Assessing soil carbonic anhydrase activity using CO₂ and COS exchange across tree–mycorrhizal associations

Carbonic anhydrases (CAs) are ubiquitous metalloenzymes found in plants and soil microbes. They play an important role in the cycling of carbon within terrestrial ecosystems by catalyzing the rapid conversion of CO₂ into bicarbonate. CAs also catalyse the irreversible hydrolysis of carbonyl sulfide (COS) and the oxygen isotope exchange between atmospheric CO₂ and terrestrial water pools (CO¹⁸O + H₂O <-> CO₂ + H₂¹⁸O). Soil CA activity and its drivers can therefore be studied by using gas exchange systems that measure soil-air fluxes of COS and CO¹⁸O.

Soil CA activity is commonly quantified using such gas exchange systems that allow the retrieval of macroscopic rates of COS hydrolysis (k_h) and ¹⁸O exchange between CO₂ and soil water (k_iso). These macroscopic rates can be related to soil properties such as soil moisture and temperature, as well as average enzyme kinetic parameters at the soil microbial community level. These ‘community-level’ enzymatic parameters (k_max/K_m) are expected to vary across contrasted ecosystems and soil microbial communities. However, microbial communities could also vary at a finer scale, between tree species, and particularly between different types of mycorrhizal symbioses, which reflect contrasting nutrient acquisition strategies and metabolic pathways.

In this study, we tested this hypothesis by measuring COS and CO¹⁸O fluxes in intact soil monoliths collected in a common garden under different tree species growing within the same climate and pedological context. This approach allowed us to characterize how soil CA activity (k_h and k_iso) vary between tree species and mycorrhizal types.

Contrary to our initial hypothesis, we found no difference between ‘community-level’ enzymatic parameters from different tree species or types of mycorrhizal associations. By measuring the monoliths at different soil temperature and soil moisture levels, we were also able to validate for the first time how the macroscopic rates kh and kiso can be related to these two abiotic factors, and estimate ‘community-level’ kmax/Km for this particular ecosystem.

This improved understanding of soil CA activity can help refine the representation of soil processes in large-scale models and better constrain the contribution of soils to the global CO₂ and COS mass balance.