Impacts of Rising Nitrogen Deposition on N Exports from Forests to Surface Waters in the Chesapeake Bay Watershed

In this study, we applied a process-based forest ecosystem model, PnET-CN, to estimate inorganic N (nitrate) loading and retention under chronic increases of atmospheric N deposition in the Chesapeake Bay (CPB) watershed. The results indicated that the average N leaching loss from forested lands in the CPB watershed is 1.23 kg N ha−1 y−1 at current N deposition levels, suggesting approximately 88% of N is retained by forest ecosystems. Total dissolved inorganic N exported from the forested watersheds was 11,617 Mg N yr−1. The predicted rates of the nitrate losses are well validated by the United States Geological Survey–National Water-Quality Assessment data measured from the gauged stations for forested drainages within the CPB watershed, and are also compatible with the field data of N loads associated with forests in the CPB watershed. If N deposition were twice current levels, the retention by forests would drop to 81%. Total N leaching loss to surface waters would then increase more than threefold. A nonlinear increase in N loads from forests under the extreme scenario of atmospheric N deposition shows the symptom of N saturation and an accelerated decline of forest functioning to retain atmospheric N deposition in the CPB watershed with rising levels of nitrogen deposition.