The world's soils store more carbon than is present in biomass and in the atmosphere(1). Little is known, however, about the factors controlling the stability of soil organic carbon stocks(2-4) and the response of the soil carbon pool to climate change remains uncertain(5,6). We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6-0.8 m depth) stimulated the microbial mineralization of 2,567 +/- 226-year-old carbon. Our results support the previously suggested idea(7) that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile(1,8,9) could however stimulate the loss of ancient buried carbon.