Phosphorus Fractionation in Biosolids‐Amended Soils Relationship to Soluble and Desorbable Phosphorus

Abstract

Phosphorus has been identified as a major factor involved in decreasing water quality through its role in eutrophication, and there is now a focus on controlling nonpoint agricultural P sources. This work was conducted to identify how biosolids applications under current regulations have affected the forms and release potential of P in agricultural soils. We collected samples from eight farms with a history of biosolids amendments, selecting fields that had setback areas (where biosolids applications were not permitted) to allow comparison of amended and unamended soils. We analyzed these soils for P fractions (soluble P, Al‐P, Fe‐P, reductant soluble P, and Ca‐P; their sum equals total P), sequentially desorbable P (Fe‐strip), oxalate P, Al and Fe, Mehlich‐1 P, and the degree of P saturation. Our results show that following a N‐based biosolids nutrient management plan can significantly increase total P (from 403 to 738 mg kg−1) and initially desorbable P (from 32 to 61 mg kg−1). The main soil components associated with P retention (Alox and Feox) also tended to be increased by biosolids amendment and this may help mitigate P release. Biosolids amendment significantly increased Fe‐P (from 137 to 311 mg kg−1), probably due to Fe added to biosolids during production, and there was also a strong trend for higher Al‐P where biosolids had been applied. Desorbable P was initially greatest from biosolids sites, but with increasing extractions, the release converged towards that from the setback areas. Mehlich‐1 P and Pox were good predictors of desorbable P release, as measured by one and five sequential extractions with Fe‐strips. Desorbable P, by both one and five Fe‐strip extractions, was more closely correlated with Al‐P than Fe‐P, especially in setback areas, indicating that Al‐P is probably the most important source of desorbable P independent of biosolids amendment. This work indicates the importance of considering P availability at agricultural biosolids application sites and of maintaining setback areas near water bodies, where no biosolids may be applied, to reduce the risk of P losses.