Forest nitrogen sinks in large eastern U.S. watersheds: estimates from forest inventory and an ecosystem model

Abstract

The eastern U.S. receives elevated rates of N deposition compared to preindustrial times, yet relatively little of this N is exported in drainage waters. Net uptake of N into forest biomass and soils could account for a substantial portion of the difference between N deposition and solution exports. We quantified forest N sinks in biomass accumulation and harvest export for 16 large river basins in the eastern U.S. with two separate approaches: (1) using growth data from the USDA Forest Service’s Forest Inventory and Analysis (FIA) program, and (2) using a model of forest nitrogen cycling (PnET-CN) linked to FIA information on forest age-class structure. The model was also used to quantify N sinks in soil and dead wood, and nitrate losses below the rooting zone. Both methods agreed that net growth rates were highest in the relatively young forests on the Schuylkill watershed, and lowest in the cool forests of northern Maine. Across the 16 watersheds, wood export removed an average of 2.7 kg N ha⁻¹ yr⁻¹ (range: 1–5 kg N ha⁻¹ yr⁻¹), and standing stocks increased by 4.0 kg N ha⁻¹ yr⁻¹ (−3 to 8 kg N ha⁻¹ yr⁻¹). Together, these sinks for N in woody biomass amounted to a mean of 6.7 kg N ha⁻¹ yr⁻¹ (2–9 kg N ha⁻¹ yr⁻¹), or 73% (15–115%) of atmospheric N deposition. Modeled rates of net N sinks in dead wood and soil were small; soils were only a significant net sink for N during simulations of reforestation of degraded agricultural sites. Predicted losses of nitrate depended on the combined effects of N deposition, and both short- and long-term effects of disturbance. Linking the model with forest inventory information on age-class structure provided a useful step toward incorporating realistic patterns of forest disturbance status across the landscape.