Phosphorus immobilization in biosolids-impacted soils: Influence of amendment type and soil chemistry on environmental and agronomic outcomes

Biosolids application on farmland provides a sustainable means of nutrient reuse while supporting crop production; however, it can promote phosphorus (P) accumulation and leaching, increasing the risk of eutrophication and harmful algal blooms. In the St. Johns River Basin (Florida, USA), biosolids application has contributed to severe ecological degradation. This study evaluated the effectiveness of several Immobilizing Phosphorus Treatments (IPTs) in mitigating P accumulation and release in soils with three levels of biosolids impacts, while maintaining sufficient P for pasture productivity and assessing potential impacts on soil chemistry. The IPTs included dolomite, lime, Ca-drinking water treatment residual (DWTR), a commercial blend (unknown formulation), Al-DWTR, aluminum processing residue (APR), and pine biochar, applied at 0, 1, and 2% (w/w) under laboratory incubation. Treatment effectiveness was evaluated using water-soluble phosphorus (WSP), iron oxide strip-P (FeO-P; plant-available P), and changes in macro- and micronutrients as well as trace metals. Al/Fe-based IPTs consistently achieved the greatest reduction in WSP across all biosolids-impacted soils, with an average decrease of approximately 67%, compared to 44% for pine biochar and 19% for Ca/Mg-based materials; however, some treatments and rates reduced P to potentially crop-limiting levels, highlighting the need to balance environmental protection and productivity. Pine biochar emerges as a promising amendment for P remediation, as it effectively reduces P availability without compromising agronomic parameters or posing environmental risks, though like all IPTs, its effectiveness may depend on soil properties and site-specific conditions. Canonical discriminant analysis distinguished the three biosolids-impacted soils based on key elements controlling P dynamics. These findings underscore the complexity of P remediation in biosolids-impacted soils and the difficulty of overcoming inherent soil chemistry constraints using amendments alone.