Declines in concentrations of NO₃ stream water have been reported for several forest ecosystems in the eastern United States.  This study examined long-term patterns of net nitrogen (N) mineralization and nitrification in soils of control and treatment watersheds at Fernow Experimental Forest, West Virginia, and specifically compared temporal patterns of net nitrification with patterns of stream NO₃ for one N-treated [35 kg N ha^-1 yr^-1 as (NH₄)₂SO₄] and two untreated watersheds.  Stream water was sampled weekly from all three watersheds from 1983 to 2006 and analyzed for NO₃. Mineral soil was collected to 5 cm at five random points within each of seven plots per watershed; these were composited, mixed, and placed in two polyethylene bags.  One bag from each pair was brought back to the laboratory for immediate extraction and analysis; the other bag was incubated in situ 5 cm beneath the mineral soil surface for various monthly intervals in the years 1993-1995, 2002, 2005, and 2007.  

Results/Conclusions

Temporal patterns for net N mineralization and nitrification were similar for all study watersheds, generally increasing during from 1993 to 1995 and then decreasing thereafter.  By 2007, rates of both processes had declined on all watersheds to ~0.8 µg N g^-1 soil d^-1, or an overall decrease of ~75% of maximum of ~3.0 µg N g^-1 soil d^-1.  Despite these fluctuations in net N mineralization and nitrification from 1993-2007, relative nitrification remained close to 100% on all watersheds.  Temporal patterns of net N mineralization and nitrification were not significantly correlated with any ambient precipitation or temperature variables. Significant correlation between mean annual stream NO₃ and mean net nitrification supports the hypothesis that recent declines in stream NO₃ are caused, in part, by N-mediated declines in net nitrification.