Effect of Soil Properties and Reclamation Practices on Phosphorus Dynamics in Reclaimed Calcareous Marsh Soils from the Guadalquivir Valley, SW Spain

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Saline and sodic soils pose major agronomic problems in arid and semiarid regions. Marsh soils in the Guadalquivir Valley are representative of saline and sodic soils in estuarine environments, and are frequently reclaimed for agricultural use. Flooding affects the nature of P sorbent surfaces in soils; it increases the proportion of poorly crystalline Fe oxides relative to highly crystalline ones, which predominate in the calcareous soils typical of the region. Poorly crystalline Fe oxides and silicate clays appear to be the main P sorbent surfaces in these soils. Carbonates seemingly have adverse effects on short-term P sorption, probably as a result of Fe oxides being partly occluded in them. Salinity, sodicity, and soil amendments can affect P forms and P dynamics in soil. Olsen P is negatively correlated (r = -0.81**) with the Na/Ca mole ratio, and positively correlated with sulfate concentration in the 1:1 extract (r = 0.75**), indicating that soils that receive large amounts of Ca amendment (e.g., phosphogypsum) exhibit increased contents in available P. The increased Ca saturation as a result of the amendment raises the short-term sorption capacity of sorbent surfaces and results in a major contribution of Ca phosphate precipitation to long-term P sorption, as suggests the relationship between d (exponent of time in the modified Freundlich equation) and the Na/Ca mole ratio in the 1:1 extract (Y = 0.26 X-0.38,R 2 = 0.66**). Resins can simulate P release to a solution with a near-zero P concentration and hence provide information about the P release potential of soil, which is agronomically and environmentally relevant. The release potential of a soil increases with increasing degree of P enrichment. Released P appears to be related to P fractions containing adsorbed P and the more soluble (citratebicarbonate extractable) Ca phosphates, and can be estimated by using a single-extraction method (Olsen P).