Linking Nitrate Production to Chemical and Biotic Controls in Soils Developed Beneath Hardwood and Conifers.

Anthropogenic sources of nitrogen (N) have altered the global N cycle to such an extent as to nearly double the rate of N that enters many terrestrial ecosystems.   However, predicting the fate of N inputs continues to present challenges, as a multitude of environmental factors play major roles in determining N pathways.  This research investigates soil chemical and biotic factors as they relate to the production of NO₃^- from soils within two adjacent watersheds in the USDA Fernow Experimental Forest, WV.  These watersheds receive high loads of N deposition and have identical management histories, varying only in vegetation cover, where one watershed is a planted monoculture of Norway spruce (Picea abies) following clearcut felling 40 years ago, and the other has regenerated to native Appalachian hardwood.  Long-term stream chemistry indicates that the hardwood stand has approached N-saturation, with a very large stream export of N (15 kg NO₃^--N/ha/yr), whereas the spruce stand exhibits virtually no export of N to the stream.  A soil incubation study was performed, utilizing soils collected from sites within each watershed.  Soils were incubated in mixed ratios of hardwood:spruce soils (ranging from pure hardwood soil to 0.75: 0.25, 0.5: 0.5, 0.25: 0.75 mixes, to pure spruce soil) in order to generate a gradient of soil chemical and biological properties.  Incubated soils were assayed for NO₃^- production over 28 days, along with a multitude of chemical and biotic measures, including pH, exchangeable aluminum, nutrient stochiometry, microbial populations, organic acid concentrations, and C and N fractions.  A correlation analysis was used to determine factors most related to the amount of NO₃^- that was produced.  Results from this study may be useful in two ways:  for the prediction of watersheds predisposed to N saturation, and for the refinement of downstream ecosystem water quality models.