Mitigating soluble phosphorus loss in legacy phosphorus soils: Biosolids characterization and phosphorus-transformation studies

Legacy phosphorus (P) from agricultural soils continues to leak into surface waters. We investigated how biosolids treated with metals (Fe, Al, and Ca) immobilize soluble P (water-extractable P, WEP) in soils while maintaining plant-available P (Mehlich 3-P, M3-P). Two complementary incubations (3–8 weeks; 25 °C) in non-legacy and legacy-P soils (M3-P: 58–139 mg kg⁻¹; 230–953 mg kg⁻¹) tracked WEP, M3-P, and M3-P saturation ratio (M3-PSR). Poultry litter and compost produced the largest WEP and the highest M3-P time-weighted means (TWM: 0.31 and 0.21 mg kg⁻¹ per kg of P), reflecting rapid P release. Fe/Al-treated biosolids (e.g., Bloom ±WTR)) showed small or negative WEP and lower M3-P TWM that declined with increasing application rate (0.12–0.04 mg kg⁻¹ from 1x to 4x). Thermo-hydrolyzed biosolid (low Fe) had a mid-range M3-P TWM (∼0.19 mg kg⁻¹) but the largest delta (Δ) over 8 weeks, consistent with sustained mineralization. WEP i TWM was –0.002–0.027 across products; increasing Bloom rates reduced WEP i TWM ( 0.007 to –0.0002 mg kg⁻¹) and Δ (–0.007 to –0.0005 mg kg⁻¹), confirming Fe/Al control of P solubility. M3-PSR changes mirrored product chemistry, with ΔPSR near zero for Bloom and minimal or negative for Fe/Al-treatments. Overall, Fe/Al-treated biosolids effectively reduced soluble P, while some products (e.g., thermo-hydrolyzed, lime-stabilized+WTR) gradually increased plant-available P. We suggest prioritizing Fe/Al-treated biosolids (±WTR) on legacy-P or sandy soils to immobilize soluble P, and applying labile materials (poultry litter, compost, thermo-hydrolyzed biosolids) in split doses on P-deficient soils, guided by M3-P, texture, WEPᵢ, and pH. • Fe/Al-rich biosolids curb soluble P and stabilize P saturation ratio in legacy-P soils. • Compost and thermo-hydrolyzed biosolids (low Fe+Al) build plant-available P and limit short-term soluble P pulses. • Lime-stabilized and WTR blends (high Ca+Al) deliver slow-release P with minimal increases in soluble P. • Matching biosolid chemistry to soil P levels and texture is a key to reducing soluble P losses.