Factors Affecting Anion Movement and Retention in Four Forest Soils

Three hypotheses concerning the movement and retention of anions in forest soils were tested in a series of laboratory and field studies on two Tennessee Ultisols with mixed deciduous forest cover and two Washington Inceptisols, one with deciduous (red alder Alnus rubra Bong.) and one with coniferous [Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco] forest cover. The first hypothesis, that sulfate and phosphate retention was related to adsorption to free Fe and Al oxides, which were in turn related to soil parent material and degree of weathering, was not supported by results of laboratory and field studies. The young, relatively unweathered Washington Inceptisols adsorbed more phosphate and sulfate than the older, highly weathered Tennessee Ultisols, apparently because of greater amorphous (oxalate-extractable) Fe and Al in the former. The second hypothesis, that NO-3 retention was governed primarily by biological uptake, was supported. Nitrate adsorption by soils in laboratory column studies was negligible, but subsequent field studies showed that tree uptake in field plots greatly reduced the leaching of applied NO-3 in all but the N-rich red alder plot. The third hypothesis, that inputs of mobile anions will reduce pH and concentrations of bicarbonate and adsorbing anions (e.g., sulfate), were supported by application of chloride as both acid and Na and Ca salts to soil columns in the laboratory. Sulfate concentration as well as bicarbonate concentrations in soil solutions were sensitive to solution pH