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Terrestrial ecosystems remove about 30 per cent of the carbon dioxide (CO.sub.2) emitted by human activities each year.sup.1, yet the persistence of this carbon sink depends partly on how plant biomass and soil organic carbon (SOC) stocks respond to future increases in atmospheric CO.sub.2 (refs. .sup.2,3). Although plant biomass often increases in elevated CO.sub.2 (eCO.sub.2) experiments.sup.4-6, SOC has been observed to increase, remain unchanged or even decline.sup.7. The mechanisms that drive this variation across experiments remain poorly understood, creating uncertainty in climate projections.sup.8,9. Here we synthesized data from 108 eCO.sub.2 experiments and found that the effect of eCO.sub.2 on SOC stocks is best explained by a negative relationship with plant biomass: when plant biomass is strongly stimulated by eCO.sub.2, SOC storage declines; conversely, when biomass is weakly stimulated, SOC storage increases. This trade-off appears to be related to plant nutrient acquisition, in which plants increase their biomass by mining the soil for nutrients, which decreases SOC storage. We found that, overall, SOC stocks increase with eCO.sub.2 in grasslands (8 [plus or minus] 2 per cent) but not in forests (0 [plus or minus] 2 per cent), even though plant biomass in grasslands increase less (9 [plus or minus] 3 per cent) than in forests (23 [plus or minus] 2 per cent). Ecosystem models do not reproduce this trade-off, which implies that projections of SOC may need to be revised. A synthesis of elevated carbon dioxide experiments reveals that when plant biomass is strongly stimulated by elevated carbon dioxide levels, soil carbon storage declines, and where biomass is weakly stimulated, soil carbon accumulates., Author(s): C. Terrer [sup.1] [sup.2] , R. P. Phillips [sup.3] , B. A. Hungate [sup.4] [sup.5] , J. Rosende [sup.6] , J. Pett-Ridge [sup.1] , M. E. Craig [sup.3] [sup.7] [...]

The global terrestrial carbon sink is increasing 1-3 , offsetting roughly a third of anthropogenic CO 2 released into the atmosphere each decade 1 , and thus serving to slow 4 the growth of atmospheric CO 2 . It has been suggested that a CO 2 -induced long-term increase in global photosynthesis, a process known as CO 2 fertilization, is responsible for a large proportion of the current terrestrial carbon sink 4-7 . The estimated magnitude of the historic increase in photosynthesis as result of increasing atmospheric CO 2 concentrations, however, differs by an order of magnitude between long-term proxies and terrestrial biosphere models 7-13 . Here we quantify the historic effect of CO 2 on global photosynthesis by identifying an emergent constraint 14-16 that combines terrestrial biosphere models with global carbon budget estimates. Our analysis suggests that CO 2 fertilization increased global annual photosynthesis by 11.85 ± 1.4%, or 13.98 ± 1.63 petagrams carbon (mean ± 95% confidence interval) between 1981 and 2020. Our results help resolve conflicting estimates of the historic sensitivity of global photosynthesis to CO 2 , and highlight the large impact anthropogenic emissions have had on ecosystems worldwide., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)