Phosphorus speciation and transformation in long-term fertilized soil: evidence from chemical fractionation and P K-edge XANES spectroscopy

Knowledge of speciation and transformation of phosphorus (P) in soil following high application rates of chemical and organic fertilizers is essential for improving P management in Chinese agricultural ecosystems because P fertilizers have been increasingly overapplied in China. Phosphorus speciation of the soil in three long-term fertilization experiments established in 1990 was investigated jointly with a sequential fractionation scheme and P K-edge X-ray absorption near edge structure (XANES) spectroscopy. Both chemical fractionation and XANES spectroscopy confirmed that P species in the topsoils (0-20 cm) were mainly composed of iron phosphate, calcium phosphate and organic P, regardless of soil pH and mineralogy. The continuous application of nitrogen, phosphorus and potassium fertilizers (NPK) had little effect on the distribution of P species in the topsoils compared with that in the control and 1990 baseline topsoils. In contrast, the application of NPK plus organic manures (MNPK) changed significantly the speciation distribution of P by increasing distinctly available P in the soil. The transportation and transformation of P species depended on soil properties including P levels, organic carbon concentrations and mineral types. The long-term application of MNPK facilitated the transportation of P into lower horizons and the accumulation of organic P in the soil. The XANES results provided spectroscopic support for the P species identified by the fractionation scheme, and the combination of the two techniques provided complementary information on the speciation and transformation of P in soil.