Long-term biogeochemical consequences of rewetting iron and sulphur-rich peatlands

Peatlands host the largest store of terrestrial carbon on Earth and it is widely accepted that reversal of their widespread degradation is required to meet emissions targets. Thus, significant action is underway globally to encourage their rewetting and restoration. However, restoration success can be complicated by geological factors in the local environment. Peatlands in regions with iron sulphide-rich rocks and sediments experience drastic drops in pH following drainage and release high concentrations of iron and toxic metals. Accumulation of iron and sulphur in the peat during this time will fundamentally alter biogeochemical cycling, yet we have little understanding of the extent to which these effects can be reversed following the raising of water tables. Furthermore, the long-term impacts on resident microbial communities responsible for dictating the nature and scale of green-house gas emissions from such sites is unknown.

We have compared two neighbouring fens in southern England underlain by glauconite- and pyrite-rich sandstone which are within the same hydrological regime but have experienced differing degrees of historical drainage and degradation. Both fens were designated for conservation and rewetted in the 1970s. Porewater nutrient and greenhouse gas profiles, peat geochemistry, mineralogy and microbial community analyses collectively suggest lasting differences in redox state and element cycling between the two areas. Wolferton Fen, which experienced less historical land disturbance, had returned to a near-natural state in 2022. However, Dersingham Fen, which was historically deeply drained and experienced significant peat loss, had a low pH, thick crusts of iron (oxyhydr)oxides remaining on the surface, and very high porewater iron and sulphate concentrations. High abundances of these alternative terminal electron acceptors inhibit methanogens in Dersingham Fen, which continues to be a source of CO₂ despite anoxia.

These results suggest that iron and sulphur-rich peatlands can tolerate some degree of degradation, but extensive drainage and peat loss will likely lead to permanent contamination which remains following rewetting. However, there may be a lot to gain from restoration of such sites as rewetting can protect remaining peat and reduce CO₂ emissions whilst methane production would remain low.