Soil CO2 Flux Measurements -- Using the LI-8100 System to Collect Air Samples for Estimating Soil Trace Gas Flux


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Air samples can be collected from the same air stream used to measure soil CO2 flux with the LI-8100 Automated Soil CO2 Flux System, for estimating the flux of trace gas, such as methane (CH4), ammonia (NH3), nitrous oxide (N2O), isotopic species, etc. This application note describes a simple method for obtaining air samples from the LI-8100 in the field. The concentration of the gas species for those collected air samples can be subsequently analyzed with gas chromatography or mass-spectroscopy methods in a laboratory. The flux of the gas species can be estimated based on the rate of change of the gas species concentration inside the chamber and from information on other required variables, which can be obtained from the output file of the LI-8100.



The figure below shows a simple method for collecting air samples in the field. In this figure, the LI-8100 Analyzer Control Unit is connected directly to the 8100-103 Survey Chamber. A T-fitting with a septum has been installed between the LI-8100 and chamber. This method can be used with all LI-8100 chambers, and does not require modifications to the LI-8100 or the chambers. Part number 8100-664 is an accessory which allows the user to implement this method. The 8100-664 accessory is an assembled section of air line with a T-fitting and septum, and quick connectors at each end. This piece fits between the end of a chamber air line and the LI-8100 “Air In” port.


A 2-minute observation length is adequate for soil CO2 flux measurements, but for other trace gases, the observation length may need to be much longer due to their low flux rates. The observation length of the LI-8100 system can be set to any value. The appropriate observation length depends on how many trace gas samples are required and the time interval between them. Five to six samples during a period of 30 minutes may be sufficient for good curve fitting. The first air sample must not be taken until at least 20 s after the chamber is closed. Our experience suggests that good mixing in the system may not be established before 20 seconds have elapsed. When this air sampling method is used on our LI-8150 Multiplexer, the first air sample should be taken at least 30 seconds after the chamber is closed. Collected air samples can then be analyzed with gas chromatography or mass-spectroscopy.

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