Inorganic nitrogen retention by watersheds at Fernow Experimental Forest and Coweeta Hydrologic Laboratory
Because elevated N loading can impair both terrestrial and aquatic ecosystems, understanding the abiotic and biotic controls over retention and export of dissolved inorganic N (DIN) is crucial. Long-term research has been conducted on experimental watersheds at two U.S. Forest Service experimental forests in the Appalachian region: Fernow Experimental Forest (FEF) in West Virginia and Coweeta Hydrologic Laboratory (CHL) in North Carolina. While similar in vegetation and research history, FEF and CHL differ in climate, historic DIN deposition, and soils. We evaluated long-term patterns of DIN inputs and exports from three watersheds at each location with similar treatments including clear-cut harvest, conversion to conifer plantation (Norway spruce [Picea abies (L.) H. Karst.] at FEF and white pine [Pinus strobus L.] at CHL), as well as reference watersheds. We examined DIN export and retention in these watersheds, comparing treated and reference watersheds within each experimental forest and comparing similarly treated watersheds between the experimental forests. Despite current similar levels of N deposition, stream water DIN concentrations and exports were generally greater at FEF by almost an order of magnitude. We found differences between FEF and CHL in stream DIN concentrations, watershed export, and retention of DIN inputs not only in the untreated reference watersheds but also in the watersheds with similar disturbance treatment. We hypothesize that these differences are the result of site and vegetation differences as well as site history including long-term patterns of DIN deposition. We document the switch from biogeochemical to hydrologic controls that occurred when N availability exceeded N immobilization, due to either N deposition or biological N inputs.