Open AccessA Transient Model of Vadose Zone Reaction Rates Using Oxygen Isotopes and Carbon Dioxide
Author(s) -
Birkham Tyler K.,
Hendry M. Jim,
Wassenaar Leonard I.,
Mendoza Carl A.
Publication year - 2007
Publication title -
vadose zone journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.036
H-Index - 81
ISSN - 1539-1663
DOI - 10.2136/vzj2006.0005
Subject(s) - vadose zone , biogeochemical cycle , water content , carbon dioxide , moisture , oxygen , chemistry , environmental chemistry , soil respiration , soil science , environmental science , soil water , geology , geotechnical engineering , organic chemistry
The importance of identifying and quantifying subsurface geochemical reaction rates and processes by monitoring and modeling CO 2 and O 2 concentrations is well established. These parameters, however, are typically studied independently under presumed steady‐state conditions. Here we present models of seasonally variable vadose zone CO 2 and O 2 concentrations that use δ 18 O of O 2 as a constraint to create a dynamic link between these three parameters under transient conditions. The gas transport modeling was used to quantify the controls of biogeochemical processes and parameters (i.e., temperature and moisture content) on vadose zone distributions of CO 2 and O 2 gas concentrations. The investigation was conducted on a 3‐m‐thick, unvegetated, fine‐sand vadose zone located in northern Alberta, Canada (56°40′N, 111°07′W). Using the modeled molar ratio of surface fluxes for O 2 and CO 2 , the change in reaction rate for a temperature change of 10°C ( Q 10 ), moisture content at maximum reaction rates, and biogeochemical discrimination against consumption of 18 O 16 O (α k ), we determined that organic C oxidation by microbial respiration was the predominant mechanism consuming O 2 and producing CO 2 The mean α k was determined to be 0.973, suggesting that subsurface respiration was via the alternative oxidase pathway, which may be common in cold climates. Modeling revealed that the moisture content of a moist, surficial clayey sand layer (0.1–0.3 m thick) had a dramatic effect on pore‐gas CO 2 and O 2 concentrations and on δ 18 O O2 The vadose zone in this study was at an unvegetated site to simplify the model application; however, it can be modified to include root respiration and applied to natural vadose zones to help quantify the role of subsurface respiration in global O 2 and C budgets.