Patterns of nitrogen availability within a forested watershed exhibiting symptoms of nitrogen saturation

Abstract

Watershed 4 (WS 4) at the Fernow experimental forest in West Virginia shows several symptoms of N saturation. Surprisingly, however, past measurements of N 2O production suggest that a portion of WS 4 (the south-facing slopes) may still be strongly limited by the lack of available N. To examine this possibility more fully, we measured four indices of N availability in two sections of WS 4-slopes with easterly aspects that are located on one side of the main stream channel and slopes with southerly aspects that are located on the opposite side of the stream. Compared to the portion of WS 4 with more east-facing slopes (78% of the total area), we found that south-facing slopes had lower extractable NO 3 − pools (14 versus 554 mg N m −2), lower rates of net nitrification (2 versus 35 mg N m −2 day −1), a greater response of root growth to N-rich microsites (4.5× versus 1.6× > controls), and lower concentrations of NO 3 − in water leaching below the A, B, and C soil horizons (ca. 0.066 versus 2.3 mg N l −1). The two sections of WS 4 also differed in the composition of their woody vegetation. Slopes with more easterly aspects had a greater relative importance of Acer saccharum and Prunus serotina. The south-facing slopes were characterized by a greater importance of Nyssa sylvatica and Fagus grandifolia. From these results we hypothesize that aspect-related differences in species composition can strongly influence the susceptibility of a forested stand to the early onset of N saturation. If this hypothesis is proven, then community composition may account for a significant proportion of the variable response of forested watersheds to similar levels of elevated N deposition. It would also imply that management practices which favor certain species might delay or accelerate the onset of N saturation and the potentially negative changes associated with this process.