Molecular Speciation of Phosphorus Present in Readily Dispersible Colloids from Agricultural Soils

Speciation of colloidal P (Pcoll) is vital yet little known. For the first time ever, the P species in readily released colloids from agricultural soils were determined by P K-edge X-ray absorption near-edge structure (XANES) and solution 31P nuclear magnetic resonance (P-NMR) spectroscopy. Water-dispersible Pcoll was the dominant fraction of readily released P (<1 μm) from the studied soils cultivated with rice (Oryza sativa L.) (RS; 80.9%) and vegetables (VS; 55.1%). The Pcoll in these samples was predominantly in inorganic form, which XANES showed to be moderately labile Fe- and Al-associated P (total 70.4–83.3%) and nonlabile hydroxyapatite (16.8–19.7%). The P-NMR analysis showed that the dominant organic P compound class in colloids from RS was orthophosphate monoesters, of which inositol hexakisphosphate was the largest component. These results strongly suggested that colloids are richer in stable P forms and poorer in labile and mineralizable P than the bulk soils.