A new method for continuously measuring the δ ¹³ C of soil CO ₂ concentrations at different depths by laser spectrometry

Summary Soil carbon dioxide ( CO 2 ) efflux is an important component of the carbon ( C ) cycle but the biological and physical processes involved in soil CO 2 production and transport are not fully understood. To improve our knowledge, we present a new approach to measure simultaneously soil CO 2 concentrations and efflux, and their respective isotopic signatures ( δ 13 C‐CO 2 ). To quantify soil air 13 CO 2 and 12 CO 2 concentrations, we adapted a method based on CO 2 diffusion from soil pores into tubes with a highly gas‐permeable membrane wall. These tubes were placed horizontally at different depths in the soil. Air was sampled automatically from the tubes and injected through a diluting system into a tuneable diode laser absorption spectrometer. The CO 2 and δ 13 C‐CO 2 vertical profiles were thus obtained at hourly intervals. Our tests demonstrated the absence of fractionation in the membrane tubes for δ 13 C‐CO 2 . Subsequently, we set up field experiments for two forest soils, which showed that natural soil CO 2 concentrations and δ 13 C‐CO 2 were not affected significantly by the measurement system. While δ 13 C‐CO 2 in air‐filled pores below 5 cm was constant over 3 days, we observed large diurnal variations in δ 13 C‐CO 2 efflux. However, the average difference between the two measurements was close to −4.4‰, which supports steady‐state diffusion over this 3‐day period. This new method seems to be a very effective way to measure the δ 13 C‐CO 2 profile of the soil atmosphere, and demonstrates that the fractionation that occurs during diffusion is the main transport process that affects the δ 13 C‐CO 2 of the soil CO 2 efflux on a daily timescale while advection may account for within‐day variations.