Chemical speciation of phosphorus in farmland soils and soil aggregates around mining areas

Phosphorus (P) speciation determines its bioavailability, mobility, and environmental risk in soils. Farmland soils around mining areas are worldwide distributed, yet the P speciation in such soils remains poorly understood. In this study, we determined the P speciation in the farmland soils and soil aggregates around mining areas from China and evaluated their relationship with soil pH and Fe/Al fractions using chemical extraction, P K-edge X-ray absorption near-edge (XANES) spectroscopy, and correlation analysis. Chemical extraction revealed that soil P was dominated by inorganic P (Pi) (68.0 - 82.5%) largely associated with pedogenic Fe/Al oxides (43.4% - 94.8%). The P was mainly adsorbed and occluded by Fe/Al oxides in acidic soils, while predominantly existed as Ca-P and occluded species in alkaline soils. The presence of heavy metals appeared to have no significant effect on P speciation in these soils. Compared with the macroaggregates, the available P, total P, and their ratio all increased dramatically in the soil colloids (< 0.002 mm) due to the accumulation of Fe/Al oxides. The XANES analysis indicated that the P speciation in the acidic soils and soil aggregates was dominated by Fe-P, followed by Al-P, and little Ca-P; in contrast, the proportions of Ca-P increased but Fe-P decreased in the alkaline soils and soil aggregates. Compared with the macroaggregates, the proportions of Fe-P increased while Ca-P decreased in the soil colloids. Correlation analysis indicated that amorphous Al and isomorphic substitution Al played critical roles in P immobilization by pedogenic Fe/Al oxides. These new insights into the P speciation are essential for environmental risk assessment and efficient utilization of P in the farmland soils.