Implications of soil inorganic carbon dynamics in California desert ecosystems

Background/Question/Methods

Soil inorganic carbon (SIC), in the form of CaCO₃, is among the largest pools of carbon in the western US and is ubiquitous in arid regions worldwide. Caliche, a secondary carbonate as CaCO₃, precipitates when meteoric water, respired CO₂, and calcium react. Caliche is assumed to be stable, but the potential effects of disturbance on caliche as a long-term carbon pool has received little attention. Renewable energy developments are projected to be deployed over desert wildland areas with deep caliche deposits which will eliminate deep-rooted vegetation. Removal of vegetation may systemically alter SIC, because respired CO₂ is the carbon source in caliche. We sought to understand how the vegetation affects caliche pools through the use of modeling and stable isotope techniques. Existing models of CaCO₃ dynamics incorporate simulated soil moisture, CO₂ concentrations, and atmospheric temperatures. We collected field data of soil CO₂, temperature, and moisture in the Mojave Desert, CA, and incorporated these into a process-based model.  Additionally, we analyzed vegetation and soil ¹³C and ¹⁸O to further investigate carbon and water dynamics of caliche. We hypothesized that caliche formation and degradation would be more dynamic and susceptible to disturbance than previously recognized.

Results/Conclusions

Modeled and stable isotope analyses suggest that caliche is relatively dynamic. Modeled outputs indicated that CaCO₃ can go in and out of solution on a daily time scale, especially after rain events in which soil moisture drastically increases and soil CO₂ reaches concentrations as high as 8,000 ppm. Consistent with previous studies, δ¹⁸O revealed that oxygen came from meteoric water. However, δ¹³C isotope values of caliche were above those expected from soil organic matter respiration. Whereas previous researchers sampled soils at depths of 50-90 cm, where the source of soil CO₂ was largely respiration, our soil samples were collected at a relatively shallow depth (10-15cm) where there was likely a higher proportion of atmospheric CO₂. These results suggest that carbon may readily exchange between the atmosphere and caliche at shallow soil depths. As desert ecosystems continue to undergo major land use changes, disturbances to vegetation and soil may lead to the exposure of deep, protected caliche layers to the atmosphere, causing them to become susceptible to erosion and carbon loss.