Nitrogen biogeochemistry of three hardwood ecosystems in the Adirondack Region of New York

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The biogeochemistry of nitrogen (N) was evaluated for three forest ecosystems (Woods Lake (WL), Pancake-Hall Creek (PHC) and Huntington Forest (HF)) in the Adiron- dack region of New York, U.S.A. to evaluate the response of a range of N atmospheric inputs and experimental N additions. Bulk N deposition was higher at sites in the west than those in the central and eastern Adirondacks. These higher atmospheric N inputs were reflected in higher bulk throughfall fluxes of N (WL and PHC, 10.1 and 12.0 kg N ha −1 yr −1 , respec- tively) in the western Adirondacks than at HF (4.6 kg N ha −1 yr −1 ) in the central Adirondacks. Nitrogen was added to plots as (NH4)2SO4 at 14 and 28 kg Nh a −1 yr −1 or as HNO3 at 14 kg N ha −1 yr −1 . Litter decomposition rates of Fagus grandifolia and Acer rubrum were substantially higher at WL and PHC compared to HF but were not affected by experimental N additions. Results using mineral soil bags showed no effects of N addition on N and C concentrations in soil organic matter, but C and N concentration increases were less at WL and PHC compared to HF. Soil solution nitrate (NO − ) concentrations at 15-cm depth in the reference plots were higher at PHC than at WL and HF while at 50-cm concentrations were higher at PHC and WL than at HF. The reference plots at the two sites (WL and PHC) with the highest atmospheric inputs of N exhibited lower N retention (53 and 33%, respectively) than HF (68%) in reference plots. The greatest increase in NO − loss in response to the exper- imental treatments occurred at HF where the HNO3 additions resulted in the highest NO − concentrations and lowest N retentions. In contrast, at WL and PHC increases in soil water NO − were not evident in response to experimental N additions. The results suggest that the two sites (WL and PHC) in the western Adirondacks did not respond to additional N inputs although they have experienced elevated atmospheric N inputs and higher N drainage losses in reference plots than the HF site in the central Adirondacks. Some of these differences in site response may have also been a function of stand age of WL and PHC that were younger (24 and 33 years, respectively) than the HF (age ∼ 70). Highest NO − fluxes in the reference