Complementary Phosphorus Speciation in Agricultural Soils by Sequential Fractionation, Solution 31P Nuclear Magnetic Resonance, and Phosphorus K-edge X-ray Absorption Near-Edge Structure Spectroscopy

Ultisols in China need phosphorus (P) fertilization to sustain crop production but are prone to P loss in runoff. Balancing P inputs and loss requires detailed information about soil P forms because P speciation influences P cycling. Analytical methods vary in the information they provide on P speciation; thus, we used sequential fractionation (SF), solution P-31 nuclear magnetic resonance (P-NMR), and P K-edge X-ray absorption near-edge structure (XANES) spectroscopy to investigate organic P (P-o) and inorganic P (P-i) species in Chinese Ultisols managed for different crops and with different fertilizer inputs in the first study to combine these techniques to characterize soil P. Sequential fractionation showed that moderately labile NaOH-P was the largest P pool in these soils, P-o varied from 20 to 47%, and residual P ranged from 9 to 31%. Deoxyribonucleic acid (1-5%) and myo-inositol hexakisphosphate (myo-IHP, 4-10%) were the major P-o forms from P-NMR. Orthophosphate diesters determined by NMR were significantly correlated with labile NaHCO3-P-o in SF (r > 0.981; P < 0.001). Soil P-i was shown to be predominantly associated with iron and soluble calcium (Ca) by XANES. Furthermore, XANES identified hydroxyapatite in the soil receiving the highest rates of Ca-phosphate fertilizer, which had the highest HCl-P pool by SF, and also identified IHP (7%) in the soil with the highest proportion of myo-IHP from P-NMR. These results strongly suggest that a combined use of SF, solution P-NMR, and P K-edge XANES spectroscopy will provide the comprehensive information about soil P species needed for effective soil P management.