Simplifi ed Method for Quantifying Theoretical Underestimation of Chamber-Based Trace Gas Fluxes

Closed chambers used to measure soil-atmosphere exchange of trace gases including nitrous oxide (N2O) and carbon dioxide (CO2) generate errors due to suppression of the gas concentration gradient at the soil-atmosphere interface. A method is described here for estimating the magnitude of fl ux underestimation arising from chamber deployment. Th e technique is based on previously established gas transport theory and has been simplifi ed to facilitate application while preserving the fundamental physical relationships. Th e method avoids the use of nonlinear regression but requires knowledge of soil properties including texture, bulk density, water content, temperature, and pH. Two options are presented: a numerical technique which is easily adapted to spreadsheet application, and a graphical method requiring minimal calculation. In both cases, the magnitude of theoretical fl ux underestimation (TFU) is determined, taking into account eff ects of chamber geometry and deployment time, the fl ux-calculation scheme, and properties of the soil and gas under consideration. Application to actual data and recent studies confi rmed that TFU can vary widely within and across sites. Th e analysis also revealed a highly linear correlation between soil water content and TFU, suggesting that previously observed relationships between water content and trace gas fl ux may in part refl ect artifacts of chamber methodology. Th e method described here provides a practical means of improving the absolute accuracy of fl ux estimates and normalizing data obtained using diff erent chamber designs in diff erent soils