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

The biogeochemistry of nitrogen (N) was evaluated for three forest ecosystems [Woods Lake (WL), Pancake-Hall Creek (PHC) and Huntington Forest (HF)] in the Adirondack 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, respectively) 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 N ha^-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 (NO₃^-) 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 NO₃^- loss in response to the experimental treatments occurred at HF where the HNO₃ additions resulted in the highest NO₃^- concentrations and lowest N retentions. In contrast, at WL and PHC increases in soil water NO₃ ^- 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 NO₃^- fluxes in the reference plots across the sites corresponded to higher δ¹⁵N values in soil and plants. An experimental addition experiment at PHC found that the forest floor and the mineral soil were the largest sinks for experimentally added N.