The isotopic composition of soil and soil-respired CO 2

Abstract

The isotopic composition of soil CO 2 has been investigated for the past three decades by earth scientists in a variety of disciplines. Carbon dioxide is produced in soil through biological processes (organic matter decomposition and root respiration) and is transported to the overlying atmosphere via molecular diffusion. Research has focused on both the isotopic composition of respired CO 2 (CO 2 that diffuses across the soil–atmosphere interface) and soil CO 2 (CO 2 that exists at any given soil depth). It is recognized that most soils are at quasi-steady state, and that the C isotopic composition of respired CO 2 is the same as that of the biological sources in the soil. Research on the isotopic composition of soil CO 2 has successfully used steady-state production/diffusion models to explain the common observation that the isotopic composition of soil CO 2 generally shows a gradual change from atmospheric isotopic values at the soil surface to values closer to biological sources with increasing depth. The complexity of the production/diffusion models needed to describe soil CO 2 is simplest with stable C isotopes, and increases in complexity and uncertainty with radiocarbon and O isotopes. For radiocarbon and O isotopes this is due to the fact that there can be a variety of isotopic sources within a given soil. Furthermore, for O isotopes, equilibration processes can alter the composition of the CO 2 as it diffuses through the soil. While much remains to be learned about the natural variations in the isotopic composition of soil and respired CO 2, interest in this topic will likely increase due to the magnitude of soil respiration on a global scale, and its role in the processes controlling the isotopic composition of atmospheric CO 2.