Nitrate uptake kinetics in streams: Is carbon the driver?

The underlying mechanisms driving the coupled interaction of organic matter quantity and uptake of inorganic nitrogen are not well understood, particularly in surface waters. To determine the relationship between background dissolved organic carbon (DOC) quantity and nitrate (NO3-) uptake kinetics in streams, a series of NO3- TASCC additions were performed in four sites within the Lamprey River Watershed, New Hampshire, with a wide range in background DOC concentrations (1 mg C/L - 8 mg C/L). Experiments were performed between the 2013 and 2014 growing seasons. Across all sites and experimental dates, ambient and dynamic uptake velocity (Vf) correlated negatively with NO3- concentrations and positively with increasing DOC concentrations and DOC:NO3- ratios. Ambient NO3- Vf was unrelated to pH, SUVA, PAR, temperature, and dissolved oxygen. Although there are general tendencies across the Lamprey River Watershed, individual sites behave differently in their uptake kinetics. Variation in uptake kinetics consisted of saturation and efficiency loss models, hysteresis, undetectable uptake, and increasing Vf with higher NO3- concentrations (biostimulation model). NO3- uptake dynamics in the Lamprey River Watershed are most influenced by DOC concentrations rather than NO3- concentrations or physico-chemical parameters. Understanding the fundamental relationship between organic matter and inorganic nutrients will be important as global and climatic changes influence the delivery and production of DOC and NO3- in aquatic ecosystems