Nitrogen is a critical nutrient for forests, and scarcity of nitrogen often limits forest growth. Theoretical considerations in ecology indicate that as a forest ages and its growth slows, its uptake of nitrogen from the soil should also slow, leading to increased loss of nitrogen in drainage water. Long-term measurements of nitrogen in streams draining the Hubbard Brook Experimental Forest in New Hampshire contradict this expected pattern. This project seeks to understand the discrepancy between the theory and the data by testing the hypothesis that young forests extract nitrogen from deep soils, but as the forest ages, re-accumulation of the nitrogen in the soil delays the loss of nitrogen in drainage water. The hypothesis will be tested by a field experiment to determine the propensity for nitrogen accumulation in soil beneath forest stands of different ages, and careful accounting of the amount of nitrogen in a sequence of forests ranging from 15 to over 200 years old. In addition, a computer simulation model of forest ecosystems will be revised to incorporate these new observations. This work is important for management of forests to minimize loss of nitrogen, a key plant nutrient. Air pollution adds excess nitrogen to the forest, and if it is lost in drainage water, it can pollute streams and lakes. The project will involve training of two graduate students and two postdoctoral associates, and a teacher training module will help two high school science teachers develop a unit for teaching about forest ecology in their classrooms.
