A new method for continuously measuring the δ13C 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 (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 5cm was constant over 3days, 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. © 2013 The Authors. Journal compilation © 2013 British Society of Soil Science.