Phosphorus availability and sorption under alternating waterlogged and drying conditions

Abstract

The effect of alternating waterlogged and drying conditions on phosphorus (P) availability and sorption was studied in three soils of contrasting chemical and physical properties. Soils were treated with two levels of P (0 and 50 mg kg‐1; P0 and P50), waterlogged for 21 days, then allowed to dry at room temperature for 14 days. The availability of P, iron (Fe), and manganese (Mn) over the waterlogged and drying periods was determined by shaking samples of each soil with 1M NaOAc (pH 3). Increasing concentrations of 1M NaOAc (pH 3) extractable P (Pac) over the waterlogged period was attributed to solubilization of Fe(OH)3 materials under reducing conditions with the release of sorbed and occluded P. The released P appeared to be resorbed by freshly precipitated amorphous Fe(OH)2 material since earlier studies showed that water soluble P concentrations in these soils were reduced to negligible levels under waterlogged conditions. The Fe(OH)2 material remained readily extractable with 1M NaOAc (pH 3) since Feac increased dramatically with waterlogging. Drying the waterlogged soils caused a rapid decrease in Pac, Feac and Mnac suggesting the Fe(OH)2 may have been transformed into more stable forms [e.g., Fe(OH)3]. Much of the changes in Pac appeared to be due to changes in Feac, with limited influence from Mnac. and mineralization of organic P. Phosphate sorption isotherms were determined using the standard batch technique for air‐dry, waterlogged (with and without ponded water), and waterlogged/dried conditions. Sorption isotherms were not affected by waterlogging and subsequent drying. Most soils sorbed all of the added phosphate irrespective of moisture treatment.