Modeling the response of forest and aquatic ecosystems to changes in atmospheric deposition

An integrated biogeochemical model (PnET-BGC), which operates on a monthly tine step, was formulated to simulate chemical transformations of vegetation, soil and drainage water in northern forest ecosystems. The validity of the model was tested against soil and stream data at the Hubbard Brook Experimental Forest (HBEF), New Hampshire. Predictions of concentrations and fluxes of major elements generally agreed reasonably well with measured values. Model output of soil base saturation and stream acid neutralizing capacity (ANC) were sensitive to parameter values of soil partial pressure of carbon dioxide (CO 2 ), soil mass, soil cation exchange capacity (CEC) and soil selectivity coefficients of calcium (Ca 2+ ) and aluminum (Al n+ ). PnET-BGC simulation of sulfur (S) dynamics at the HBEF using a measured dryto-bulk deposition ratio of 0.21 resulted in an underprediction of soil S pools and stream sulfate (SO 4 2- ) concentrations, and assumption on the additional source of S affects model forecasts of the future response to anticipated decreases in S deposition. Long-term simulations showed that elevated S deposition has increased stream SO 4 2- concentrations, and the enhanced incorporation of Sin adsorbed SO 4 2- and organic S soil pools. Following the implementation of the 1970 Amendment to the Clean Air Act, there has been a net release of S from soil pools. Simulation results also indicate that atmospheric deposition of both strong acids and basic cations, and historical forest cutting practices have resulted in changes in the soil base saturation, the stream pH and ANC from pre-industrial estimates of ∼20%, 6.3 and 43 μeq L -1 , respectively, to current values of ∼10%, ∼5.0 and ∼-5 μeq L -1 , respectively. While historical forest cutting and vegetation accumulation had little effect on the long-term depletion of base cations from soil exchange pools, model predictions of exchangeable cation pools and stream chemistry were markedly affected by deposition of strong acids and basic cation, with the deposition of strong acid anions having the most impact. Simulations of previous and anticipated controls on emissions indicate that the extent of recovery of the forest soils and stream waters depends on the degree of the emission controls imposed. Model predictions suggest that ecological indicators of the sensitivity to soil and water to acid deposition at the HBEF are within the critical thresholds suggested to have adverse effects on forest growth