Diurnally variable δ¹⁸O signatures of soil CO₂ fluxes indicate carbonic anhydrase activity in a forest soil

[1] Oxygen isotopes are valuable tools for studying the gas exchange between terrestrial ecosystems and the atmosphere. We determined the δ¹⁸O signatures of soil CO₂ fluxes from soil chamber measurements over the diurnal cycle in September 2000, May 2001 and July 2001 in a Sitka spruce plantation in Scotland. Concurrent estimates of the δ¹⁸O composition of soil water were obtained from soil samples collected in the vicinity of the chambers. The observed δ¹⁸O signatures of net soil CO₂ fluxes were diurnally variable and strongly depleted compared to those expected from a simple evasion of respired CO₂ at isotopic equilibrium with soil water. We then simulated the δ¹⁸O signatures of soil CO₂ fluxes using a model of soil gas exchange that includes atmospheric invasion of CO₂ with concurrent isotopic equilibration with soil water and evasion of the equilibrated CO₂. This brought the modeled δ¹⁸O signatures closer to the observations, but complete agreement was only achieved when acceleration of isotopic exchange between CO₂ and soil water by carbonic anhydrase activity was included. We hypothesize that carbonic anhydrase is present in the litter or surface soil layers. This introduces a feedback that can result in diurnally variable δ¹⁸O signatures of net soil CO₂ fluxes. Such effects can only be captured in models that have an explicit description of the canopy air space with a variable δ¹⁸O signature of CO₂.