Simplified 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 (N(2)O) and carbon dioxide (CO(2)) 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 flux underestimation arising from chamber deployment. The technique is based on previously established gas transport theory and has been simplified to facilitate application while preserving the fundamental physical relationships. The 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 flux underestimation (TFU) is determined, taking into account effects of chamber geometry and deployment time, the flux-calculation scheme, and properties of the soil and gas under consideration. Application to actual data and recent studies confirmed that TFU can vary widely within and across sites. The analysis also revealed a highly linear correlation between soil water content and TFU, suggesting that previously observed relationships between water content and trace gas flux may in part reflect artifacts of chamber methodology. The method described here provides a practical means of improving the absolute accuracy of flux estimates and normalizing data obtained using different chamber designs in different soils.