The full carbon balance of a rewetted cropland fen and a conservation-managed fen

On a global scale, the release of greenhouse gases (GHG) from peatland drainage and cultivation are believed to account for ∼5% of estimated anthropogenic GHG emissions. Drainage generally leads to peat subsidence and extensive soil loss, resulting in a diminishing store of soil carbon (C). This is a challenge for maintaining drainage-based agriculture, as such practices will eventually lead to the loss of organic soils that arable cultivation depends on. The conversion of croplands on peat to semi-natural grasslands, alongside raising water tables, is one possible way to reduce the loss of these valuable C stores. Here, we report the net ecosystem carbon balances (NECB) of two lowland peatlands in East Anglia, south-east UK. One site is a relic conservation-managed fen on deep peat, subject to active hydrological management to maintain water levels, and dominated by Cladium and Phragmites sedge and reed beds, whilst the other is a former cropland that has been converted to seasonally-inundated grazed grassland. Despite occasionally experiencing severe water table drawdown, the conservation-managed fen was a strong C sink of -104 g C m−2 yr−1. In contrast, the grassland was a C source of 133 g C m−2 yr−1, with gaseous carbon dioxide (CO2) emissions being the main loss pathway, due to low water tables exposing the soil profile in summer. At each site, ditch emissions of CO2 were moderately large (22 and 37 g C m−2 yr−1), whilst ditch methane (CH4) emissions (0.2 and 1.8 g C m−2 yr−1) made a negligible contribution to the NECB, but are important when considering the ecosystem GHG balance in terms of CO2 equivalents. Excluding dissolved inorganic carbon (DIC), fluvial C losses were 6 g C m−2 yr−1 for the conservation-managed fen and 12 g C m−2 yr−1 for the former cropland, and were dominated by dissolved organic carbon (DOC). The small fluvial C loss is the result of both sites being hydrologically isolated from the surrounding agricultural landscapes. Although the partially re-wetted cropland was still acting as a net C source, our estimates suggest that seasonal rewetting has reduced net annual C losses to ∼20% of their former cropland values. Maintaining high water tables year round would potentially further reduce C losses, and shallow inundation might allow the return of wetland species such as Phragmites and Typha, perhaps as floating rafts.