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Responses of Northern U.S. Forests to Environmental Change
ISBN 0-387-98900-5

Chapter 9: Nitrogen Saturation in Experimental Forested Watersheds

Ivan J. Fernandez and Mary Beth Adams

In the U.S the concept of “nitrogen saturation” is variously defined as (1) the absence of a growth response in vegetation to N additions, (2) the initiation of NO₃ leaching, or (3) the lack of net N accumulation in ecosystems as evidenced by an equivalence between inputs and losses. The region of the U.S. that is most susceptible to N saturation, due to high inputs of N, is the Northeast and in particular high elevation sites where deposition is the greatest. Many factors affect the susceptibility of a forest site to N saturation, including vegetation type, soil characteristics, and land use history.

Watershed-scale additions of N at both Bear Brook in Maine and the Fernow Experimental Forest in West Virginia have clearly shown that the majority of deposited N even in saturated ecosystems is retained. Somewhat scanty evidence suggests that some of the N is retained in vegetation, and that more is likely retained in the soil. However, the mechanisms responsible for N retention are unknown, as are the consequences for forest management and the potential for recovery if N inputs are reduced. There is no conclusive evidence that increased N inputs have resulted in increased vegetative growth

(a) Stream of NO₃ concentration time series for BBWM.

(b) Stream of NO₃ concentration time series for Fernow.

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