CO2 efflux from a calcareous Mojave Desert soil: isotopic results from a laboratory and field study

Soil inorganic carbon (SIC) represents a substantial C pool in arid ecosystems. The contribution of the SIC pool to net ecosystem C flux is poorly understood but has gained attention because there have been reports of anomalous C fluxes in some arid environments. In the context of climate change, altered precipitation patterns and changes in soil pCO₂ values (from changes in vegetation density, plant water use efficiency, and belowground respiration) could potentially affect SIC storage in some ecosystems. The stable carbon isotope values of organic and inorganic carbon (e.g. carbonates) can differ substantially and may be useful in determining whether PIC influences C fluxes. However, variable rates of heterotrophic and root respiration and diffusion of atmospheric CO₂ into the soil as well as the variation and complexity of the CaCO₃-CO₂-H₂O system at different soil depths can complicate interpretation of isotopic data. We monitored soil CO₂ concentrations and CO₂ efflux from irrigated and non-irrigated plots in a calcareous soil at the Mojave Global Change Facility (MGCF). The site is on the northern part of the Mojave Desert with a mean annual precipitation of 71 mm and vegetation characterized by a Larrea tridentata, Lycium spp., Ambrosia dumosa plant community. We used a Keeling plot approach to determine source δ¹³C values from effluxed CO₂-¹³C in a laboratory incubation experiment and from direct field measurements of soil CO₂-¹³C. Data from the laboratory incubation experiment suggested that there was a contribution of PIC on effluxed CO₂-¹³C in a closed system, but results from the field measurements were much more difficult to interpret and did not support a large contribution of SIC to CO₂ fluxes in these soils. We discuss the usefulness of isotopic measure of CO₂ on CO₂ efflux in the context of the MGCF experiment.