Riparian zone heterogeneity influences the production and fate of biodegradable dissolved organic carbon across land-water interfaces

Transport of biodegradable organic carbon (bDOC) across land-water interfaces supports the ecological and biogeochemical functioning of northern freshwater ecosystems. Yet, we know little about how the generation and supply of terrestrial bDOC to boreal headwaters is regulated by the physical, biological, and hydrological properties of the riparian interface. We used 7-, 14- and 28- day bDOC incubations on eight occasions during the northern growing season to assess how terrestrial and aquatic bDOC concentrations differ along flowpaths connecting riparian soils to a headwater stream. We found that bDOC quantity declined along the transition from land to water, and that riparian soils had higher concentrations of bDOC compared to aquatic landscape components. Additionally, these differences corresponded to changes in the optical and chemical properties of the dissolved organic matter pool. Further, the generation of bDOC in riparian soils varied across interface types and reflected hydrogeomorphically determined differences in soil organic matter storage, groundwater level dynamics and soil microbial activity. In particular, the potential transfer of bDOC from soils to groundwater appeared largely regulated by the degree of contact between soils and lateral subsurface flowpaths. Riparian interfaces with near-constant opportunity to deliver resources laterally to streams by shallow, preferential groundwater flowpaths were found to have a relatively poor capacity to generate bDOC within local soils. At the same time, groundwater within these same interfaces had higher concentrations of bulk DOC and bDOC, likely due to connections with larger contributing hillslopes which serve as important support systems to streams during baseflow periods. Collectively, our results show that boreal headwaters are comprised of a continuum of interface types that differ in capacity to generate bDOC in near-stream soils, and in opportunity to mobilize and convey bDOC laterally. Ultimately this leads to wider variability in when and where within the broader stream network these inputs may be most important to aquatic ecosystems.