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 nuclear magnetic resonance (P-NMR), and P K-edge X-ray absorption near-edge structure (XANES) spectroscopy to investigate organic P (P) and inorganic P (P) 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 varied from 20 to 47%, and residual P ranged from 9 to 31%. Deoxyribonucleic acid (1-5%) and -inositol hexakisphosphate (-IHP, 4-10%) were the major P forms from P-NMR. Orthophosphate diesters determined by NMR were significantly correlated with labile NaHCO-P in SF ( > 0.981; < 0.001). Soil P 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 -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.

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Purpose Colloid-facilitated migration of phosphorus (P) is a widely accepted phenomenon in surface and subsurface environment. Release and migration of colloidal P (P coll ) in agricultural fields are closely related to P fertilization regimes. In this study, a site-specific experiment with rice/oilseed rape rotation was conducted to determine the export potential of P coll from the field and literatures reporting the impact of P fertilization regimes on release and migration of P coll in other agricultural fields were compared. Materials and methods In this 2-year field experiment, four P fertilization regimes (no fertilizer control, inorganic P fertilizer of low and high rates, and swine manure treatment) with three replicates were conducted. Floodwater and runoff samples were collected in flooding season and the 100-cm-depth soil samples were collected after both crops' harvest seasons. Colloidal particles were separated by microfiltration and ultracentrifugation processes and determined gravimetrically. The P coll value was calculated as the difference between the concentration of total P in non-ultracentrifuged and ultracentrifuged samples. The same method was applied for the colloidal mineral elements (Fe and Al) and organic carbon.Results and discussion Total P concentration in paddy floodwater significantly increased after fertilization but decreased quickly in the following days, maintaining at 6.0 mg m −2 . In soil extracts, concentration of P coll was low but stable, which ranged from 6 to 22 % of total P after oilseed rape season and from 7 to 18 % after rice season. In runoff samples, there were positive correlations between P coll , colloidal Fe (Fe coll ), colloidal Al (Al coll ), and colloidal TOC (TOC coll ); the majority of P forms was molybdate reactive P. In both crops' seasons, the amount of colloids increased with soil depth. Content of soil P coll was low and occupied 0.1-2 % of total P. The literature review showed that P coll in soil solution, runoff, and leachate ranged from 1.4 to 94 % of total P. Conclusions These results suggested that although the concentrations of P coll were not high, they widely distributed in paddy floodwater, runoff, and soil profile. Fertilization regimes and planting systems had a significant influence on the contents of P coll . Moreover, the P coll binding with Fe/Al minerals and organic carbon might be an alternative route of P loss in paddy field.

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