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 toaccount for ∼5% of estimated anthropogenic GHG emissions. Drainage generally leads to peat subsidence andextensive soil loss, resulting in a diminishing store of soil carbon (C). This is a challenge for maintaining drai-nage-based agriculture, as such practices will eventually lead to the loss of organic soils that arable cultivationdepends on. The conversion of croplands on peat to semi-natural grasslands, alongside raising water tables, isone 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 ondeep peat, subject to active hydrological management to maintain water levels, and dominated by Cladium andPhragmites 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−2yr−1. In contrast, the grassland was a C source of 133 g C m−2yr−1, with gaseous carbon dioxide (CO2) emissions being the main loss pathway, due to low water tables ex-posing the soil profile in summer. At each site, ditch emissions of CO2were moderately large (22 and 37 g C m−2yr−1), whilst ditch methane (CH4) emissions (0.2 and 1.8 g C m−2yr−1) made a negligible contribution to theNECB, but are important when considering the ecosystem GHG balance in terms of CO2equivalents. Excludingdissolved inorganic carbon (DIC), fluvial C losses were 6 g C m−2yr−1for the conservation-managed fen and12 g C m−2yr−1for the former cropland, and were dominated by dissolved organic carbon (DOC). The smallfluvial C loss is the result of both sites being hydrologically isolated from the surrounding agricultural land-scapes. Although the partially re-wetted cropland was still acting as a net C source, our estimates suggest thatseasonal rewetting has reduced net annual C losses to ∼20% of their former cropland values. Maintaining highwater tables year round would potentially further reduce C losses, and shallow inundation might allow thereturn of wetland species such as Phragmites and Typha, perhaps as floating rafts