Interaction of atmospheric nitrogen regulation, climate change, and elevated CO2 on the long term productivity of forested ecosystems in the Chesapeake Bay watershed

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John Hom
Yude Pan
Kevin Mccullough
Richard Birdsey

The Chesapeake Bay watershed is the largest estuary in the United States. It is the first estuary in the U.S. targeted for intensive government efforts to reduce nutrient loading into the Chesapeake Bay. Upland forested watersheds make up 60% of the land area and provide ecosystem services in retaining N from leaching into the estuary. Our objective is to understand how forest productivity and N retention will change under different N regulation scenarios and long term N deposition using the PnET-CN ecosystem model in a factorial design (with the interaction of CO2, O3, N, and climate). Using a constant N deposition scenario held at the year 2000 level, N retention decreases from 88% to 84% by 2050, and declines to 77% N retention by the year 2100, indicating that forest watersheds will start to saturate in the future under constant 10 kg ha-1 yr-1 scenario.
With decreasing N deposition under current regulation scenarios, N retention will stay at 88% in 2050, however retention begins to decrease by 2100 to 77%-81%. All N only scenarios result in small increases in forest NPP with additional N. Adding CO2 to the N regulation scenarios through 2050 resulted in ~36% increase in net primary productivity (NPP), with forests able to use and retain more atmospheric N.
The response of Chesapeake Bay forested watersheds to N deposition is a dynamic process, influenced by changing air quality regulations and multiple factors such as rising CO2.