The use of headwater catchments to understand the age, availability, and fate of organic matter exported from terrestrial to fluvial systems

Abstract Headwater streams provide a critical interface between terrestrial and aquatic ecosystems. Significant quantities of dissolved organic carbon are stored in soils and a portion of this is transported through river systems on an annual basis. This organic carbon is an important energy source for microbial communities and its quality has important implications for its availability and transport to downstream reaches. Prior studies have focused on dissolved organic carbon in large river systems. While important, these studies can be affected by anthropogenic influences and by large-scale heterogeneity in geology and vegetation across large watersheds. Here we provide insight into the export, quality, and fate of dissolved organic carbon in headwater streams. We utilized small catchments with varying land use types in Pennsylvania to assess changes to the quality of dissolved organic carbon during storm events and nutrient addition experiments. The agricultural site exports significantly higher concentrations of DOC per watershed area. During storm events we observed little change in the fluorescence organic carbon signal and an increase in SUVA254. Biolability decreased with increasing discharge in the forested watersheds. Nutrient addition experiments indicated an increase in microbial respiration with both nitrogen and phosphorous. Batch culture experiments show that much of the organic carbon transported in headwater streams is respired to the atmosphere as CO2. Land use is important to DOC export, DOC biolability, and bacterial growth efficiency, but has minimal influence on the export of aged DOC. Hydrology played a more important role than land use in controlling the export, availability, and fate of dissolved organic carbon in headwater streams.