Biosolids-derived phosphorus persists in Florida sandy soils

Abstract

Biosolids, the solid byproduct from sewage treatment plants, are utilized as soil conditioners and fertilizers due to their rich organic matter and nutrient content. However, there are concerns about heavy metals and high levels of phosphorus in biosolids which can lead to environmental issues such as increased biotoxicity and eutrophication of water bodies. This study examines the effects of historical biosolids applications on sandy soils at a former ranch in Seminole County, Florida, focusing on the persistence of legacy phosphorus and heavy metals 14-years post-application. Our results showed that concentrations of heavy metals such as chromium, copper, nickel, lead, and zinc in the top 15 cm of soil were well below the Environmental Protection Agency and European Union recommended limits, indicating no significant long-term accumulation. In contrast, phosphorus levels were notably higher in biosolids-treated soils compared to untreated samples, demonstrating the persistence of legacy phosphorus and raising concerns about potential eutrophication in water bodies. Examination of the vertical movement of biosolids-derived materials revealed elevated phosphorus and calcium levels in both topsoil and subsoil horizons, suggesting that phosphorus was translocating with calcium. These findings highlight the persistent residual effects of legacy phosphorus, evident even in sandy, leachable soils 14 years post-application, suggesting that legacy phosphorus may have a longer half-life than currently assumed and that its role in long-term biosolids application projects could be underestimated.