AIR-DRYING AND PRETREATMENT EFFECTS ON SOIL SULFATE SORPTION

Drying, freezing, and refrigeration are commonly employed to facilitate the handling and storage of soil samples on which chemical, biological, and physical analyses are to be performed. These laboratory protocol have the potential to alter soil chemical characteristics and may result in unrealistic estimates of in situ chemical processes. We determined the effect of air drying, storage temperature, and time on SO4-sorption characteristics. Sulfate-sorption experiments were conducted in the field on moist samples and compared with laboratory results for the same soils that had been kept frozen (-5-degrees-C), refrigerated (4-degrees-C) for 14 or 60 d, or air dried. Only air drying significantly altered SO4-sorption characteristics. The impact of air drying on SO4-sorption capacities was then determined on 29 pedons (84 horizons) from four northeastern USA (NE) and three southern Blue Ridge province states. A suite of physico-chemical properties of soils whose sorption capacities were affected by drying were also measured. The SO4-sorption capacity increased as much as 320% when dried soils were compared with field-moist samples. Soils whose sorption capacities were most affected by air drying included the NE soils group, which had a higher percentage of the clay fraction in the form of amorphous minerals and were high in exchangeable acidity and total organic C. Air drying caused significant decreases in ethylenediaminetetraacetic acid (EDTA) extractable Fe and Al, oxalate-extractable Al, and KCl-extractable Al. This suggests the formation of new solids on drying, which increase SO4 sorption. Development of a multivariate model, however, indicated that total organic C, change in Fe extracted by pyrophosphate, dry minus moist; change in Fe extracted by oxalate, dry minus moist; and H+ solution concentration in 0.01 M CaCl2, dry minus moist were the best predictors of the change in SO4 sorption capacity with drying.