The oxygen isotope composition of atmospheric carbon dioxide (CO 2) is intimately linked to largescale variations in the cycling of CO 2 and water across the Earth's surface. Understanding the role the biosphere plays in modifying the oxygen isotope composition of atmospheric CO 2 is particularly important as this isotopic tracer has the potential to constrain estimates of important processes such as gross primary production at large scales. However, constraining the atmospheric mass budget for the oxygen isotope composition of CO 2 also requires that we understand better the contribution of soil communities and how they influence the rate of oxygen isotope exchange between soil water and CO 2 (k iso) across a wide range of soil types and climatic zones. As the carbonic anhydrases (CAs) group of enzymes enhances the rate of CO 2 hydration within the water-filled pore spaces of soils, it is important to develop understanding of how environmental drivers can impact k iso through changes in their activity. Here we estimate k iso and measure associated soil properties in laboratory incubation experiments using 44 soils sampled from sites across western Eurasia and northeastern Australia. Observed values for k iso always exceeded theoretically derived uncatalysed rates, indicating a significant influence of CAs on the variability of k iso across the soils studied. We identify soil pH as the principal source of variation, with greater k iso under alkaline conditions suggesting that shifts in microbial community composition or intra-extracellular dissolved inorganic carbon gradients induce the expression of more or higher activity forms of CAs. We also show for the first time in soils that the presence of nitrate under naturally acidic conditions reduces k iso , potentially reflecting a direct or indirect inhibition of CAs. This effect appears to be supported by a supplementary ammonium nitrate fertilisation experiment conducted on a subset of the soils. Greater microbial biomass also increased k iso under a given set of chemical conditions, highlighting a putative link between CA expression and the abundance of soil microbes. These data provide the most extensive analysis of spatial variations in soil k iso to date and indicate the key soil trait datasets required to predict variations in k iso at large spatial scales, a necessary next step to constrain the important role of soil communities in the atmospheric mass budget of the oxygen isotope composition of CO 2 .Read less <