The influence of soils on the oxygen isotope ratio of atmospheric CO2

The oxygen isotope ratio of atmospheric CO2 is controlled by the isotope ratio of fluxes between the atmosphere and plants, soils and oceans. The 18O/16O ratios of the CO2 fluxes from these reservoirs reflect the 18O/16O ratios of the water in them. The influence of soils on the 18O/16O ratio of atmospheric CO2 depends on the size of the CO2 flux from soils to the atmosphere and the isotope ratio of this flux, which in turn depends on the isotope ratio of soil water and the extent of isotopic exchange between the CO2 and soil water. Soil water has the lowest 18O/16O ratio of these three pools, but tends to be enriched in 18O near the soil surface because of soil water evaporation, so the isotope ratio of the CO2 flux out of a soil will depend on the kinetics of the soil water-soil CO2 isotope exchange. Our field measurements and modelling indicate that the rate constant for soil water-CO2 oxygen isotope exchange averages 10-3 to 10-4s-1. The relevant size of the CO2 flux from soils to the atmosphere depends not only on the net flux of CO2 due to soil respiration but also on the amount of atmospheric CO2 which permeates the soil, undergoes isotope exchange with soil water and diffuses back to the atmosphere. The effects of these gross fluxes of CO2 between soil and the atmosphere are significant for estimating the soil component in the global atmospheric CO18O budget, especially in soils with low respiration rates. (author)