The largest terrestrial-to-atmosphere carbon flux is respired CO₂. However, the partitioning of soil and plant sources, understanding of contributory mechanisms, and their response to climate change are uncertain. A plant removal experiment was established within a peatland located in the UK uplands to quantify respiration derived from recently-fixed plant carbon and that derived from decomposition of soil organic matter, using natural abundance ¹³C and bomb-¹⁴C as tracers. Soil and plant respiration sources were found respectively to contribute ~ 36% and between 41 and 54% of the total ecosystem CO₂ flux. Respired CO₂ produced in the clipped ('soil') plots had a mean age of ~ 15 years since fixation from the atmosphere, whereas the ¹⁴C content of ecosystem CO₂ was statistically indistinguishable from the contemporary atmosphere. Results of carbon mass balance modelling showed that, in addition to respiration from bulk soil and plant respired CO₂, a third, much older source of CO₂ existed. This source, which we suggest is CO₂ derived from the catotelm constituted between ~ 10 and 23% of total ecosystem respiration and had a mean radiocarbon age of between several hundred and ~ 2000 years before present (BP). These findings show that plant-mediated transport of CO₂ produced in the catotelm may form a considerable component of peatland ecosystem respiration. The implication of this discovery is that current assumptions in terrestrial carbon models need to be re-evaluated to consider the climate sensitivity of this third source of peatland CO₂.