Decomposition 'hotspots' in a rewetted peatland: implications for water quality and carbon cycling

Restoration of drained peatlands has been promoted to reduce gaseous and aquatic carbon losses; however, there are conflicting reports as to its effectiveness. Here we report ‘‘hotspots’’ of organic matter decomposition as a result of rewetting a drained peatland in Wales, at the field-scale, in the medium/long-term with implications for water quality and greenhouse gas emissions. Low soil moisture levels, that characterise these hotspots before rewetting, regenerate electron acceptors and provide carbon and nutrients which stimulate phenol oxidase-mediated release of phenolic compounds from the peat matrix upon waterlogging. Electron acceptors are then consumed sequentially, eventually favouring CH4 production and rising pH, despite accumulating SO4 levels. The latter two processes promote positive feedback to increased phenol oxidase activities and the release of even more dissolved organic carbon (DOC) and CH4 from the peat matrix. Hotspot formation therefore represents an inextricably linked physico-chemical and biological positive feedback mechanism. Such hotspots account for a large proportion of the mean increase in carbon loss due to rewetting of this naturally drained peatland (e.g. at maximum mean DOC concentrations: with hotspot 997%; without hotspot 102%) and are not ‘‘outliers’’ but important drivers of biogeochemical fluxes that should be included in budgets for carbon and other elements (e.g. sulphur). As such, understanding hotspot formation should allow improved management strategies for restoration, carbon stocks, drinking water quality and even future geo-engineering options in the face of changes in climate and atmospheric chemistry.