Author
|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 9/4/2009 Publication Date: 1/1/2010 Citation: Venterea, R.T. 2010. Simplified Method for Quantifying Theoretical Underestimation of Chamber-Based Trace Gas Fluxes. Journal of Environmental Quality. 39(1):126-135. Interpretive Summary: Closed chambers placed on the soil surface is the most common method for determining the rate of exchange of trace gases including nitrous oxide (N2O) and carbon dioxide (CO2). It is widely recognized that this method generates substantial errors in flux estimates due to the so-called “chamber effect”. However, to date there have been no easily applicable techniques to quantify the magnitude of flux-underestimation arising from this effect. The method described here is based on previously developed theory and has been simplified to facilitate application. Two options are presented: a graphical method requiring minimal calculation, and a more precise numerical technique which is easily adapted to spreadsheet application. In both cases, the theoretical magnitude of flux underestimation is determined, taking into account the effects of chamber geometry and deployment time, properties of the soil and gas under consideration (N2O or CO2), and the particular flux-calculation scheme that is used. The method will be useful to scientists for designing chamber measurement protocols and interpreting chamber-based gas flux data. The techniques described here can also be useful to modelers and policy makers engaged in synthesizing data from diverse studies in order to normalize the data to account for different chamber methodologies applied under differing soil conditions. Technical Abstract: The vast majority of studies examining soil-atmosphere exchange of nitrous oxide (N2O) have utilized chambers placed on the soil surface. Chambers are also used to measure soil respiration and carbon dioxide (CO2) exchange. It is widely recognized that chambers generate substantial errors in flux estimates due to suppression of the gas concentration gradient at the soil-atmosphere interface. Due to wide variation in chamber methodology, the extent of the so-called “chamber effect” can also vary widely, leading to decreased confidence in trace gas emissions estimates. To date there have been no easily applicable techniques to quantify the magnitude of flux-underestimation arising from chamber deployment. The method described here is based on previously developed gas transport theory and has been simplified to facilitate application while preserving the fundamental physical relationships. Two options are presented: a graphical method requiring minimal calculation, and a more precise numerical technique which is easily adapted to spreadsheet application. In both cases, the theoretical underestimation (TFU) of the flux is determined, taking into account the effects of chamber geometry and deployment time, properties of the soil and trace gas under consideration (N2O or CO2), and the particular flux-calculation scheme that is used. The method can be used for designing chamber measurement protocols to minimize chamber-induced errors and for interpreting chamber-based gas flux data. An analysis of recently published studies indicates that TFU can vary widely, and also that previously observed relationships between soil water content and trace gas flux may in part reflect artifacts of chamber methodology. |
