NITRATE SORPTION IN A VARIABLE-CHARGE FOREST SOIL OF THE PACIFIC NORTHWEST

Abstract

Despite the general acceptance that soils of the Pacific Northwest (United States) have a high retention capacity for phosphate (PO₄⁻³), and perhaps even sulfate (SO₄⁻²), few studies in the region have investigated the potential of physicochemical mechanisms to retain nitrate (NO₃⁻). The specific objectives of this study were to (i) determine the capacity of different horizons of a mesic, Typic Fulvudand under intensive forest management in southwestern Washington to sorb NO₃⁻, (ii) determine the point of zero net charge for each horizon of this soil, and (iii) relate specific mineralogical characteristics to the physicochemical soil properties. Five soil pits were excavated to a depth of approximately 150 cm, and soil samples were composited by genetic horizon, including A, AB, 2Bw1, and 2Bw2 horizons. Through batch equilibration, NO₃⁻ sorption isotherms were created for each horizon and showed an increase in sorption with both depth and increased NO₃-N solution concentrations. The point of zero net charge of the two Bw horizons was determined to exist between a pH range of 3.5 to 3.6. Selective dissolution techniques of the mineral soil were used to determine the presence of crystalline and noncrystalline aluminosilicates. Allophane and imogolite contents of the less than the 2-mm mineral soil fraction ranged from 0.6% to 3.0% across all observations, although noncrystalline forms of Fe and Al comprised a majority of the subsurface horizons, and increased with depth. The presence of variable charge soil components on this site, coupled with the acidic soil pH regime associated with the coniferous forest stand, allows for the concentration-dependent sorption of NO₃⁻, which may serve to retain a significant proportion of an otherwise highly mobile form of an essential plant nutrient.