Environmental and Vegetative Controls on Soil CO2 Efflux in Three Semiarid Ecosystems

Abstract

Soil CO2 efflux (F-soil) is a major component of the ecosystem carbon balance. Globally expansive semiarid ecosystems have been shown to influence the trend and interannual variability of the terrestrial carbon sink. Modeling F-soil in water-limited ecosystems remains relatively difficult due to high spatial and temporal variability associated with dynamics in moisture availability and biological activity. Measurements of the processes underlying variability in F-soil can help evaluate F-soil models for water-limited ecosystems. Here we combine automated soil chamber and flux tower data with models to investigate how soil temperature (T-s), soil moisture (theta), and gross ecosystem photosynthesis (GEP) control F-soil in semiarid ecosystems with similar climates and different vegetation types. Across grassland, shrubland, and savanna sites, theta regulated the relationship between F-soil and T-s, and GEP influenced F-soil magnitude. Thus, the combination of T-s, theta, and GEP controlled rates and patterns of F-soil. In a root exclusion experiment at the grassland, we found that growing season autotrophic respiration accounted for 45% of F-soil. Our modeling results indicate that a combination of T-s, theta, and GEP terms is required to model spatial and temporal dynamics in F-soil, particularly in deeper-rooted shrublands and savannas where coupling between GEP and shallow theta is weaker than in grasslands. Together, these results highlight that including theta and GEP in F-soil models can help reduce uncertainty in semiarid ecosystem carbon dynamics.