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

摘要

The biogeochemistry of nitrogen (N)was evaluated for three forest ecosystems[Woods Lake (WL), Pancake-Hall Creek (PHC) andHuntington Forest (HF)] in the Adirondackregion of New York, U.S.A. to evaluate theresponse of a range of N atmospheric inputsand experimental N additions. Bulk Ndeposition was higher at sites in the westthan those in the central and easternAdirondacks. These higher atmospheric N inputswere reflected in higher bulk throughfallfluxes of N (WL and PHC, 10.1 and 12.0 kg Nha−1 yr−1, respectively) in thewestern Adirondacks than at HF (4.6 kg Nha−1 yr−1) in the centralAdirondacks. Nitrogen was added to plots as(NH4)2SO4 at 14 and 28 kg Nha−1 yr−1 or as HNO3 at 14 kg Nha−1 yr−1. Litter decompositionrates of Fagus grandifolia and Acerrubrum were substantially higher at WL andPHC compared to HF but were not affected byexperimental N additions. Results usingmineral soil bags showed no effects of Naddition on N and C concentrations in soilorganic matter, but C and N concentrationincreases were less at WL and PHC compared toHF. Soil solution nitrate (NO3 −)concentrations at 15-cm depth in the referenceplots were higher at PHC than at WL and HFwhile at 50-cm concentrations were higher atPHC and WL than at HF. The reference plots atthe two sites (WL and PHC) with the highestatmospheric inputs of N exhibited lower Nretention (53 and 33%, respectively) than HF(68%) in reference plots. The greatestincrease in NO3 − loss in response tothe experimental treatments occurred at HFwhere the HNO3 additions resulted in thehighest NO3 − concentrations andlowest N retentions. In contrast, at WL andPHC increases in soil water NO3 −were not evident in response to experimental Nadditions. The results suggest that the twosites (WL and PHC) in the western Adirondacksdid not respond to additional N inputsalthough they have experienced elevatedatmospheric N inputs and higher N drainagelosses in reference plots than the HF site inthe central Adirondacks. Some of thesedifferences in site response may have alsobeen a function of stand age of WL and PHCthat were younger (24 and 33 years,respectively) than the HF (age ∼ 70).Highest NO3 − fluxes in thereference plots across the sites correspondedto higher δ15N values in soil andplants. An experimental addition experimentat PHC found that the forest floor and themineral soil were the largest sinks forexperimentally added N.