Atmospheric carbon dioxide enrichment (eCO 2) can enhance plant carbon uptake and growth 1,2,3,4,5 , thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO 2 concentration 6. While evidence gathered from young aggrading forests has generally indicated a strong CO 2 fertilization effect on biomass growth 3,4,5 , it is unclear whether mature forests respond to eCO 2 in a similar way. In mature trees and forest stands 7,8,9,10 , photosynthetic uptake has been found to increase under eCO 2 without any apparent accompanying growth response, leaving an open question about the fate of additional carbon fixed under eCO 2 4,5,7,8,9,10,11. Here, using data from the first ecosystemscale Free-Air CO 2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responds to four years of eCO 2 exposure. We show that, although the eCO 2 treatment of ambient +150 ppm (+38%) induced a 12% (+247 g C m-2 yr-1) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone accounting for ~50% of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO 2 , and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO 2 fertilization as a driver of increased carbon sinks in global forests. Main text Globally, forests act as a large carbon sink, absorbing a significant portion of the anthropogenic CO 2 emissions 1,12 , an ecosystem service that has tremendous social and
