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. The oxygen isotope composition of atmospheric carbon dioxide(CO2) is intimately linked to large-scale variations in the cyclingof CO2 and water across the Earth's surface. Understanding the rolethe biosphere plays in modifying the oxygen isotope composition of atmosphericCO2 is particularly important as this isotopic tracer has thepotential to constrain estimates of important processes such as gross primaryproduction at large scales. However, constraining the atmospheric mass budget for the oxygen isotope composition of CO2 also requires that weunderstand better the contribution of soil communities and how they influencethe rate of oxygen isotope exchange between soil water and CO2(kiso) across a wide range of soil types and climatic zones. Asthe carbonic anhydrases (CAs) group of enzymes enhances the rate ofCO2 hydration within the water-filled pore spaces of soils, it is important to develop understanding of how environmental drivers can impactkiso through changes in their activity. Here we estimatekiso and measure associated soil properties in laboratoryincubation experiments using 44 soils sampled from sites across westernEurasia and north-eastern Australia. Observed values for kiso always exceeded theoretically derived uncatalysed rates, indicating a significant influence of CAs on the variability of kiso across thesoils studied. We identify soil pH as the principal source of variation, withgreater kiso under alkaline conditions suggesting that shifts inmicrobial community composition or intra–extra-cellular dissolved inorganic carbon gradients induce the expression of more or higher activity forms ofCAs. We also show for the first time in soils that the presence of nitrateunder naturally acidic conditions reduces kiso, potentiallyreflecting a direct or indirect inhibition of CAs. This effect appears to besupported by a supplementary ammonium nitrate fertilisation experimentconducted on a subset of the soils. Greater microbial biomass also increasedkiso under a given set of chemical conditions, highlighting a putative link between CA expression and the abundance of soil microbes. Thesedata provide the most extensive analysis of spatial variations in soilkiso to date and indicate the key soil trait datasets required topredict variations in kiso at large spatial scales, a necessarynext step to constrain the important role of soil communities in theatmospheric mass budget of the oxygen isotope composition of CO2.

soil

Oxygen isotope exchange between water and carbon dioxide in soils is controlled by pH, nitrate and microbial biomass through links to carbonic anhydrase activity