ABSTRACT:

Rivers and streams are control points for CO2 evasion to the air (fCO2), with rates often exceeding internal metabolic production (net ecosystem production, NEP). The difference is attributed to groundwater inputs enriched in CO2 from upland soil respiration, but this implies a terrestrial-to-aquatic C transfer far larger than estimated by terrestrial mass balance. One explanation is that riparian zones, neglected in most terrestrial mass balances, contribute a disproportionate fraction of observed fCO2, highlighting the integral role of river corridors (i.e., streams plus their adjacent wetlands) in landscape C export dynamics. To test this hypothesis, we measured fCO2, NEP, and the lateral CO2 contributions from both terrestrial uplands (TER) and riparian wetlands (RIP) for seven mid-order reaches in a lowland river network in north Florida, USA. NEP contributed nearly half of fCO2 on average, but the remaining CO2 evaded by the stream was generally far larger than measured TER, suggesting principally river corridor (RIP) origins. The relative importance of RIP vs. TER varied markedly between contrasting hydrogeologic settings: RIP contributed 60% of fCO2 where geologic confinement forces lateral drainage through riparian soils, but only 12% where unconfined karst results in deeper groundwater flowpaths that largely bypass riparian zones. On a unit area basis, the relatively narrow riparian corridor yielded 40 times more CO2 than the terrestrial uplands (33.77 vs. 1.38 g-C m-2 yr-1), resulting in river corridors sourcing the majority of fCO2 (NEP + RIP = 85%) to streams. Including riparian zones in the conceptual model for terrestrial-to-aquatic C transfer implies that true terrestrial CO2 subsidies to streams are smaller than previously estimated.