Abrupt shifts in the concentration, composition, and reactivity of dissolved organic carbon from terrestrial to aquatic compartments across boreal watersheds.

Dissolved organic carbon (DOC) plays a critical role in the boreal aquatic carbon cycle, serving as a vital link between land and water ecosystems. Despite rapidly increasing knowledge of the carbon cycling in the terrestrial and aquatic ends of watersheds, understanding of the biogeochemical processing of DOC pools along land-water continua remains poor. We determined the concentration, composition, and reactivity of DOC in key watershed compartments (forest soil water, riparian zones (RZ) soil water, streams, and lakes) of 16 watersheds in two regions of boreal Quebec. Our findings show a substantial decline (3.3 times) in DOC concentration across the land-water interface from soils to lakes. We also found higher spatial variability in DOC concentration in the terrestrial compared to the aquatic compartments of watersheds. Despite these variations, the percentage of bio- and photo-degradable DOC remained consistent along the land-water continuum. We further found high concentrations of protein-like DOM in the forest soil water and RZ samples, yet these concentrations diminished significantly in the aquatic environment, consistent with efficient loss of this DOM in standardized experiments. These findings suggest a terrestrial source of highly bio-reactive protein-like components and a rapid loss of terrestrial DOC as it leaves land and is drained into streams and lakes.