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Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/5/2009 Publication Date: 5/1/2009 Citation: Venterea, R.T., Spokas, K.A., Baker, J.M. 2009. Accuracy and Precision Analysis of Chamber-Based Nitrous Oxide Gas Flux Estimates. Soil Science Society of America Journal. 73(4):1087-1093. Interpretive Summary: Chamber-based methods for determining nitrous oxide (N2O) gas flux are known to be subject to substantial errors which generally result in an underestimate of the true rate of transfer of N2O from soil to atmosphere. Methods have been previously developed to estimate the magnitude of these errors so that more accurate assessment of greenhouse gas (GHG) impacts of agro-ecosystems can be made. The current study extends previous error evaluation techniques by considering the impacts of soil biological uptake and measurement error on resulting flux estimates. This analysis concludes that under most circumstances, N2O flux-estimate errors can be approximated using techniques that assume no soil uptake. Measurement error associated with three different analytical systems for determining N2O concentration were characterized. A spreadsheet-based chamber error analysis tool (CEAT) was developed that quantifies effects of measurement error, chamber protocols, and soil properties on resulting flux estimates. The analysis revealed critical trade-offs that need to be considered in designing and applying chamber methods. Consideration of these issues will enable scientists to make more accurate measurements of N2O fluxes from soils to the atmosphere. Improved measurements will in turn enable policy makers and regulators to make better estimates of state, regional and national scale GHG emissions associated with agricultural activities. Technical Abstract: Previous analysis of chamber-based nitrous oxide (N2O) gas flux estimate errors has not considered the impacts of soil biological uptake and has not closely examined the influence of measurement error on resulting flux estimates. Simulation modeling is used here to demonstrate that chamber N2O concentrations are not likely to be affected by soil uptake except under extreme conditions. Thus, in most circumstances, N2O flux-estimate errors can be approximated using numerical and analytical techniques that assume no soil uptake. Measurement error associated with three different analytical systems for determining N2O concentration were characterized. The analytical standard deviation (sA) varied as smooth functions of concentration and also varied among systems. While sA was generally less than 5% of the concentration, Monte Carlo analysis indicated that the overall flux measurement standard deviation (sM) was magnified. A spreadsheet-based chamber error analysis tool (CEAT) was developed that quantifies effects of measurement and calculation protocols, soil properties, and sA on resulting flux estimates. The analysis revealed critical trade-offs. While maximization of chamber height, minimization of deployment time, and use of non-linear flux-calculation schemes reduced errors in estimating pre-deployment fluxes on an average basis, each of these practices also caused substantially increased sM and, in some cases, positively skewed flux-estimate distributions. Thus, analytical error can be critical in selection of a flux measurement protocol. It is shown here how characterization of analytical system performance in combination with tools like CEAT can be useful in evaluating alternative chamber protocols. |
