Summary
Heavy nitrogen loads can seriously affect water quality of streams and rivers, public water supplies, and downstream waters such as the Chesapeake Bay and the Gulf of Mexico, where "dead zones" can occur as a result of eutrophication (Eutrophication is the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen, phosphorus, or both). To manage forests for clean water, it is imperative that the cycling and retention of nutrients can be understood. Research conducted at two experimental forests in the Appalachian region, Coweeta Hydrologic Laboratory in North Carolina and the Fernow Experimental Forest in West Virginia, used long-term stream chemistry records and experimentally manipulated watersheds to explore the retention of nitrogen in forested watersheds. Differences in nitrogen retention were found between locations (Fernow vs. Coweeta), and also between watersheds that had received similar disturbance treatments. These differences are not only due to site and vegetation differences, but site history, particularly long-term atmospheric inputs. High levels of biological nitrogen inputs from nitrogen-fixing black locust trees can mimic the effects of many years of atmospheric deposition and create situations of high nitrogen availability within a watershed. When high levels of available nitrogen exist, the controls on nitrogen export from the watershed switch from biogeochemical to hydrologic processes, allowing the nitrogen to travel further downstream.
