Oxygen isotopic equilibrium between carbon dioxide and water in soils

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 O-18 isotopic balance of atmospheric CO2. The soil-to-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-re-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.