Stage III N-saturated forested watershed rapidly responds to declining atmospheric N deposition

Abstract

This study used a mass balance approach by characterizing the input, output, and sink rates of N in order to assess a declared "stage III N-saturated forest" response to decreased atmospheric N deposition in western Maryland. Relying on the conceptual model of kinetic N-saturation to holistically link stream, vegetative, soil, and atmospheric compartments and the use of a novel stable isotopic technique, the study demonstrated dynamic soil NO₃-N pools, unprocessed atmospheric NO₃-N in base flow, and significant reductions in NO₃-N yield in response to decreased atmospheric N deposition. A lumped conceptual model, incorporating a dormant season NO₃-N flush, was proposed that explains forest response to decreased deposition and sheds light on the hydrologic processes that govern the storage/release of NO₃-N among years. It is proposed that this flushing mechanism prevents forests from attaining higher stages of N-saturation and predicts forests will be responsive to further reductions in N deposition.