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SOIL CHEMISTRY

Overestimation of Phosphorus Adsorption Capacity in Reduced Soils: An Artifact of Typical Batch Adsorption Experiments

^a Dep. of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew Univ. of Jerusalem, Rehovot 76100, Israel
^b Dep. of Environmental Sciences, Tel-Hai Academic College, Upper Galilee 12210, Israel

^* Corresponding author (Shenker{at}agri.huji.ac.il).

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 N₂–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.

Abbreviations: CBD, citrate–bicarbonate–dithionite • CDP, cultivated degraded peat soil • EC, electrical conductivity • EPC₀, equilibrium P concentration at zero adsorption • FDP, fallow degraded peat soil • Fe_d and Fe_ox, Fe extracted by citrate–bicarbonate–dithionite and oxalate, respectively • NDNCP, nondegraded noncalcareous peat soil • OM, organic matter • S_max, maximum P adsorption