Citric acid-mediated mobilization of legacy phosphorus in fertilized grey desert soil

Understanding soil phosphorus (P) characteristics and release potential is important for using the accumulated soil P (legacy-P). A long-term fertilization experiment in grey desert soil was performed to investigate the release potential and characteristics of soil legacy-P after mobilization with a low molecular-weight organic acid (i.e., citric acid). Grey desert soil was untreated (CK), treated with commercial fertilization (NPK) or organic manure plus commercial fertilization combination (NPKM), and extracted with citric acid at high (10 mmol·L^-1) or low (2 mmol·L^-1) concentrations. The results showed that the legacy-P release potential differed among the variable fertilization treatments and was consistent with soil total P concentrations (NPK > NPKM > CK). The legacy-P release potentials after citric acid extraction were similar at high and low concentrations and made up more than 80% of the soil total P. Rapid soil P release was accompanied by a sharp decrease in soil pH, indicating that acid dissolution is the primary release mobilization mechanism. Under NPK treatment, the mobilized P was greater than that under NPKM from the beginning to the end of mobilization. NPKM released more P at the beginning of mobilization with low citric acid concentration, similar to soil magnesium. These results suggest that magnesium-associated P played a major role in P mobilization under NPKM treatment. The high citric acid concentration primarily mobilized the inorganic Ca₈-P pool, followed by the inorganic Ca₁₀-P pool. Citric acid significantly facilitated P mobilization in fertilized grey desert soil. The high release potential indicates that grey desert soil legacy-P can be accessed by reducing fertilization and efficient P management practices.