The Dual Isotopic Composition of Nitrate Exported From Temperate Watersheds of Different Land Uses

Non-point sources are repeatedly found to contribute the majority of nitrogen to most coastal watersheds. While the dominance of non-point sources is not true of all systems, determining their roles in the total nitrogen export to any estuary is an important step towards effectively managing nitrogen loads. This research focuses on determining the relative importance of inputs and processes on watershed nitrogen export by measuring the dual isotopic composition of nitrate (δ15N and δ18O) from watersheds of different sizes and land uses. Seasonal differences are attributed to changing inputs to the landscape and/or variation in the biogeochemical processing of nitrogen inputs in both the terrestrial and aquatic environment. In watersheds almost entirely altered by humans (e.g. suburbia) seasonal isotopic shifts have not been observed, suggesting that the inputs and/or processes in these catchments do not change significantly on this time scale. In contrast, the 15N18O3- exported from forested sites suggests that hydrologic conditions and biological demands change throughout the year, thus altering its isotopic composition. Similarly, 15N18O3- in agricultural streams changes systematically with season due to the variation in inputs and hydrologic regime. These differences will allow us to decipher the various land use origins of non-point nitrogen loading within the tributaries of the Connecticut River.