A recently developed isotopomer method 26, 27 enables reconstruction of metabolic C fluxes by analyzing cell wall carbohydrates from Sphagnum remnants. Previous attempts to estimate responses of Sphagnum to increased atmospheric CO 2 were either based on free-air CO 2 enrichment (FACE) or greenhouse experiments 20– 25. To our knowledge, responses of Sphagnum photosynthetic C fluxes to the recent increase in atmospheric CO 2 have never been explored on the global scale. Understanding how Sphagnum C fluxes respond to recent and projected increases in atmospheric CO 2 is therefore crucial for predicting future peat C fluxes. However, currently it is not clear whether increases in Sphagnum C accumulation driven by ongoing and projected global warming will outweigh increases in the rate of microbial peat decomposition 9, 18, 19. Therefore, C accumulation and storage in the form of Sphagnum remains generally exceeds C losses from microbial decay. Compared to vascular plants, Sphagnum remnants are highly resistant to microbial decay, which is vital for peat C accumulation 16, 17. Sphagnum peat mosses are primarily responsible for the accumulation of peat C because they often constitute 80–100% of the ground cover in northern peatlands 15. In addition to rising atmospheric CO 2 levels, ongoing climatic changes such as increases in temperature and changes in precipitation are hypothesized to influence peatland C fluxes 12– 14. Multiple observations indicate that recent increases in atmospheric CO 2 have affected peat C accumulation rates: (i) the variation in acrotelm peat accumulation was mainly driven by photosynthesis 9, (ii) peat C accumulation in Alaskan mires increased about threefold during the twentieth century 10, and (iii) the variation in net ecosystem exchange between mires was mainly controlled by differences in leaf area index 11. Since the beginning of the industrial revolution in the early nineteenth century, CO 2 concentrations have risen from ca. During the early and mid-Holocene, the accumulation of peat C was largely determined by the retreat of the northern ice sheet and the rise in temperature because atmospheric CO 2 concentrations were relatively stable at 275 ± 8 ppm (SD) 5– 7. Changes in climate are expected to have strong effects on peatland C sequestration 1, 3, 4. Over one third of global soil carbon (C) is stored in boreal mires 1, 2, making peat C accumulation an essential part of the global C budget. The global suppression of photorespiration in Sphagnum suggests an increased net primary production potential in response to the ongoing rise in atmospheric CO 2, in particular for mire structures with intermediate water table depths. Further, we showed that the photorespiration to photosynthesis ratio varied between Sphagnum subgenera, indicating differences in their photosynthetic capacity. ![]() By estimating the changes in water table depth, temperature, and precipitation during the twentieth century, we excluded potential effects of these climate parameters on the observed isotopomer responses. ![]() Rising CO 2 levels generally suppressed photorespiration relative to photosynthesis but the magnitude of suppression depended on the current water table depth. Here we investigate the response of the photorespiration to photosynthesis ratio in Sphagnum mosses to recent CO 2 increases by comparing deuterium isotopomers of historical and contemporary Sphagnum tissues collected from 36 peat cores from five continents. Net carbon assimilation is strongly reduced by photorespiration, a process that depends on the CO 2 to O 2 ratio. ![]() Atmospheric CO 2 levels have increased dramatically during the twentieth century, from 280 to > 400 ppm, which has affected plant carbon dynamics. Natural peatlands contribute significantly to global carbon sequestration and storage of biomass, most of which derives from Sphagnum peat mosses.
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