Investigating the Transformation of Soil Legacy Phosphorus in Long-Term Agricultural Fields Using Multiple Advanced Techniques.

Understanding long-term transformations of soil legacy phosphorus (P) is important to reduce excess fertilization and possible P loss. Using well-characterized wheat plots receiving 10 kg P ha^-1 yr^-1 since 1967 as an example, we investigated P speciation in surface (0-7.5 cm) and subsurface (7.5-15 cm) soils collected in 1995 and 2010 from subplots with continuous P fertilization (+P) or fertilizer cessation (-P) from 1995. There were no significant differences in crop yield in 2010 for +P and –P plots, and no significant differences in total P, which indicated the soil legacy P was sufficient for wheat cultivation from 1995 to 2010. However, bicarbonate-extractable P was significantly decreased in the 2010 -P soils compared to the 1995 soils at the surface layer. P K-edge XANES analysis further revealed these reduced P pools were reflected by the decreased inorganic P associated with Al (hydr)oxides (Al-P_i), which was indirectly supported by reduced inorganic Al-P fraction by sequential fractionation. These differences were not observed between the 2010 +P and 1995 soils. Furthermore, XANES analysis revealed the transformation of stable hydroxyapatite to metastable Ca₃(PO₄)₂ and an accumulation of P bound to Fe (hydr)oxides (Fe-P) in the 2010 +P and –P surface soils relative to the 1995 soils. The P-NMR results showed decreased orthophosphate and increased organic P forms in -P surface soils compared to the +P soils from 2010 and to the 1995 soils. The transformation of legacy P speciation in the subsurface soils was less impacted than in the surface soils. These results suggest that when fertilizer P is withdrawn in well-fertilized P soils, plants will utilize legacy fertilizer and mineralizing organic P. These results provide important information about legacy P in croplands to improve agricultural sustainability while mitigating water quality deterioration.