Distinct phosphorus release from fresh and legacy biosolids-amended sandy soils

Biosolids are commonly used as soil amendments; however, repeated application of biosolids results in phosphorus (P) accumulation, elevating environmental risks by increasing P loss through runoff and leaching. Predicting soil P loss after ceasing biosolids application remains challenging. In a laboratory experiment, 80 leaching events were applied to sandy soils with biosolids application histories from active use to 15 years post-application. Soils with recent applications showed an early peak in P release that later stabilized, while those with legacy applications exhibited lower but more consistent P release. These results suggest that fresh biosolids contained a highly mobile P fraction that depletes over time, leaving behind more fixed P that may persist for decades. These P release trajectories could be reasonably modeled by two-pool exponential decay models. Overall, these findings highlight the importance of biosolids aging in regulating P dynamics and identify the hot moments in P loss in biosolids-impacted systems.