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

Abstract

Abstract. The oxygen isotope composition (δ18O) of atmospheric carbon dioxide (CO2) can be used to estimate gross primary production at the ecosystem-scale and above. Understanding how and why the rate of oxygen isotope exchange between soil water and CO2 (kiso) varies can help to reduce uncertainty in the retrieval of such estimates. The expression and activity of carbonic anhydrases in soils are important drivers of variations in kiso. Here we estimate kiso and measure associated soil properties in laboratory incubation experiments using 44 soils sampled from sites across western Eurasia and northeastern Australia. Observed kiso exceeded theoretical uncatalysed rates indicating the significant influence of carbonic anhydrases on the variability observed among the soils studied. We identify soil pH as the principal source of variation, with greater kiso 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 carbonic anhydrases. We also show for the first time in soils that the presence of nitrate under acidic conditions reduces kiso, potentially reflecting the direct or indirect inhibition of carbonic anhydrases. This effect was confirmed by a supplementary ammonium nitrate fertilisation experiment conducted on a subset of the soils. Future changes in atmospheric nitrogen deposition or land-use may thus influence carbonic anhydrase activity. Greater microbial biomass also increased kiso under a given set of chemical conditions likely highlighting the ubiquity of carbonic anhydrase expression by soil microbial communities. These data provide the most extensive analysis of spatial variations in soil kiso to date and indicate key controls required to predict variations in kiso at the scales needed to improve efforts to constrain gross primary productivity using the δ18O of atmospheric CO2.