Aquatic nitrogen source elucidation using stable isotope analysis in the North Carolina Coastal Plain

Abstract

Eastern North Carolina has been subjected to widespread water quality degradation for decades, notably throughout the Cape Fear River Watershed, owing largely to the magnitude of concentrated animal feeding operations (CAFOs) in the region. Long-term nutrient monitoring data from numerous locations throughout southeastern North Carolina have shown significantly elevated organic nitrogen (Org-N) concentrations starting around the year 2000—a concerning development, as labile Org-N can stimulate algal blooms and subsequent bacterial production, thus enhancing eutrophication in freshwater systems. By measuring the stable isotope signatures (δ¹³C, δ¹⁵N) of particulate organic matter sampled from a range of southeastern North Carolina waters, the predominant sources to the observed Org-N loadings were elucidated. Isotope data from across the Cape Fear River watershed indicated a large gradient of livestock waste-N contributions, with hog waste-N contributions consistently higher in the Northeast Cape Fear River watershed and with fertilizer-N contributions higher in the Black River watershed—findings that are consistent with each sub-basin's land usage. %N sediment content was positively correlated with hog waste-N contributions in the Black River watershed, indicating that sediments in CAFO-dense regions are reservoirs for agricultural nutrient pollution. Hog waste-N source contributions and %N sample contents for Black River sediments were strongly correlated with regional swine and poultry CAFO densities, establishing a strong connection between industrial animal production and stream sediment nutrient loads. Collectively, these findings suggest a major role of livestock waste, as well as human sewage, in driving the long-term Org-N increase in eastern North Carolina water bodies.