Water quality dynamics following re-wetting of agriculturally drained peatlands: a mesocosm experiment

Peatland rewetting is an effective restoration strategy to reduce carbon losses, halt soil subsidence and restore ecosystem functioning. However, rewetting drained and degraded peatlands may release stored nutrients and potentially toxic elements into surface waters, raising concerns about eutrophication and metal mobilisation. Conversely, rewetting might have positive effects, whereby contaminants in source waters are retained within the peat itself. To assess these effects, we conducted a mesocosm experiment using peat cores from five agricultural lowland peatlands (raised bogs and fens) across England. The cores were subjected to three hydrological regimes: drained (control), raised (rewetted) and raised but fluctuating water table conditions. We monitored water quality determinands, including organic and inorganic nitrogen and phosphorus, potassium, sulphate, calcium, magnesium, sodium, chloride and fluoride, in the mesocosm surface water prior to treatment initiation and in mesocosm pore water (as an indicator of mobility) over nine weeks. Overall, pore water chemistry remained relatively consistent over the duration of the experiment. There were no site-wide effects of the water table treatments, except on nitrate concentrations, which were highest under drained conditions. Responses for individual determinands varied among sites, likely due to differences in peat type (bog vs fen), peat chemistry and land use history. Our results suggest that rewetting is unlikely to trigger widespread downstream water quality deterioration, at least not for the determinands we measured, though the effects vary by site and solute. These findings highlight the importance of site-specific assessments, particularly if nitrogen, prior to rewetting.