Towards Sustainable Mine Restoration: Environmental Safety Assessment of Phosphorus Migration Following Sewage Sludge Compost Application to Abandoned Phosphate Mines in Southwestern China

Repurposing waste-derived organic amendments for degraded land restoration supports the transition toward a circular economy, yet the associated environmental risks must be rigorously evaluated to ensure long-term sustainability. Sewage sludge compost (SSC) is a promising soil amendment for the ecological restoration of abandoned phosphate-mine sites (APMSs). However, SSC application may alter phosphorus (P) migration and release, thereby posing a risk of secondary pollution that could undermine the sustainability of mine reclamation efforts. Here, dynamic leaching-column experiments simulating three years of natural rainfall infiltration were conducted to investigate P-release characteristics under different fertilization scenarios, including application rate, application method, and compost maturity. Ecological risk was evaluated using a modified phosphorus pollution index (PPI), and the likely mechanisms governing P leaching and release in APMS soils were explored. Dissolved reactive P was the predominant released form in all treatments, accounting for 67.9–85.6% of total released P. Increasing the SSC application rate reduced the cumulative total phosphorus (TP) release ratio (on a soil TP content basis) from 0.0348% in the unfertilized control to 0.0264–0.0299%. Deep full-profile incorporation within the upper 20 cm of APMS soil and subsurface application reduced TP release by 36.8% and 34.1%, respectively, relative to shallow full-profile incorporation within the upper 10 cm of APMS soil. Primary sewage sludge compost (SSC*) controlled P release only when cumulative rainfall infiltration was ≤669 mm; by the third year, the ecological risk increased from slight to the severe-pollution level. SSC applied at 120 t ha−1 under the shallow full-profile fertilization mode reduced the PPI of APMS1 and APMS2 by 30.8–44.8% and 5.6–12.3%, respectively. P dissolution was positively associated with Ca release. Humic substances likely formed larger, less mobile, non-rigid colloidal complexes through Ca2+ bridging with carboxyl and phenolic groups, thereby immobilizing phosphate. These findings under the experimental conditions provide a scientifically grounded technical basis for the subsequent environmentally safe and sustainable use of SSC as a waste-to-resource strategy in APMS reclamation, contributing to the sustainable management of both municipal solid waste streams and degraded mining landscapes.