For millennia, peatlands have served as an important sink for atmospheric CO(2) and today represent a large soil carbon reservoir. While recent land use and wildfires have reduced carbon sequestration in tropical peatlands, the influence of disturbance on boreal peatlands is uncertain, yet it is important for predicting the fate of northern high-latitude carbon reserves. Here we quantify rates of organic matter storage and combustion losses in a boreal peatland subjected to long-term experimental drainage, a portion of which subsequently burned during a wildfire. We show that drainage doubled rates of organic matter accumulation in the soils of unburned plots. However, drainage also increased carbon losses during wildfire ninefold to 16.8±0.2 kg C m(-2), equivalent to a loss of more than 450 years of peat accumulation. Interactions between peatland drainage and fire are likely to cause long-term carbon emissions to far exceed rates of carbon uptake, diminishing the northern peatland carbon sink.