Stable Isotope Tracers of Nitrogen Sources to the Neuse River, North Carolina

摘要

Stable nitrogen isotopic analyses were conducted on dissolved nitrate in surface water samples collected over a two-year period from the lower Neuse River, North Carolina, to assess temporal variation of nitrate sources to the watershed. These nitrogen isotopic values correspond well to literature values published for point and non-point nitrate sources, respectively, and confirm that there is a well defined isotopic difference between point and non-point nitrate inputs to the Neuse River Basin. The nitrogen isotopic values of nitrate from the lower Neuse, which integrates the nutrient loading trends over the entire basin, show an annual cycle. Over the entire annual hydrological cycle, surface water nitrate nitrogen isotopic values are exponentially related to river discharge rate. The relationship suggests that a residence time delay factor in an intermediate reservoir (groundwater, wetlands) plays an important role in the transfer of non-point source nitrate to the riverine system during the falling discharge (spring) period. The enhanced fertilizer loading in wet years during the critical spring period can influence the biological species succession and would therefore play a significant role in enhancing nuisance algae bloom potential in late summer.