Determining the relative importance of fluxes and processes to nitrogen and carbon export from temperate watersheds

Humans have severely altered the nitrogen and carbon cycles through the production of fertilizers, burning of fossil fuels, and development of the landscape. Rivers and streams effectively integrate the physical, biological, chemical and anthropogenic processes and provide an important link between terrestrial and oceanic nutrient pools. By sampling streams across a land use gradient within the southern portions of the Connecticut River watershed the relative importance of anthropogenic fluxes and processes to nitrogen and carbon exports were determined. Simple models in conjunction with the dual isotopic composition of nitrate (delta15N-NO3 - and delta18O-NO3-) revealed that while the nitrate (NO3-) exported by streams draining developed watersheds reflected the sources of nitrogen to the system, the small amount of nitrate exported by forested watersheds was derived from internal processes indicating that the system was retaining, removing or reprocessing the vast majority of NO3- delivered to them. Anthropogenic fluxes dominated the altered system signal and due to the enriched 15N nature of sources such as manure and septic in the agricultural and urban watersheds, resulting in significant correlations between NO 3- concentrations and delta15N- NO 3-. In addition, the temporal variation in the isotopic composition of NO3- revealed that biotic processes significantly altered the amount of NO3- exported throughout the year.;Modeling results suggest that significant reprocessing and removal was occurring within the developed watersheds, attenuating up to 65% of the possible NO3- yield. Anthropogenic changes to the land cover not only affect nitrate yields, they also significantly alter the rate of chemical weathering and dissolved inorganic carbon (DIC) yields. DIC yields were significantly correlated with the amount of urbanization within a watershed and results indicated that both urbanized and agricultural watersheds exported significantly more DIC than undeveloped systems. Greater yields were attributed to enhanced weathering and CO2 production within developed systems associated with crop cultivation, urban green spaces, and organic matter loading. Increased nitrification within agricultural catchments led to the dissolution of lime, furthering increasing DIC yields. Calculated anthropogenic yields (5-10 g C m-2 yr-1) suggest that up to 70% of global DIC flux may be attributable to human activities.