Southern Research Station

Title: Quantitative comparison of in situ soil CO₂ flux measurement methods

Author: Knoepp, Jennifer D.; Vose, James M.

Date: 2002

Source: Res. Pap. SRS-28.Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 11p.

Station ID: RP-SRS-028

Description: Development of reliable regional or global carbon budgets requires accurate measurement of soil CO₂ flux. We conducted laboratory and field studies to determine the accuracy and comparability of methods commonly used to measure in situ soil CO₂ fluxes. Methods compared included CO₂ traps; a multichamber open dynamic system (OC) with an infrared gas analyzer; and a headspace analysis system, with gas chromatograph with circulating fan (GCF) and gas chromatograph without circulating fan (GC), measuring changes in headspace CO₂ concentration using a gas chromatograph. In the laboratory, we compared NaOH, OC, GCF, and GC using sand-filled cylinders to simulate a soil system. Three concentrations of standard CO₂ gas (representing low, medium, and high soil-CO₂ flux rates) entered the cylinders through individually monitored flow meters. Flux rates measured using the OC method equaled the actual CO₂ flux at all three CO₂ concentrations. Flux rates determined with GC and GCF were < 55 percent of the actual flux rate. The percent of actual CO₂ flux measured increased with increasing CO₂ concentrations. At all concentrations, NaOH collected between 40 and 47 percent of CO₂ entering the system. A field study was conducted to verify laboratory results and allow comparison with the soda lime trap (SODA) method. In laboratory and field studies, all methods detected significant differences in flux rates among the standard CO₂ concentrations and field sites. Regression analyses showed good relationships between NaOH, SODA, and GC methods with flux rates measured using the OC methods (r² >= 0.78). Slope values for these regression equations ranged from 0.34 for NaOH to 0.54 for GC and SODA. These results suggest that data collected using the other methods could be standardized to OC flux rates. However, because methodological differences significantly affect CO₂ flux measurements, care should be used in applying these relationships.

Keywords: CO₂ traps, flux rates, gas chromatograph, infrared gas analyzer, NaOH base trap, open dynamic chamber, soda lime trap, soil respiration.

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Citation

Knoepp, Jennifer D.; Vose, James M.  2002.  Quantitative comparison of in situ soil CO₂ flux measurement methods.   Res. Pap. SRS-28.Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 11p.