Ambient dissolved organic nitrogen (DON) pool response to in situ manipulation of organic solutes

Dissolved organic matter (DOM) provides a major link between terrestrial and aquatic ecosystems serving as both an energy and nutrient source. Understanding this duality is challenging due the inherent molecular diversity of the ambient pool of DOM. Here we use in-situ manipulations to study the ecological role of dissolved organic nitrogen (DON). We performed a series of pulse additions of organic solutes (acetate and glycine) in forested headwater streams across New England with a wide range of DOC and DON concentrations (4.41-27.71 mg L^-1 and 0.09-0.47 mg L^-1, respectively). Samples collected from each addition were analyzed for concentrations of acetate, NO₃^-, NH₄⁺, DOC, and major cations and anions as well as specific ultraviolet absorbance (SUVA). Glycine concentrations were measured through the implementation of a novel analytical chemical analysis using High-Performance Liquid Chromatography (HPLC) and the analysis of individual amino acids using the total hydrolysable amino acids (THAA) method. Pulse additions of acetate are expected to stimulate nitrogen uptake, causing DON concentrations to decrease, while we expect that the additions of glycine will demonstrate the duality of DON as an energy and nutrient source. These patterns will help us understand the ecological role of DOM in streams, associated biogeochemical controls, and DON's ultimate fate in aquatic ecosystems.