Low stream nitrate concentrations associated with oak forests on the Allegheny High Plateau of Pennsylvania

Stream nitrate (NO3-) concentrations were 2.6-7.0 times lower in watersheds in which red oak (Quercus rubra) stands were present than in other forested watersheds in northwestern Pennsylvania. Also, seasonal variations in NO3- concentrations were smaller in watersheds with oak stands than in watersheds without oak stands. Stream water concentrations of calcium, magnesium, potassium, sodium, chloride, and sulfate did not differ significantly between the two groups of watersheds. The variability in NO3- concentrations among streams did not appear to be due to differences in atmospheric nitrogen deposition among watersheds. Low stream NO3- concentrations occurred even if only 10-20% of a watershed was covered by oak stands. We suggest that oaks, rather than directly controlling NO3- losses, instead merely occur on soils which have low NO3- losses.