ANION RETENTION MECHANISMS IN FOUR FOREST SOILS--RELEVANCE TO THE LEACHING PROCESS (WASHINGTON, TENNESSEE)

The primary objective of this study was to evaluate the importance of anion retention in the regulation of cation leaching of four soils in two contrasting soil weathering status and fertility levels. The anions examined were nirate, chloride, sulfate, and bicarbonate. The soils tested reflected this wide range in soil properties. In Washington an Alderwood soil (Dystric Durochrept) with a low fertility (Douglas-fir forest cover) and a high fertility (alder forest cover) was selected. These soils having a low weathering status were in turn compared to two heavily weathered soils in Tennessee, a low fertility Fullerton soil (Typic Paleudult) and a high fertility Tarklin soil (Typic Fragiudult).;The results support the hypotheses. Anion retention was increased by the increasing crystalline and amorphous forms of Fe and Al, and carbon containing competing organic materials for anion adsorption sites decreased anion retention by the soils. The proportion of sulfate and phosphate decreased in leachates in lower horizons through the leaching process. Soils of a higher adsorption capacity had proportionally more phosphate and sulfate adsorption in the surface horizon.;Bicarbonate competes with the other anions for adsorption sites. Its retention capacity was lower than phosphate but higher than sulfate, chloride, and nitrate. The leaching of nitrate did not affect the leaching of sulfate and phosphate, however, as the corollary hypothesis suggested. Chemical adsorption was the dominant mechanism in regulation of phosphate and sulfate leaching in Douglas-fir and red alder soils.;The main hypotheses were: (1) retention capacity of anions (or mobility of anions) relates to Fe and Al content in soils, and organic matter depresses the retention capacity of anions. The corollary hypotheses for (1) were: high adsorption capacity of soils for sulfate and phosphate causes the proportion of sulfate and phosphate to decrease in leachates in lower horizons through the leaching process: and high available nitrogen increases phosphate and sulfate leaching due to greater nitrate leaching (produced by nitrification); (2) bicarbonate anions with anions from nitrate, chloride, sulfate, and phosphate for retention sites; its retention capacity was stronger than nitrate and chloride, but lower than phosphate and sulfate.