Variation in Forest Soil Properties along a Great Lakes Air Pollution Gradient

Abstract

Abstract A pronounced air‐pollution gradient exists across the Great Lakes region, with deposition of SO 4 , NO 3 , and H increasing from northern Minnesota to southern lower Michigan. Soils at six northern hardwood sites along this gradient were examined to characterize soil physical and chemical properties relevant to retention of pollutants, and to investigate the impact of differences in pollutant loading on soil chemical properties. Three randomly located pedons at each site were described and sampled. Soils at all sites were classified in closely related subgroups within the Spodosol order (Entic Haplorthods, Typic Haplorthods, Alfic Haplorthods, and Alfic Fragiorthods). Cation‐exchange capacity and SO 4 ‐adsorption potential tended to decrease from north to south along the gradient, largely related to trends in inherent soil properties. In upper B horizons, additional significant positive deposition effects on water‐soluble and adsorbed SO 4 , and negative deposition effects on SO 4 ‐adsorbing potentials were consistent with hypothesized pollutant impacts on soil SO 4 pools and soil capacities to retain additional atmospheric SO 4 . Exchangeable nutrient‐cation contents (Ca, Mg, K) in upper B horizons tended to decrease as pollutant deposition increased. Regression analyses suggested that observed trends in nutrient‐cation contents were primarily related to inherent soil properties such as clay, organic matter, and cation‐exchange capacity. Differential effects of pollutant deposition and natural acidification processes on nutrient‐cation reserves could not be separated at this time.