A laboratory comparison of two methods used to estimate the isotopic composition of soil delta13CO2 efflux at steady state.

The stable isotopic composition of soil (13)CO(2) flux is important for monitoring soil biological and physical processes. While several methods exist to measure the isotopic composition of soil flux, we do not know how effective each method is at achieving this goal. To provide clear evidence of the accuracy of current measurement techniques we created a column filled with quartz sand through which a gas of known isotopic composition (-34.2 per thousand) and concentration (3,000 ppm) diffused for 7 h. We used a static chamber at equilibrium and a soil probe technique to test whether they could identify the isotopic signature of the known gas source. The static chamber is designed to identify the source gas isotopic composition when in equilibrium with the soil gas, and the soil probe method relies on a mixing model of samples withdrawn from three gas wells at different depths to identify the gas source. We sampled from ports installed along the side of the sand column to describe the isotopic and concentration gradient as well as to serve as a control for the soil probe. The soil probe produced similar isotopic and concentration values as the control ports, as well as Keeling intercepts. The static chamber at equilibrium did not identify the source gas but, when applied in a two end-member mixing model, did produce a similar Keeling intercept produced from the control ports. Neither of the methods was able to identify the source gas via the Keeling plot method probably because CO(2) profiles did not reach isotopic steady state. Our results showed that the static chamber at equilibrium should be used only with a Keeling plot approach and that the soil probe is able to provide estimates of uncertainty for the isotopic composition of soil gas as well as information pertinent to the soil profile.