SIMULATED EFFECTS OF ATMOSPHERIC SULFUR DEPOSITION ON NUTRIENT CYCLING IN A MIXED DECIDUOUS FOREST

The effects of three S deposition scenarios - 50% reduction, no change , and 100% increase - on the cycles of N, P, S, K, Ca, and Mg in a mix ed deciduous forest at Coweeta, North Carolina, were simulated using t he Nutrient Cycling model (NuCM). The purpose of this exercise was to compare NuCM's output to observed soil and streamwater chemical change s and to explore NuCM's response to varying S deposition scenarios. Ec osystem S content and SO42- leaching were controlled almost entirely b y soil SO42- adsorption in the simulations, which was in turn governed by the nature of the Langmuir isotherm set in the model. Both the sim ulations and the 20-year trends in streamwater SO42- concentration sug gest that the ecosystem is slowly becoming S saturated. The streamwate r data suggest S saturation is occurring at a slower rate than indicat ed by the simulations, perhaps because of underestimation of organic S retention in the model. Both the simulations and geld data indicated substantial declines in exchangeable bases in A and BA soil horizons, primarily due to vegetation uptake. The correspondence of model output with field data in this case was a result of after-the-fact calibrati on (i.e. setting weathering rates to very low values) rather than pred iction, however. Model output suggests that soil exchangeable cation p ools change rapidly, undergoing annual cycles and multi-decade fluctua tions. Varying S deposition had very little effect upon simulated vege tation growth, nutrient uptake, or N cycling. Varying S deposition str ongly affected simulated Ca2+, Mg2+, K+, and P leaching but caused lit tle change in soil exchangeable pools of Ca2+ K+, or P because soil ex changeable pools were large relative to fluxes. Soil exchangeable Mg2 pools were reduced by high rates of S deposition but remained well ab ove levels sufficient for tree growth. Although the total soil pools o f exchangeable Ca2+ and K+ were only slightly affected by S deposition , there was a redistribution of Ca2+ and K+ from upper to lower horizo ns with increasing S deposition, causing increased base saturation in the deepest (BC) horizon. The 100% increased S deposition scenario cau sed increasing peaks in simulated Al3+ concentrations in A horizons af ter 25 years as a result of large seasonal pulses of SO42- and lowered base saturation. Simulated soil solution Al3+ concentrations remained well below toxicity thresholds for selected tree species at the site.