NUMERICAL EVALUATION OF STATIC-CHAMBER MEASUREMENTS OF SOIL-ATMOSPHERE GAS-EXCHANGE - IDENTIFICATION OF PHYSICAL PROCESSES

The exchange of gases between soil and atmosphere is an important proc ess that affects atmospheric chemistry and therefore climate. The stat ic-chamber method is the most commonly used technique for estimating t he rate of that exchange. We examined the method under hypothetical fi eld conditions where diffusion was the only mechanism for gas transpor t and the atmosphere outside the chamber was maintained at a fixed con centration. Analytical and numerical solutions to the soil gas diffusi on equation in one and three dimensions demonstrated that gas flux den sity to a static chamber deployed on the soil surface was less in magn itude than the ambient exchange rate in the absence of the chamber. Th is discrepancy, which increased with chamber deployment time and air-f illed porosity of soil, is attributed to two physical factors: distort ion of the soil gas concentration gradient (the magnitude was decrease d in the vertical component and increased in the radial component) and the slow transport rate of diffusion relative to mixing within the ch amber. Instantaneous flux density to a chamber decreased continuously with time; steepest decreases occurred so quickly following deployment and in response to such slight changes in mean chamber headspace conc entration that they would likely go undetected by most field procedure s. Adverse influences of these factors were reduced by mixing the cham ber headspace, minimizing deployment time, maximizing the height and r adius of the chamber, and pushing the rim of the chamber into the soil . Nonlinear models were superior to a linear regression model for esti mating flux densities from mean headspace concentrations, suggesting t hat linearity of headspace concentration with time was not necessarily a good indicator of measurement accuracy.