Long-term nitrogen dynamics of Coweeta forested watersheds in the southeastern United States of America

Abstract

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 ozone events) on reference watersheds. Reference watersheds were characterized as highly conservative of inorganic N with deposition < 9.0 kg ha-1 yr-1 and stream water exports below 0.25 kg ha-1 yr-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 NO3- in stream water and increases in the seasonal amplitude and 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 distributed watershed at Coweeta (hardwood converted to grass, fertilized, limed, treated with herbicide, and subsequendy characterized by successional vegetation). Compared to other intensive management practices, prescribed burning had little effect on stream water NO3- concentrations, and stream NO3- losses associated with natural disturbances are small and short-lived.