The role of soil pH on soil carbonic anhydrase activity

Carbonic anhydrases (CAs) are metalloenzymes present in plants and microorganisms that catalyse the inter-conversion of CO2 and water to bicarbonate and protons. Because oxygen isotopes are also exchanged during this reaction, the presence of CA also modifies the contribution of soil and plant (COO)-O-18 fluxes to the global budget of atmospheric (COO)-O-18. The oxygen isotope signatures (delta O-18) of these fluxes differ as leaf water pools are usually more enriched than soil water pools, and this difference is used to partition the net CO2 flux over land into soil respiration and plant photosynthesis. Nonetheless, the use of atmospheric (COO)-O-18 as a tracer of land surface CO2 fluxes requires a good knowledge of soil CA activity. Previous studies have shown that significant differences in soil CA activity are found in different biomes and seasons, but our understanding of the environmental and ecological drivers responsible for the spatial and temporal patterns observed in soil CA activity is still limited. One factor that has been overlooked so far is pH. Soil pH is known to strongly influence microbial community composition, richness and diversity in addition to governing the speciation of CO2 between the different carbonate forms. In this study we investigated the CO2-H2O isotopic exchange rate (k(iso)) in six soils with pH varying from 4.5 to 8.5. We also artificially increased the soil CA concentration to test how pH and other soil properties (texture and phosphate content) affected the relationship between k(iso) and CA concentration. We found that soil pH was the primary driver of k(iso) after CA addition and that the chemical composition (i.e. phosphate content) played only a secondary role. We also found an offset between the delta O-18 of the water pool with which CO2 equilibrates and total soil water (i.e. water extracted by vacuum distillation) that varied with soil texture. The reasons for this offset are still unknown.