The oxygen isotope composition of atmospheric carbon dioxide (CO₂) is intimately linked to large-scale variations in the cycling of CO₂ and water across the Earth's surface. Understanding the role the biosphere plays in modifying the oxygen isotope composition of atmospheric CO₂ 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 CO₂ 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 CO₂ (k_iso) across a wide range of soil types and climatic zones. As the carbonic anhydrases (CAs) group of enzymes enhances the rate of CO₂ 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 north-eastern 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–extra-cellular 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 CO₂.

中文翻译：

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土壤中水和二氧化碳之间的氧同位素交换由 pH 值、硝酸盐和微生物生物量通过与碳酸酐酶活性的联系来控制

大气二氧化碳 ( CO ₂ )的氧同位素组成与地球表面CO ₂和水循环的大规模变化密切相关。了解生物圈在改变大气CO ₂氧同位素组成方面所起的作用 尤为重要，因为这种同位素示踪剂有可能限制对重要过程（例如大规模初级生产总量）的估计。然而，限制了CO ₂氧同位素组成的大气质量预算还要求我们更好地了解土壤群落的贡献以及它们如何影响土壤水和CO ₂ ( k _iso )之间的氧同位素交换速率，跨越范围广泛的土壤类型和气候带。由于碳酸酐酶 (CA) 酶组提高了土壤充满水的孔隙空间内CO ₂水合的速率 ，因此了解环境驱动因素如何通过其活动的变化影响k _iso是很重要的 。这里我们估计 k _iso并使用从欧亚大陆西部和澳大利亚东北部的地点取样的 44 种土壤在实验室孵化实验中测量相关的土壤特性。观察到的k _iso值总是超过理论推导的未催化速率，表明 CA 对所研究土壤中k _iso的变异性有显着影响。我们将土壤 pH 值确定为变化的主要来源，具有更大的k _iso在碱性条件下，表明微生物群落组成的变化或细胞外溶解的无机碳梯度会诱导更多或更高活性形式的 CA 的表达。我们还首次在土壤中表明，在自然酸性条件下硝酸盐的存在会降低k _iso，这可能反映了对 CA 的直接或间接抑制。这种效果似乎得到了在土壤子集上进行的补充硝酸铵施肥实验的支持。更大的微生物生物量也增加了 k _iso在一组给定的化学条件下，突出了 CA 表达与土壤微生物丰度之间的假定联系。这些数据提供了迄今为止对土壤k _iso空间变化最广泛的分析， 并指出了在大空间尺度上预测k _iso变化所需的关键土壤性状数据集，这是限制土壤群落在大气中的重要作用的必要下一步。CO ₂的氧同位素组成的质量预算。