Advances in Soil CO2 Flux Measurement.

This paper will discuss measurements of trace gas flux from the soil using chamber methods.  The focus will be on CO₂ fluxes but many of the principles also apply to other gases.  Measuring soil gas fluxes with chambers looks simple but is sensitive to chamber design and measurement techniques.  Diffusion from the porous soil is sensitive to gas concentration in the chamber headspace and to small pressure fluctuations due to wind.  Increases in chamber CO₂ during a measurement reduce the gradient between chamber and soil atmospheres suppressing the flux and often leading to non-linear plots of CO₂ concentration with time.  Extent of this suppression depends upon soil porosity, which partly explains why plots of CO₂ concentration with time are sometimes linear and sometimes not.  Elevated chamber CO₂ can also cause elevated CO₂ concentration in the soil atmosphere beneath the chamber, which can affect subsequent flux measurements.  These effects will be discussed as they relate to chamber design, data analysis methods, and soil porosity.  Evidence will be presented showing similar effects in nature.  Elevated CO₂ concentrations in the atmosphere on calm nights can reduce instantaneous fluxes at night and cause changes in soil CO₂ storage that affect fluxes the following day.