The basic design features of LI-COR soil CO2 flux chambers were first published by Norman et. al., (1992), and the various factors known to influence the measurement of ground CO2 flux were summarized in LI-COR Application Note #124 and Welles, et. al., (2001). One of these factors is the concentration of CO2 in the chamber head space relative to the ambient atmospheric CO2 concentration. CO2 diffuses out of the soil in response to the concentration gradient between the soil pores and the chamber head space. If the chamber CO2 concentration is allowed to rise above the ambient CO2 concentration, then the CO2 flux is suppressed to a level below the natural undisturbed value. For this reason, soil flux measurements made with the LI-6400 Portable Photosynthesis System (used with the 6400-09 Soil CO2 Flux Chamber) are carried out near the ambient CO2 concentration. This is equally important for measurements with the LI-8100 Automated Soil CO2 Flux System, but we use a different approach to overcome it. In this application note, we outline the theory underlying the two systems and compare results obtained using them.
The protocol for determining soil CO2 flux with the LI-6400 is to first lay the chamber on its side on top of the soil and measure the ambient atmospheric CO2 concentration. The chamber is then placed on a soil collar and the chamber CO2 concentration is scrubbed to a level just below the ambient atmospheric concentration. When the scrubber is turned off, CO2 flux from the soil causes the chamber CO2 concentration to rise until it reaches a set point that is just above the ambient atmospheric concentration, completing the measurement cycle. As the chamber CO2 concentration rises, rates of change (∂Cc/ ∂t) are calculated every 2.5 seconds on about 10 points from the previous 7.5 seconds using linear regression. This produces a sequence of rates from which corresponding shortinterval CO2 fluxes are computed. The final estimate of soil CO2 flux is calculated by performing a linear regression of the short-interval fluxes against the average chamber CO2 concentration computed for each time interval, and using the slope and intercept of the regression to obtain the flux at the ambient CO2 concentration. More details are presented on page 3-4 of the 6400-09 Soil CO2 Flux Chamber Manual (2003).
The LI-6400 is designed for survey measurements, so using chemicals to scrub CO2 works fine; however, the LI-8100 Automated Soil CO2 Flux System is designed for long-term unattended operation where the use of chemicals is undesirable. Eliminating chemicals to scrub CO2 has made it necessary to change the procedure for estimating the soil CO2 flux rate at ambient CO2 concentration. At the start of a measurement, the LI-8100 chamber is held open above the soil collar and the system measures the ambient CO2 concentration (Cc(0)).