₂ emissions from soil respiration by 0.87 Mg CO₂-C ha^-1. CH₄ fluxes changed from uptake to emissions with a raise in the water table depth, and the maximum mean annual emission rate was 11 kg CH₄-C. Nitrous oxide emissions ranged from 2 to 33 kg N₂O-N ha^-1 year; they were high from bare soil in the beginning of the experiment but decreased towards the end of the experiment. Short rotation cropping of willow reached net sequestration of carbon before harvest, but all treatments and years showed net loss of carbon based on the net ecosystem carbon balance. Overall, the short rotation coppice of willow had the most favourable carbon and greenhouse gas balance over the years (10 Mg CO₂ eq. on the average over four years). The total greenhouse gas balance of the forage and set-aside treatments did not go under 27 Mg CO₂ eq. ha^-1 year^-1 highlighting the challenge in curbing peat decomposition in highly degraded cultivated peatlands.