Environmental and vegetative controls on soil co₂ efflux in three semiarid ecosystems

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 (θ), and gross ecosystem photosynthesis (GEP) control F_soil in semiarid ecosystems with similar climates and different vegetation types. Across grassland, shrubland, and savanna sites, θ regulated the relationship between F_soil and T_s, and GEP influenced F_soil magnitude. Thus, the combination of T_s, θ, 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, θ, 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 θ is weaker than in grasslands. Together, these results highlight that including θ and GEP in F_soil models can help reduce uncertainty in semiarid ecosystem carbon dynamics.