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SOIL BIOLOGY & BIOCHEMISTRY

A Subsurface, Closed-Loop System for Soil Carbon Dioxide and Its Application to the Gradient Efflux Approach

^a Dep. of Soil Science, Walster Hall, North Dakota State Univ., Fargo, ND 58105, formerly with, USDA-ARS National Soil Tilth Lab., Ames, IA, 50011
^b USDA-ARS National Soil Tilth Lab., Ames, IA, 50011
^c Soil Science Dep., Williams Hall, North Carolina State Univ., Raleigh, NC, 27695

^* Corresponding author (thomas.desutter{at}ndsu.edu).

Carbon dioxide concentrations in the soil can vary both temporally and spatially. Methodology was developed to semicontinuously measure subsurface concentrations of CO₂ using expanded, porous Teflon (ePTFE) tubing. Lengths of ePTFE tubing (7.6 m) were buried at 0.02, 0.1, and 0.18 m below the soil surface in a Harps loam soil (fine-loamy, mixed, superactive, mesic Typic Calciaquoll) in central Iowa, and also positioned directly on the soil surface (0 m). Soil atmospheric gases that diffused through the walls of the tubing were circulated in a closed-loop design through solid-state CO₂ sensors to determine the concentration of CO₂ at each depth. Independent measures of CO₂ concentrations were also determined by sampling the in-line gas stream of the ePTFE system and from samples extracted from gas wells positioned near the buried tubing. Good agreement (r² > 0.95) was observed between the ePTFE system and the independent measures, with the ePTFE having biases of 1.2 and 1.37 times greater than the in-line and gas well samples, respectively. The soil-gas diffusion coefficient of CO₂ (D_s) was determined using intact soil cores and values were about 2.5 times less than two popular models used to predict D_s in soil. Estimates of CO₂ flux using Fick's Law, six approaches to determine the vertical CO₂ concentration gradient, and three methods to determine D_s ranged from >800 to <1 µmol m^–2 s^–1 on Day of the Year 239.5. Although Fick's Law is commonly used to estimate CO₂ flux from soil, the approach used to determine the vertical CO₂ concentration gradient and method used to determine D_s can both include sources of uncertainty.

Abbreviations: AFP, air-filled porosity • DOY, day of the year • ePTFE, expanded, porous Teflon • PE, ultra-high-molecular-weight polyethylene