Soil carbon and water dynamics: ¹³C and ¹⁸O as system tracers

Abstract

The goal of this research was to determine whether measurements of the stable isotopic composition of soil CO2 could be used to separate the net terrestrial CO2 flux to the atmosphere into the component fluxes of photosynthesis and respiration. Knowledge of these gross fluxes is needed to quantify terrestrial carbon storage. I measured the isotopic composition of soil CO2 along two bioclimatic gradients in the Rocky Mountains of Colorado. Sites were selected based on the state factor approach such that variability in results could be attributed primarily to differences in climate, vegetation type, and soil texture. Sites were sampled weekly (one site) or every three weeks over the growing season, with additional sampling before bud-break and after plant senescence. Soil organic matter and soil gas were analyzed for δ13C, and soil gas and soil water for δ18O, via continuous-flow mass spectrometry. One-dimensional, linear regression, and full equilibration-diffusion models were employed to calculate the isotopic composition of the soil CO2 flux. Through my study I identified serious flaws in the "equilibrium-only" approach to calculation of the δ18O value of soil-respired CO2. Data suggest that under some field conditions CO2 and water are not in isotopic equilibrium. Diffusion is central to the correct calculation of the δ18O value of soil-respired CO2, as shown by comparison of linear regression model output with that from a full diffusion-equilibration model. In addition, the isotopic composition of soil-respired CO2 depends on soil physical conditions. For example, some of the variability in expressed kinetic fractionation was explained by soil moisture. When moisture content was high, kinetic fractionation was as great as 7.5‰, but it was as low as 3.4‰ under drier conditions. Third, simpler models were shown to be incapable of reproducing the observed temporal variability in the isotopic composition of the soil CO2 flux, which was as large as 8‰ over the course of the growing season at a single location. Finally, the δ13C value of the soil CO2 efflux was controlled primarily by the proportion of C4 vegetation at a site, with no significant differences in δ13CO2 across pure C3 sites.