Controlling phosphorus mobility in biosolids-amended soils: The role of phosphate mineralogy and P:Ca and P:Mg ratios in amendment performance.

Phosphorus (P) release from biosolids-amended soils poses risks to surface and groundwater quality, yet the controlling factors remain insufficiently quantified. This study compared biosolids from two wastewater treatment plants in South Florida and North Florida to identify chemical indicators of P stability and evaluate amendment strategies for reducing P losses. Controlled batch experiments were conducted in triplicate using calcium- and aluminum-based drinking water treatment residuals (Ca-DWTR and Al-DWTR), biochars, and commercial blends. Biosolids differed in P release behavior, which was associated with calcium (Ca) and magnesium (Mg) content and phosphate mineralogy. Materials containing less soluble Ca- and Mg-associated phosphate phases released less P, whereas those dominated by more soluble phases exhibited higher mobility. Lower solution P:Ca and P:Mg molar ratios were consistently associated with reduced P release, indicating their potential as predictive indicators. Ca- and aluminum (Al)-based amendments reduced P release by up to 90%, while biochars achieved <40% reduction. These findings indicate that P release is influenced by phosphate mineralogy and its association with Ca and Mg, rather than total P content alone. The identified relationships provide a basis for predicting P release potential and selecting effective amendments. These conclusions are based on laboratory-scale batch experiments, and further validation under field conditions is needed to capture the complexities of hydrological flow and long-term soil interactions.