Abstract
<title>Abstract</title> <p> <bold>Background and Aims</bold> Whether endogenous minerals in biochar can be exploited for phosphorus (P) management without exogenous reagents remains largely unexplored. We investigated whether calcium (Ca)-rich sugarcane filter mud (SFM) biochar could achieve long-term P immobilization in acidic soil. <bold>Methods</bold> SFM biochar (pyrolyzed at 500 °C) was granulated with compound fertilizer at substitution ratios of 20–100% (w/w). An optimized 40% formulation was tested in a 21-month citrus pot experiment (initial soil pH 6.11), monitoring soil P dynamics, leaching, enzyme activities, and plant P status. Independent field corroboration (13 months, initial soil pH 4.91) was also evaluated. <bold>Results</bold> The optimized amendment increased available P by 38.7% (39.26 vs. 28.30 mg kg⁻¹) and reduced P leaching by 22.0–37.7% relative to the conventional fertilization treatment, despite a 30% fertilizer reduction. Soil water retention and organic matter content increased by ~40% and 75.6%, respectively. Leaf P concentrations and SPAD values remained comparable to full fertilization. Field data exhibited consistent but less pronounced trends. Convergent evidence—including a Ca/P molar ratio of ~1.08, elemental co-localization, sustained P retention, and concurrent leaching reduction—strongly suggests the formation of metastable Ca-phosphate phases, most probably amorphous calcium phosphate (ACP). <bold>Conclusion</bold> This 21-month study provides robust evidence for an endogenous-mineral-driven strategy that achieves durable P immobilization while valorizing a globally significant industrial waste, eliminating the need for exogenous chemical modification. </p>