Phosphorus Sorption Characteristics of Flooded Soils

Abstract

Abstract Surface soils representing Alfisols and Inceptisols were collected from various parts of Louisiana under rice ( Oryza sativa L.) cultivation and incubated under oxidized (aerobic) and reduced (anaerobic) conditions for 2 weeks in a soil to 0.01 M CaCl 2 solution ratio of 1:5. The release of native soil P and the sorption of added inorganic P was investigated under these conditions. The soils selected for study were characterized for clay content, total carbon, extractable P, pH, and oxalate‐extractable Fe, soil properties associated with P sorption. The results show that generally more soil P was released under reduced than oxidized conditions, and this increase in soluble P under reduced conditions was significantly related to Bray no. 2 extractable P, clay content, and oxalate Fe. At high levels of added P, more P was sorbed under reduced conditions than under oxidized conditions in 14 out of 20 soils under study. However, in Midland fine sandy loam essentially all of added P was recovered in the equilibrium solution under both oxidized and reduced conditions, suggesting this soil had no capacity to sorb P. The relationship between P sorbed at 500 µg/g added P and oxalate‐extractable Fe was described by fitting two linear curves with different slopes. In the reduced soil samples the slope of Region I (≤ 3,000 µg Fe/g) suggests that 5.4 Fe sorbed 1 P and that this ratio did not change until all of the added P was sorbed. In the oxidized samples this ratio was about 16 Fe/1 P for Region I (≤4,500 µg Fe/g). The slopes for Region II of the linear curves for both reduced and oxidized samples were essentially zero. A closer relationship between P sorbed and oxalate extractable Fe under reduced conditions indicates that poorly crystalline and amorphous oxides and hydroxides of Fe play a primary role in P retention by flooded soils and sediments. A probable greater surface area generated by the transformation of oxyferric hydroxide to more reactive ferrous compounds may be responsible for greater P sorption under reduced conditions.