Transformation and bioavailability of specifically sorbed phosphate on variable-charge minerals in soils

Abstract

Crop production on red soils in China is largely limited by the low availability of phosphorus, which is frequently attributed to the adsorption of phosphate by variable-charge minerals including Fe and Al oxides and kaolinite. Isotopic tracing analysis and soil incubation were carried out to investigate the desorption and microbial transformation of applied specifically sorbed P in two pH-contrasting light-textured soils. A rapid release of P from the added mineral-P surface complex in the two tested soils was observed. Most of the released P was recovered in a 0.5MNaHCO₃ extract and in soil microbial biomass. Microbial biomass-³²P was detected at early stages of incubation and reached up to 10–30% of the added ³²P. Approximately 50–70% of the added complex ³²P, varying between minerals and soils, was extractable in the 0.5MNaHCO₃ at 75 days after incubation for the acid soil but up to 120 days for the neutral soil. Microbial biomass-P plus 0.5MNaHCO₃-extractable ³²P accounted for more than 60–80% of total added complex-³²P, implying high desorption and transformation of the specifically sorbed P in the two soils. There was more inorganic ³²P than organic ³²P in the NaHCO₃ extract, suggesting that chemical release of specifically sorbed P was dominant. Ligand exchange and chemical desorption due to a change in environmental conditions such as pH and ionic strength are likely the major mechanisms responsible for the chemical release of specifically sorbed ³²P in the tested soils.