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The influence of soil carbonic anhydrase on the partitioning of gross CO2 fluxes using the oxygen isotopes of CO2 and water
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Edité par CCSD ; European Geosciences Union -
International audience. Measuring terrestrial gross CO$_2$ fluxes at large scales presents one of the main challenges in global carbon cycle research. The oxygen isotopic composition ($\delta^{18}$O) of atmospheric CO$_2$ offers the possibility to partition net CO$_2$ fluxes into photosynthesis and respiration at ecosystem, regional and global scales. This approach relies on a detailed knowledge of the $\delta^{18}$O signature of the terrestrial gross CO$_2$ fluxes. The latter reflects the $\delta^{18}$O of leaf and soil water because CO$_2$ exchanges isotopically with water. This exchange can be accelerated by the enzyme carbonic anhydrase (CA). The high CA content in leaves of plants amplifies the impact of leaf photosynthesis on the $\delta^{18}$O of atmospheric CO$_2$ (deltaa) by enhancing the equilibration of atmospheric CO$_2$ with isotopically enriched leaf water. Here, we report that the accelerated isotopic exchange between CO$_2$ and water due to CA activity may be a widespread phenomenon in soils as well. Across a range of ecosystems, we found that CO$_2$ hydration was 10 to 300 times faster than the uncatalysed rate, with highest values in the hottest ecosystems. At the global scale, accounting for soil CA activity dramatically shifts the influence of soil and leaf fluxes on $\delta_a$, thus changing the estimates of terrestrial gross CO$_2$ fluxes. At a time when new laser technologies are poised to deliver more extensive data coverage of variations in $\delta_a$, our finding indicates that deltaa signals should enable us to constrain CO$_2$ gross fluxes in regions where this information has been particularly difficult to obtain, such as in the tropics.
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