The effect of soil moisture, soil particle size, litter layer and carbonic anhydrase on the oxygen isotopic composition of soil‐released CO 2

Summary Soil respiration and photosynthesis are the two largest carbon dioxide ( CO 2 ) fluxes between terrestrial ecosystems and the atmosphere and, therefore, the dominant processes influencing the oxygen isotopic composition of atmospheric CO 2 . The characterization of temporal and spatial variations of plant and soil‐related fluxes of different oxygen isotopologues of CO 2 ( 12 C 16 O 2 ; 12 C 16 O 18 O ) is relevant to constraining the global carbon budget. The oxygen isotopic composition of soil‐respired CO 2 is controlled by its release rate, the degree of isotopic equilibrium with soil water and the diffusional transport of CO 2 . The hypothesis of this study was that, as well as soil moisture, the soil particle size, the presence of an organic litter layer and the enzyme carbonic anhydrase ( CA ) would have a significant impact on the oxygen isotopic composition of soil‐released CO 2 . We tested this hypothesis with soil microcosm experiments on columns of medium and fine sand. Soil water content and soil texture influenced the isotopic composition of soil‐released CO 2 significantly. A litter layer had a significant effect on the isotopic composition of water vapour but not on CO 2 released from soil. In the absence of CA , oxygen isotope equilibration between the CO 2 invasion flux and soil water was insignificant, whereas in the presence of CA about 55% of the CO 2 invading the soil exchanged oxygen isotopes with soil water. Our findings highlight the importance of small‐scale variability of soil attributes for the oxygen isotopic composition of soil‐released CO 2 as well as the strong impact of CA activity in soils.