Abstract:

Although historic land use is widely recognized as an important determinant of watershed N cycling, efforts to examine land use legacy effects are limited by incomplete historical data. This research evaluates N isotopes of sedimented organic matter (¹⁵N_org), in a palynological context, as a long-term proxy of changes in N source to wetland biota. N and S isotope measurements of organic sediments, fossil plant fragments, and living plants are used to explore isotope stratigraphies of wetland sediment cores. Processes potentially contributing to isotope stratigraphies are investigated including the following: a change in N source, diagenesis, and denitrification. We document the ¹⁵N_org stratigraphy of a core from the Smithsonian Environmental Research Center, MD, U.S.A. spans approximately 350 years, during which time ¹⁵N_org increases from +2” to +7”. Reconstructed population density and wastewater inputs to the watershed suggest that the increase in ¹⁵N reflects changing land use from forested conditions to increasing nutrient inputs from human waste. Our results illustrate the importance of hydrologic connectivity in delivering waste-derived N in a watershed characterized by relatively low human population density. These results also demonstrate how this approach can expand the temporal horizon over which we can assess human impacts to watershed N dynamics.