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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

View larger version (16K): [in this window] [in a new window]   Fig. 1. Schematic diagram of the closed-loop, porous Teflon tubing and instrumentation used to determine CO2 concentrations in the soil. Valves A, B, and C control N2 gas for the air dryer, removal of CO2 from the instrumentation plumbing, and calibration of the CO2 sensors, respectively.

View larger version (13K): [in this window] [in a new window]   Fig. 2. Instrumentation used to determine the soil-gas diffusion coefficient of CO2 through intact soil cores.

View larger version (12K): [in this window] [in a new window]   Fig. 3. Potential methods used to determine the vertical CO2 concentration gradient in soil from actual data: (A1) dC/dz evaluated at z = zi ; (A2) slope of the line for CO2 concentration with depth; and (A3) C/z evaluated between zi and zj.

View larger version (20K): [in this window] [in a new window]   Fig. 4. Average 24-h profiles of (A) CO2 concentration, (B) volumetric water content, and (C) temperature beneath a Harps loam soil near Ames, IA.

View larger version (16K): [in this window] [in a new window]   Fig. 5. Comparison of concentration of CO2 in the soil determined from the porous Teflon tubing, closed-loop design to CO2 concentrations from samples extracted from the in-line air stream of the closed-loop design and gas wells buried at 0.02, 0.1, and 0.18 m below the soil surface. Root mean square (rms) = [( predicted values – measured values)2/number of data]0.5.

View larger version (12K): [in this window] [in a new window]   Fig. 6. Effect of air-filled porosity () on the soil-gas diffusion coefficient of CO2 in soil (Ds) as shown using the soil-specific, Moldrup et al. (2000), and Millington and Quirk (1961) models. Error bars are standard deviations.

View larger version (16K): [in this window] [in a new window]   Fig. 7. The soil-gas diffusion coefficient of CO2 in a Harps loam soil as measured using the instrumentation in Fig. 2 and estimated using models of Moldrup et al. (2000) and Millington and Quirk (1961).

View larger version (20K): [in this window] [in a new window]   Fig. 8. Carbon dioxide concentration profiles in the soil at 1200 h between Days of the Year 237 and 244.