Soil Degassing of Carbon-14 Dioxide: Rates and Factors
Sheppard, Marsha I. | Ewing, L. L. | Hawkins, J. L.
Soil air normally contains elevated levels of CO₂ relative to the atmosphere. The primary source of soil C is plant-root and microbial respiration. The exchange of soil and atmospheric CO₂ is important to many environmental concerns, such as acid rain, global warming and waste management. Proposed disposal of high-level nuclear wastes containing primarily inorganic ¹⁴C may provide a source of ¹⁴CO₂ to the atmosphere. Field and laboratory experiments show that ¹⁴CO₂ soil degassing rate constants, the flux density (Bq·m²·s⁻¹) divided by soil inventory (Bq·m⁻²), range from −10⁻⁷ to −10⁻² s⁻¹, and that the loss of inorganic ¹⁴C is driven primarily by gaseous diffusion. These constants are affected by soil pH and porosity, with smaller influences of soil temperature, moisture and organic matter content. Degassing rate constants derived through mass balance calculations to estimate loss differ only by 20% from direct trapping methods. Frozen soil degassing rate constants were up to 25 times smaller than lab values, indicating that annual ¹⁴C loss rates in northern climates would be lower because of reduced gaseous diffusion during the winter months. Using our field data, we recommend an annual ¹⁴C soil degassing rate constant of −1 × 10⁻⁶ s⁻¹ for acidic soils and a value of −5 × 10⁻⁷ s⁻¹ for calcareous soils. For probabilistic assessment modelling, we recommend a geometric mean degassing constant of −4.3 × 10⁻⁷ s⁻¹ with a geometric standard deviation of 3.26 for three different soils. This indicates the median half-life of ¹⁴C in surface soils is 18 d, with a 99% confidence interval of 13 h and 640 d.
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