Occurrence of a nearly constant air flux through the accumulation chamber and determination of the two components of the CO2 flux from soil. I: Laboratory experiments.

Different types of laboratory experiments were carried out during this study. In type A experiments a standard gas mixture is continuously injected, at constant flux, into the accumulation chamber, mimicking the soil CO2 flux measurements performed in field surveys. In type B experiments, a standard gas mixture is initially injected into the accumulation chamber for a short lapse of time, to achieve a relatively high CO2 concentration inside the accumulation chamber; then the injection of the standard gas mixture is stopped and the CO2 concentration inside the chamber is monitored for a sufficient interval of time. In both types of experiments, the accumulation chamber appears to be flushed by a considerable flux of atmospheric air, which is virtually constant in each experiment but is different from experiment to experiment.The occurrence of this air flux through the accumulation chamber (i) has no effect on the determination of the soil CO2 flux on the basis of the initial slope (at time zero) of the CO2 concentration-time curve, but (ii) it complicates the evaluation of the two components of the soil CO2 flux, namely the CO2 molar fraction of soil gas and the flux of the soil gas mixture.A method to obtain both the CO2 molar fraction of soil gas and the flux of the soil gas mixture is presented and the implications related to the knowledge of the two components of the soil CO2 flux are discussed