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

The biogeochemistry of nitrogen (N) was evaluated for three forest ecosyste ms [Woods Lake (WL), Pancake-Hall Creek (PHC) and Huntington Forest (HF)] i n the Adirondack region of New York, U.S.A. to evaluate the response of a r ange of N atmospheric inputs and experimental N additions. Bulk N depositio n was higher at sites in the west than those in the central and eastern Adi rondacks. These higher atmospheric N inputs were reflected in higher bulk t hroughfall fluxes of N (WL and PHC, 10.1 and 12.0 kg N ha(-1) yr(-1), respe ctively) 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)(2)SO4 at 14 a nd 28 kg N ha(-1) yr(-1) or as HNO3 at 14 kg N ha(-1) yr(-1). Litter decomp osition rates of Fagus grandifolia and Acer rubrum were substantially highe r at WL and PHC compared to HF but were not affected by experimental N addi tions. 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 inc reases 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 th an at WL and HF while at 50-cm concentrations were higher at PHC and WL tha n 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%, respectiv ely) than HF (68%) in reference plots. The greatest increase in NO3- loss i n response to the experimental treatments occurred at HF where the HNO3 add itions resulted in the highest NO3- concentrations and lowest N retentions. In contrast, at WL and PHC increases in soil water NO3- were not evident i n response to experimental N additions. The results suggest that the two si tes (WL and PHC) in the western Adirondacks did not respond to additional N inputs although they have experienced elevated atmospheric N inputs and hi gher N drainage losses in reference plots than the HF site in the central A dirondacks. 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, re spectively) than the HF (age similar to 70). Highest NO3- fluxes in the ref erence plots across the sites corresponded to higher delta N-15 values in s oil and plants. An experimental addition experiment at PHC found that the f orest floor and the mineral soil were the largest sinks for experimentally added N.