Effects of Soil Physical Non-Uniformity on Chamber-Based Gas Flux Estimates

Chamber methods for measuring trace gases fluxes are prone to errors resulting in large part from alteration of near-surface concentration gradients. There is little information available for estimating the magnitude of such errors that simultaneously accounts for soil physical properties, chamber deployment methods, and flux-calculation options. The current study employed numerical modeling to examine how variations in these factors influence flux-estimate errors. Errors varied among profiles and flux-estimation techniques, resulting in potentially important biases. A theoretically-based flux model which assumes physical uniformity performed relatively well in non-uniform soils provided that precautions were taken. Errors using the theoretical model for non-uniform soils were minimized with larger effective chamber heights (h) and shorter deployment times (DT), as was the case for all flux-models. Recent studies that have recommended minimizing h and extending DT in order to enhance non-linearity of chamber data need to be reevaluated in light of these findings. Site-specific selection of chamber and flux-calculation methods should consider the physical characteristics of the soil profile as well as measurement error. It is also shown here that random measurement error can result in skewed flux-estimate errors. Techniques presented here can be used to develop soil- and method-specific error estimates, provided that errors from other sources are minimized.