A new method for continuously measuring the δ¹³C of soil CO₂ concentrations at different depths by laser spectrometry
Parent, F. | Plain, C. | Epron, D. | Maier, M. | Longdoz, B.
Soil carbon dioxide (CO₂) efflux is an important component of the carbon (C) cycle but the biological and physical processes involved in soil CO₂ production and transport are not fully understood. To improve our knowledge, we present a new approach to measure simultaneously soil CO₂ concentrations and efflux, and their respective isotopic signatures (δ¹³C‐CO₂). To quantify soil air ¹³CO₂ and ¹²CO₂ concentrations, we adapted a method based on CO₂ 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 CO₂ and δ¹³C‐CO₂ vertical profiles were thus obtained at hourly intervals. Our tests demonstrated the absence of fractionation in the membrane tubes for δ¹³C‐CO₂ . Subsequently, we set up field experiments for two forest soils, which showed that natural soil CO₂ concentrations and δ¹³C‐CO₂ were not affected significantly by the measurement system. While δ¹³C‐CO₂ in air‐filled pores below 5 cm was constant over 3 days, we observed large diurnal variations in δ¹³C‐CO₂ 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 δ¹³C‐CO₂ profile of the soil atmosphere, and demonstrates that the fractionation that occurs during diffusion is the main transport process that affects the δ¹³C‐CO₂ of the soil CO₂ efflux on a daily timescale while advection may account for within‐day variations.
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