Watershed geomorphology interacts with precipitation to influence the magnitude and source of CO 2 emissions from Alaskan streams

Boreal ecosystems contain a large fraction of the world's soil carbon and are warming rapidly, prompting efforts to understand the role of freshwaters in carbon export from these regions. We examined geomorphic controls on the magnitude of stream CO 2 emissions and sources of stream dissolved inorganic carbon (DIC) across a heterogeneous river basin in SW Alaska. We found that watershed slope and precipitation interact to control gaseous carbon fluxes from streams, with the highest flux from low‐gradient watersheds following rainstorms. Watershed slope influences C loading and stream CO 2 fluxes by controlling carbon accumulation in watersheds and to a lesser extent by determining gas transfer velocity across the stream air‐water interface. Low gas transfer velocity in flat streams offsets some of the effects of higher terrestrial C loading at those sites, resulting in lower than expected vertical CO 2 fluxes. While the isotopic composition of stream DIC (Δ 14 C and δ 13 C) was highly variable across space and time, shifts toward contemporary, terrestrial sources after precipitation were most pronounced in flat watersheds. At base flow DIC was a mixture of modern and aged sources of biogenic and geologic origin (Δ 14 C, −198.3‰ to 27.9‰, n = 23). Aged (Δ 14 C‐depleted) C sources contributed to food webs via a pathway from DIC to algae to invertebrate grazers. We observed coherent changes in stream carbon chemistry after large rainstorms despite considerable physical heterogeneity among watersheds. These patterns provide a way to extrapolate across boreal landscapes by constraining CO 2 concentration and flux estimates by local geomorphic features. Substantial variation in stream CO 2 emissions in boreal regions has been observed, but physical controls remain poorly understood We found that watershed slope and precipitation interact to control stream CO 2 fluxes via effects on C loading and gas transfer velocity Large rain events consistently mobilized younger, terrestrial dissolved inorganic carbon into streams relative to base flow conditions