Vegetation Controls on Nitrogen Cycling and Retention: Comparing Characteristics of “Tight” and “Leaky” Watersheds.

Anthropogenic sources of nitrogen (N) have altered the global N cycle to such an extent as to nearly double the rate of available N in many terrestrial ecosystems.   However, predicting the fate of N inputs has become increasingly difficult, as a multitude of environmental factors play major roles in determining N pathways.  This research investigates the role of specific vegetation and soil and forest floor characteristics in the production and export of different pools of C and N within two adjacent watersheds of the USDA Fernow Experimental Forest, WV.  These watersheds have identical management histories, varying only in vegetation cover: one watershed is a monoculture of Norway spruce (Picea abies) and the other has regenerated to native Appalachian hardwoods.  Long-term stream chemistry indicates that the hardwood stand exhibits symptoms of N-saturation, with a very large stream export of N (15 kg NO₃^--N/ha), though the spruce stream exhibits virtually no export of N.  Soil lysimeters were installed at two depths and several distances from the streams to investigate spatial and temporal patterns of water chemistry and the dynamics of multiple pools of C and N.  Water chemistry throughout the stream reach was analyzed monthly, along with multiple soil and forest floor characteristics, including C and N pools, pH, and exchangeable base cations.  Stream, soil water, and soil chemical analyses indicate that pH, NO₃^-, NH₄⁺ and total N, along with total N in mineral soil, are always much lower in the spruce system than in the hardwood.  The forest floor in the spruce has accumulated nearly twice the biomass and N per ha than the hardwood.  Thus, the forest floor in the spruce watershed may be a large sink for both C and N, through either biotic uptake of N or abiotic immobilization into humic compounds.