Atmospheric carbon dioxide enrichment (eCO₂) 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 CO₂ concentration⁶. Although evidence gathered from young aggrading forests has generally indicated a strong CO₂ fertilization effect on biomass growth^3,4,5, it is unclear whether mature forests respond to eCO₂ in a similar way. In mature trees and forest stands^7,8,9,10, photosynthetic uptake has been found to increase under eCO₂ without any apparent accompanying growth response, leaving the fate of additional carbon fixed under eCO₂ unclear^{4,5,7,8,9,10,11}. Here using data from the first ecosystem-scale Free-Air CO₂ Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responded to four years of eCO₂ exposure. We show that, although the eCO₂ treatment of +150 parts per million (+38 per cent) above ambient levels induced a 12 per cent (+247 grams of carbon per square metre per year) 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 half 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 eCO₂, and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO₂ fertilization as a driver of increased carbon sinks in global forests.

大气中的二氧化碳富集 (eCO ₂ ) 可以促进植物对碳的吸收和生长^{1,2,3,4,5 ，从而通过减缓大气中 CO} ₂浓度的增加速度⁶为气候变化提供重要的负反馈。尽管从年轻的退化森林收集的证据普遍表明，CO ₂施肥对生物量生长有很强的影响^3,4,5，但尚不清楚成熟森林是否以类似的方式对 eCO _{2作出反应。}在成熟的树木和林分^{7,8,9,10中，发现在 eCO} ₂下光合吸收增加没有任何明显的伴随生长反应，在 eCO ₂下固定的额外碳的命运不明确^{4,5,7,8,9,10,11}。在这里，我们使用来自成熟森林中第一个生态系统规模的自由空气 CO ₂富集 (FACE) 实验的数据，构建了一个全面的生态系统碳预算，以跟踪森林对四年 eCO ₂暴露的反应时碳的命运。我们表明，虽然 eCO ₂高于环境水平 150 ppm（+38%）的处理导致通过初级生产总量的碳吸收增加 12%（每平方米每年增加 247 克碳），这种额外的碳吸收并没有导致增加生态系统层面的碳固存。相反，大部分额外的碳通过几个呼吸通量排放回大气，仅增加土壤呼吸就占总吸收盈余的一半。_{我们的研究结果质疑了在eCO 2}下森林作为碳汇的能力将普遍增强的主流观点，并对依赖无处不在的CO _{2的气候减缓战略的有效性提出了挑战。} 施肥是全球森林碳汇增加的驱动因素。