A new method for continuously measuring the 13C of soil CO2 concentrations at different depths by laser spectrometry

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 (C-13-CO2). To quantify soil air (CO2)-C-13 and (CO2)-C-12 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 C-13-CO2 vertical profiles were thus obtained at hourly intervals. Our tests demonstrated the absence of fractionation in the membrane tubes for C-13-CO2. Subsequently, we set up field experiments for two forest soils, which showed that natural soil CO2 concentrations and C-13-CO2 were not affected significantly by the measurement system. While C-13-CO2 in air-filled pores below 5cm was constant over 3days, we observed large diurnal variations in C-13-CO2 efflux. However, the average difference between the two measurements was close to -4.4 parts per thousand, which supports steady-state diffusion over this 3-day period. This new method seems to be a very effective way to measure the C-13-CO2 profile of the soil atmosphere, and demonstrates that the fractionation that occurs during diffusion is the main transport process that affects the C-13-CO2 of the soil CO2 efflux on a daily timescale while advection may account for within-day variations.