Oxygen isotopic equilibrium between carbon dioxide and water in soils

Abstract

Exact mathematical solutions are presented for some simple cases of isotopic equilibration of oxygen-18 between carbon dioxide and water in soils, which is a major component of the 18O isotopic balance of atmospheric CO₂. The soilto-air exchange is made up of 2 components. First, respiratory CO₂ is produced in the soil and escapes to the atmosphere through diffusion. The lighter isotopic species escapes a little more rapidly, enriching the heavier isotopic species in the CO₂ left behind in the soil, which is then out of isotopic equilibrium with soil water. Therefore, simultaneously with the escape of gas, there is continuous re-equilibration of CO₂ with soil water. The second component of soil-to-air exchange is independent of the production of CO₂ in the soil: the invasion of atmospheric CO₂ into the soil, where it equilibrates isotopically with the soil moisture before diffusing back out. A consequence for atmospheric isotopic measurements is that, in general, for respired CO₂ a plot of the isotopic ratio versus the inverse of the concentration is curved, and that the slope in the vicinity of the ambient concentration may not point towards the isotopic ratio of the net CO₂ evolved from the soil.

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