Peatlands are an important natural store of carbon (C). Drainage of lowland peatlands for agriculture and the subsequent loss of anaerobic conditions had turned these C stores into major emitters of greenhouse gases (GHGs). Practical management strategies are needed to reduce these emissions, and ideally to reverse them to achieve net GHG removal (GGR). Here we show that a combination of enhanced C input as recalcitrant organic matter, CH4 suppression by addition of terminal electron acceptors, and suppression of decomposition by raising water levels has the potential to achieve GGR in agricultural peat. We measured GHG (CO2, N2O, and CH4) fluxes for 1 year with intensive sampling (6 times within the first 56 days) followed by monthly sampling in outdoor mesocosms with high (0 cm) and low (− 40 cm) water table treatments and five contrasting organic amendments (Miscanthus-derived biochar, Miscanthus chip, paper waste, biosolids, and barley straw) were applied to high water table cores, with and without iron sulphate (FeSO4). Biochar produced the strongest net soil C gain, suppressing both peat decomposition and CH4 emissions. No other organic amendment generated similar GGR, due to higher decomposition and CH4 production. FeSO4 application further suppressed CO2 and CH4 release following biochar addition. While we did not account for life-cycle emissions of biochar production, or its longer-term stability, our results suggest that biochar addition to re-wetted peatlands could be an effective climate mitigation strategy.