Spodosol Variability and Assessment of Response to Acidic Deposition

Variability in forest soils makes it difficult to observe short‐term changes in chemical properties under field conditions. A buried soil‐bag technique was developed to examine the chemical response of a Maine forest soil to loadings of strong acids (HNO 3 and H 2 SO 4 ). Acids were added by irrigation on 18 hardwood plots, 15 by 15 m, American beech ( Fagus grandifolia Ehrh.) dominating. Treatments, with three replicates each, were control, low S, high S, low N, high N, and N plus S. Soils were typical northeastern Spodosols, with acidic forest floors (pH 3.6) and upper mineral soils (pH 3.4‐4.0), low concentrations of base cations in mineral horizons, and enrichment of the spodic horizons with organic C, N, and S. Relative standard deviations (RSD) of horizon means for all chemical properties except pH showed large variability (e.g., RSDs for exchangeable Ca 2+ were 42‐95%) over the small (2.2 ha) area encompassed by the plots. Twenty‐five 250‐µm‐mesh nylon bags filled with 300 g of homogeneous B horizon soil (Tunbridge series, a coarse‐loamy, mixed, frigid Typic Haplorthod) were placed directly below the forest floor in each plot. After 1 yr of treatment, collection and analysis of three bags per plot showed significant ( P ≤ 0.05) differences. Mean adsorbed SO 2‐ 4 concentrations were 48 and 92 µg S g −1 in control and high‐S treatments, respectively. Exchangeable Ca 2+ and Mg 2+ were greater in the high‐S treatment than the control; base saturation increased from 5.9 to 8.5% (possibly due to displacement of cations from the forest floor). The buried soil‐bag technique detected small alterations in forest soil chemistry under field conditions, with minimal disturbance to study plots.