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A Subsurface, Closed‐Loop System for Soil Carbon Dioxide and Its Application to the Gradient Efflux Approach
Author(s) -
DeSutter T. M.,
Sauer T. J.,
Parkin T. B.,
Heitman J. L.
Publication year - 2008
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj2006.0101
Subject(s) - loam , carbon dioxide , soil science , soil water , chemistry , saturation (graph theory) , soil gas , diffusion , flux (metallurgy) , analytical chemistry (journal) , mineralogy , hydrology (agriculture) , environmental science , environmental chemistry , geology , geotechnical engineering , physics , mathematics , organic chemistry , combinatorics , thermodynamics
Carbon dioxide concentrations in the soil can vary both temporally and spatially. Methodology was developed to semicontinuously measure subsurface concentrations of CO 2 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 CO 2 sensors to determine the concentration of CO 2 at each depth. Independent measures of CO 2 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 ( r 2 > 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 CO 2 ( 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 CO 2 flux using Fick's Law, six approaches to determine the vertical CO 2 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 CO 2 flux from soil, the approach used to determine the vertical CO 2 concentration gradient and method used to determine D s can both include sources of uncertainty.