Evaporation and carbonic anhydrase activity recorded in oxygen isotope signatures of net CO 2 fluxes from a Mediterranean soil

Abstract

The oxygen stable isotope composition ( δ 18 O) of CO 2 is a valuable tool for studying the gas exchange between terrestrial ecosystems and the atmosphere. In the soil, it records the isotopic signal of water pools subjected to precipitation and evaporation events. The δ 18 O of the surface soil net CO 2 flux is dominated by the physical processes of diffusion of CO 2 into and out of the soil and the chemical reactions during CO 2 –H 2 O equilibration. Catalytic reactions by the enzyme carbonic anhydrase, reducing CO 2 hydration times, have been proposed recently to explain field observations of the δ 18 O signatures of net soil CO 2 fluxes. How important these catalytic reactions are for accurately predicting large‐scale biosphere fluxes and partitioning net ecosystem fluxes is currently uncertain because of the lack of field data. In this study, we determined the δ 18 O signatures of net soil CO 2 fluxes from soil chamber measurements in a Mediterranean forest. Over the 3 days of measurements, the observed δ 18 O signatures of net soil CO 2 fluxes became progressively enriched with a well‐characterized diurnal cycle. Model simulations indicated that the δ 18 O signatures recorded the interplay of two effects: (1) progressive enrichment of water in the upper soil by evaporation, and (2) catalytic acceleration of the isotopic exchange between CO 2 and soil water, amplifying the contributions of ‘atmospheric invasion’ to net signatures. We conclude that there is a need for better understanding of the role of enzymatic reactions, and hence soil biology, in determining the contributions of soil fluxes to oxygen isotope signals in atmospheric CO 2 .

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