Carbon Exports from Terrestrial Ecosystems: A Critical-Zone Framework

In terrestrial ecosystems, vascular plants photosynthesize and respire to produce organic matter and CO2, respectively. Fractions of these products dissolve and are processed belowground to become dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) for export in landscape drainage to receiving inland waters. This paper gathers findings from across the Earth and ecological sciences to assemble a simple framework explaining the interplay among these processes in the critical zone (CZ), the skin of Earth from the top of the vegetation canopy through soil and subsoil to the base of active groundwater circulation. In this framework, chemical weathering, that is the dissolution of soil and subsoil mineral material, is a keystone process fueling carbon and energy flow through the system. The ecologic function of weathering dynamically generates nutrients positively feeding back into biosynthesis (and DOC generation) supporting heterotrophy. Root and heterotrophic respiration in turn drive the geologic function of weathering, entraining and stabilizing CO2 in solution (DIC generation) for storage in the hydrosphere and lithosphere. The CZ framework supports straightforward explanations of spatiotemporal patterns of DOC and DIC exports from catchments, including how they differentially respond to hydrologic and ecosystem development dynamics. Mechanisms that generate and export dissolved C also fuel and affect dynamics of stream emission of CO2 to the atmosphere. At larger time scales, terrestrial C-export rates and dynamics co-evolve with CZ development and disturbance. Ultimately terrestrial C exports are plate-tectonically and thermostatically capped and floored by volcanic CO2 production and carbonate chemistry in Earth’s crust.