A New Method for In Situ Measurements of Oxygen Isotopologues of Soil Water and Carbon Dioxide with High Time Resolution

The oxygen isotope composition of atmospheric CO 2 (δ 18 O ac ) can be used to disentangle ecosystem component CO 2 fluxes, such as soil respiration and plant assimilation, because the δ 18 O composition of different water pools is transferred to CO 2 during isotopic equilibration. The oxygen isotope exchange between CO 2 and water in soils has been widely studied with theoretical models, but experimental data are scarce, albeit indispensable to characterization of the role of soils in determining δ 18 O ac . Here, we present a new methodology to monitor the δ 18 O of soil CO 2 (δ 18 O sc ) and of soil water (δ 18 O sw ) in situ at varying soil water content. Infrared laser spectroscopy was combined with gas‐permeable polypropylene (PP) tubing installed at different depths in a sand column. The permeable tubing did not lead to any isotopic fractionation and was suitable for combined δ 18 O sc and δ 18 O sw measurements. Soil water became gradually 18 O enriched from the top of the sand over several days. Measured and δ 18 O sc simulated with the model MuSICA indicated incomplete CO 2 –H 2 O isotopic equilibrium. Irrigation of the sand column with tapwater resulted in a temporary reset of δ 18 O sw along the soil column, while δ 18 O sc was only influenced when the enzyme carbonic anhydrase was added to the irrigation water. Our study demonstrates that δ 18 O sc and δ 18 O sw can now be monitored in situ and online with high time resolution with minimum disturbance. With this new tool at hand, research into the oxygen isotope exchange between soil water and CO 2 in natural soils has the potential to advance to a new stage and help to constrain the atmospheric CO 2 budget.