Long-term nitrogen dynamics of Coweeta forested watersheds in the southeastern United States of America
We analyzed long-term (23 years) data of inorganic N deposition and loss for an extensive network of mature mixed hardwood covered watersheds in the southern Appalachians of North Carolina to assess trends and dynamics of N in baseline ecosystems. We also assessed watershed N saturation in the context of altered N cycles and stream inorganic N responses associated with management practices (cutting prescriptions, species replacement, and prescribed burning) and with natural disturbances (drought and wet years, insect infestations, hurricane damage, and ou>ne events) on reference watersheds. Reference watersheds were characterized as highly conservative of inorganic N with deposition < 9.0 kg ha-1yr-1 and stream water exports below 0.25 kg ha-1yr-1. However,reference watersheds appeared to be in a transition phase between stage 0 and stage 1 of watershed N saturation as evidenced by significant time trend increases in annual flow-weighted concentrations of N3- in stream water and increases in the seasonal amplitude aud duration of NO3 concentrations during 1972-1994. These stream water chemistry trends were partially attributed to significant increases in NO3- and NH4+ concentrations in bulk precipitation over the same period and/or reduced biological demand due to forest maturation. Levels and annual patterns of stream NO3- concentrations and intra-annual seasonal patterns characteristic of latter phases of stages 1 and 2 of watershed N saturation were found for low-elevation and high-elevation clear-cut watersheds, respectively, and were related to the dynamics of microbial transformations of N and vegetation uptake. Evidence for stage 3 of N saturation, where the watershed is a net source of N rather than a N sink, was found for the most disttibutcd watershed at Coweeta (hardwood converted to grass, fertilized, limed, treat& with herbicide, and snbsequently characterized by successional vegetation). Compared to other intensive mauagement practices, prescribed burning had little effect on stream water NO3- concentrations, aud stream NO3- losses associated with natural disturbances are small and short-lived.