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A Semiautomated Instrument for Measuring Total and Radiolabeled Carbon Dioxide Evolution from Soil

USDA-ARS, Building 050, Room 100, BARC-West, Beltsville, MD 20705.

^* Corresponding author.

ABSTRACT

The design and operation of a semiautomated instrument for measuring total and radiolabeled CO₂ evolution from soil is described. In the recirculation mode, headspace gases are recirculated in a closed loop through an infrared (IR) detector. After a steady state is achieved, the concentration of total CO₂ is read and recorded. In the flush mode, compressed air is used to flush CO₂ from the headspace into test tubes containing trapping solution. Radiolabeled carbon dioxide is quantified by counting aliquots of trapping solution in a liquid scintillation counter. The variability associated with the IR detector is 3.6% (coefficient of variation). The trapping efficiency is 100% (±2.0%). The variability associated with two different methods of pesticide application to soil was also investigated. With the slurry method, aliquots of air-dried soil were added to a pesticide solution [carbofuran(2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate)] with thorough mixing. With the spray method, the carbofuran solution was applied as an aerosol to soil that had been previously wetted. There was little difference in the variability of total CO₂ production between the two methods. The variability associated with ¹⁴CO₂ evolution from soil (carbofuran hydrolysis), however, was greater with sprayed soil than with slurred soil, suggesting that the slurry method resulted in a more homogenous distribution of pesticide throughout the soil. The low analytical variability associated with measurements of total CO₂ and ¹⁴CO₂ production, as well as the large number of samples that can be accommodated (up to 94 flasks or intact soil cores) make this instrument ideal for the execution of the large screening studies needed to obtain reliable estimates of the spatial variability of rates of pesticide degradation.

Contribution from the USDA-ARS.

Received for publication December 1, 1988.