Overestimation of phosphorus adsorption capacity in reduced soils: An artifact of typical batch adsorption experiments

Although soil reduction often results in P release to soil solutions, many researchers have observed an increased maximum P adsorption (S_max) following soil reduction. We hypothesized that this result is an experimental artifact caused by exposure of the reduced soils to aerobic conditions and by the use of high P additions, which may result in precipitation. Four semiarid altered wetland soils were incubated under reduced conditions, followed by reoxidation, and their P-adsorption characteristics were measured under atmospheric and N₂-atmosphere conditions. During the reductive incubation, soluble P and Fe concentrations increased. In one of the soils, P and Fe were monitored after reoxidation and both were found to decrease. The reduction-reoxidation cycle has led to increased S_max values. Under an N₂ atmosphere, the equilibrium P concentrations at zero adsorption (EPC₀) of all soils were higher than those determined under atmospheric conditions, whereas no significant changes were observed in S _max values. Oversaturation of the equilibrating solutions with respect to P minerals suggested P precipitation and overestimation of S _max at high added P concentrations under both aerobic and N ₂-atmosphere conditions. We conclude that aerobic batch experiments of reduced soils are affected by P adsorption to newly in-tube-formed ferric oxides. In accordance, we stress the importance of the EPC₀ rather than the S_max as an informative measure of P adsorption, and the need for using low-P experiments and maintaining anaerobic conditions in evaluating P adsorption of reduced soils.