Factors Controlling Dissolved Organic Carbon (DOC) and Nitrogen (DON) Dynamics in Watersheds

Our previous work in a glaciated, forested watershed in western New York revealed distinct differences in the responses of DOC and DON during storm events. We explored if these differences also occurred at an unglaciated, forested, 12 ha watershed in the Piedmont physiographic province of Maryland. Key questions that we addressed are: What are the sources and flowpaths for DOC and DON in the watershed? Do they differ for DOC and DON? Does DOM quality influence the concentrations and transport of DOC and DON? Watershed sources that are being sampled include: rainfall, throughfall, litter, soil water groundwater, hillslope seeps and hyporheic zone water. Watershed sources are sampled manually during non-storm periods (every two weeks) while automated samplers are used for storm events. Water samples are analyzed for all cations, anions, DOC, DON, Silica and O18. DOM quality is characterized using specific ultraviolet absorbance (SUVA), Fluorescence Index (FI), Excitation-Emission Matrices (EMMs), hydrophobic and hydrophilic fractions, and concentrations of phenols, carboxylic acids, flavanoids, and amino acids. Hydrologic monitoring includes discharge and groundwater elevations. Our initial results suggest that differences in DOC and DON are not as large as that observed for the western New York site. Endmember mixing analysis (EMMA) indicates that stream chemistry is regulated by seep runoff, litter, and riparian soil water with individual endmember contributions varying with size of the storm events and wetness conditions. Highest DOC and DON concentrations were recorded for throughfall and litter layer. SUVA values were highest and most variable for litter leachate followed by riparian water. Concentrations of DOC and DON in streamflow increased dramatically during events and peaked at or after the discharge peak. While SUVA values followed a similar trend, there were slight differences among events. Additional relationships between DOM concentrations and quality are also being investigated.