Evaluation of the effects of future controls on sulfur dioxide and nitrogen oxide emissions on the acid-base status of a northern forest ecosystem

The integrated biogeochemical model, PnET-BGC, was used to simulate the response of soil and surface water at the reference watershed (W6) at the Hubbard Brook Experimental Forest, New Hampshire, to changes in atmospheric deposition. The performance of the model was assessed using two objective statistical criteria, the normalized mean absolute error, and the efficiency, in order to compare simulated results with observed values between 1980 and 1998. Model results showed good agreement with measured concentrations of stream Ca²⁺, and SO₄^2-, while stream NO₃^- and Al concentrations and soil solution Ca/Al ratios were over predicted after 1990. Model simulations showed that there was some improvement in soil and stream chemistry in response to the 1990 Amendments to the Clean Air Act (CAAA) compared to conditions without this legislation. However, the 1990 CAAA will not result in substantial changes in critical indicators (e.g. soil base saturation, soil solution Ca/Al, stream pH, acid neutralizing capacity (ANC) and Al concentrations). The slow recovery rates suggest that additional reduction in strong acid inputs will be required to significantly alleviate ecosystem stress from acidic deposition. Simulation of the impact of equivalent reductions in SO₄^2- and NO₃^- deposition indicated slightly greater recovery under the SO₄^2- reductions compared with NO₃^-. An inter-annual pattern of stream NO₃^- concentrations suggests that nitrification under snowpack is a significant source of N in the ecosystem. Vegetation N uptake during summer greatly limits NO₃^- loss, and as a result, summer utility controls of NO_x emissions will not significantly mitigate stream NO₃^- loss and associated surface water acidification.