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United States Department of Agriculture

Agricultural Research Service

Title: Models for Calculating Gas Fluxes with a Closed Chamber

Authors
item Wagner, Steve
item Reicosky, Donald
item Alessi, Randolph

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 11, 1996
Publication Date: N/A

Interpretive Summary: Portable closed chambers provide scientists with a valuable tool for measuring crop growth (photosynthesis), crop water use, and soil respiration. Typically, the chamber encloses a number of plants and the carbon dioxide and water vapor concentration changes are recorded for about a minute. The slope of the recorded gas concentration data indicates the rate of plant growth or water use. However, due to effects the measuremen system may have on the measured process (called loading error or observer effect), the data that may appear to be a straight line is really a curve; and using a simple linear model may underestimate the flux. We demonstrated the need for a model that accounts for possible nonlinearity in closed-chamber gas concentration data using field measurements from three unique studies between 1985 and 1991. We provide a model that estimates the rate at the time of chamber closure. The quadratic model provides users of well-mixed closed chambers an alternative to a simple linear model for data sets with significant nonlinearity, thus improving the accuracy of the closed chamber technique by 10 to 40 percent.

Technical Abstract: Portable closed chambers provide a valuable tool for measuring canopy photosynthesis, evapotranspiration, and soil respiration. Typically, a linear regression model is selected to estimate the rate of gas concentration change with time, because the flux is assumed constant over the relatively short measurement period. However, due to effects the measurement system may have on the measured process (i.e., loading error o observer effect), assuming a constant rate and using a linear model may underestimate the flux. The first objective was to demonstrate the need for a model that accounts for possible nonlinearity in closed-chamber gas concentration data using field measurements from three unique studies between 1985 and 1991. The second objective was to provide a model that estimates the rate at the time of chamber closure. We described and compared the linear regression model, a multiple-linear regression (quadratic) model, and a modified-rate regression technique. Generally, 6 to 100 percent of chamber measurement data sets were significantly nonlinear, causing the alternative models to yield fluxes 10 to 40 percent greater than those calculated with the linear regression model. The frequency and amount of nonlinearity were related to the measured rate and chamber volume. Closed-chamber data should be tested for nonlinearity and an appropriate model used to calculate flux. The quadratic model provides users of well-mixed closed chambers an alternative to a simple linear model for data sets with significant nonlinearity.

Last Modified: 12/22/2014
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