Mineral and Organic Matter Controls on the Sorption of Macronutrient Anions in Variable-Charge Soils

Partitioning ions between the solid and solution phase is one of the most important processes controlling nutrient mobility and bioavailability. Despite this, less research has focused on the interactions of nutrient anions at soil interfaces, although variable-charge components are present to some extent in nearly all soils. The objective of this study was to develop equations using commonly measured soil properties (particle size analysis, organic matter content, and extractable Fe and Al fractions) to predict sorption isotherms for NO₃⁻, SO₄²⁻, and H₂PO₄⁻. Six subsurface soils, ranging spatially and temporally from heavily weathered Oxisols of the tropics to a recently glaciated Entisol from the U.S. Pacific Northwest, were used to generate sorption isot herms of the three macronutrient anions using initial solution concentrations from 0.1 to 5 mmol L-1. Before batch sorption experiments, soils were saturated with KCl, rinsed free of excess salts, and adjusted to pH = 4.0 ± 0.1 to eliminate the confounding effects of competing ions or differing pH regimes. Almost all soils from temperate latitudes had a greater capacity to sorb anions than the Oxisols included in this study for comparison. This was particularly true for the soils with volcanic parent materials from the U.S. Pacific Northwest. For any given soil, the capacity to sorb the macronutrient anions was in the order H₂PO₄⁻ > SO₄²⁻ > NO₃⁻. Multiple regression analyses generally suggest that the electrostatic sorption of NO₃⁻ and SO₄²⁻ is positively related to the presence of active Al fractions and negatively correlated with organic C content.