Nitrate variability in hydrological flow paths for three mid‐Appalachian forested watersheds following a large‐scale defoliation

Abstract

Nitrate (NO3−) leakage from forested watersheds due to disturbance is a well documented but not well understood process that can contribute to the degradation of receiving waters through eutrophication. Several studies have shown that large‐scale defoliation and deforestation events in small forested watersheds in the eastern United States cause immediate and dramatic increases in NO3− flux to steams, with large differences in recovery time. Water quality and discharge data collected from 1992 to 2004 following a large‐scale gypsy moth defoliation were used to investigate hydrological controls on long‐term NO3− leakage from three forested watersheds in Shenandoah National Park, Virginia. During storm events, a conventional two‐component hydrograph separation in conjunction with an inverse solution technique was employed to determine the concentrations of NO3− in groundwater and soil water. Following defoliation, groundwater NO3− concentrations declined exponentially with a distinct seasonal pattern. A rank‐order relationship between the rate constants associated with the exponential declines in groundwater NO3− concentrations and groundwater recession constants indicates a hydrological control on long‐term watershed recovery for these defoliated systems. Comparisons to deforested systems in Hubbard Brook, New Hampshire, and Coweeta, North Carolina, indicate hydrological controls are similarly present. Biogeochemical differences, however, need to be considered to account for the more attenuated recovery observed in defoliated systems. No long‐term trend was found in the model‐derived soil water NO3− concentrations, which suggests the presence of some form of rate limitation on the transformation of the nitrogen pool introduced during the disturbance and/or reduced nutrient uptake due to tree mortality.