The impact of soil microorganisms on the global budget of δ 18 O in atmospheric CO 2

Abstract

Improved global estimates of terrestrial photosynthesis and respiration are critical for predicting the rate of change in atmospheric CO₂. The oxygen isotopic composition of atmospheric CO₂ can be used to estimate these fluxes because oxygen isotopic exchange between CO₂ and water creates distinct isotopic flux signatures. The enzyme carbonic anhydrase (CA) is known to accelerate this exchange in leaves, but the possibility of CA activity in soils is commonly neglected. Here, we report widespread accelerated soil CO₂ hydration. Exchange was 10–300 times faster than the uncatalyzed rate, consistent with typical population sizes for CA-containing soil microorganisms. Including accelerated soil hydration in global model simulations modifies contributions from soil and foliage to the global CO¹⁸O budget and eliminates persistent discrepancies existing between model and atmospheric observations. This enhanced soil hydration also increases the differences between the isotopic signatures of photosynthesis and respiration, particularly in the tropics, increasing the precision of CO₂ gross fluxes obtained by using the δ¹⁸O of atmospheric CO₂ by 50%.