Environmental and vegetative controls on soil CO2 efflux in three semiarid ecosystems

Authors: ROBY, M.C. - University Of Arizona; Scott, Russell - Russ; BARRON-GAFFORD, G.A. - University Of Arizona; Hamerlynck, Erik; MOORE, D.J.P. - University Of Arizona

Submitted to: Soil Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2019
Publication Date: 1/8/2019
Citation: Roby, M., Scott, R.L., Barron-Gafford, G., Hamerlynck, E.P., Moore, D. 2019. Environmental and vegetative controls on soil CO2 efflux in three semiarid ecosystems. Soil Systems. 3(1):6. https://doi.org/10.3390/soilsystems3010006.
DOI: https://doi.org/10.3390/soilsystems3010006

Interpretive Summary: Soil carbon dioxide (CO2) efflux (Fsoil) represents the net CO2 efflux due to belowground plant and microbial respiration and biogeochemical processes, and is a major component of the total amount of carbon moving from the land surface to the atmosphere. Increased understanding of the processes underlying Fsoil in globally expansive semiarid ecosystems is necessary to reduce uncertainty in terrestrial carbon dynamics. We combined field measurements with statistical models to investigate how soil temperature, soil moisture, and gross ecosystem photosynthesis control Fsoil in three semiarid ecosystems with similar climate but with different vegetation type. Across grassland, shrubland, and savanna sites, soil moisture regulate the relationship between Fsoil and soil temperature, and photosynthesis influenced Fsoil magnitude. Our modeling results indicates that the combination of soil moisture, soil temperature and photosynthesis are required to appropriately model spatial and temporal dynamics in Fsoil, particularly in the deeper-rooted shrublands and savannas where coupling between photosynthesis and shallow soil moisture is weaker than in grasslands. These results show that accounting for the interactive effects of the three drivers on Fsoil will be important to determine the response of water-limited ecosystems to changes in climate and land cover.

Technical Abstract: Soil CO2 efflux (Fsoil) is a major control on ecosystem carbon balance. Globally expansive semiarid ecosystems have been shown to influence the trend and interannual variability of the terrestrial carbon sink. Modeling Fsoil in water-limited ecosystems remains relatively difficult due to high spatial and temporal variability associated with dynamics in moisture availability and biological activity. Measurements that examine the processes underlying variability in Fsoil can help evaluate Fsoil models for water-limited ecosystems. Here we combine automated soil chamber and flux tower data with models to investigate how soil temperature (Ts), soil moisture ('), and gross ecosystem photosynthesis (GEP) control Fsoil in semiarid ecosystems with similar climate and different vegetation type. Across grassland, shrubland, and savanna sites,(theta)regulated the relationship between Fsoil and Ts, and GEP influenced Fsoil magnitude. Thus, the combination of Ts, (theta), and GEP controlled rates and patterns of Fsoil. In a root exclusion experiment at the grassland, we found that growing season autotrophic respiration accounted for 45% of Fsoil. Our modeling results indicate that the combination of Ts, (theta), and GEP terms is required to appropriately model spatial and temporal dynamics in Fsoil, 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 Fsoil models can be useful to reduce uncertainty in semiarid ecosystem carbon dynamics.