Phosphorus fractionation in biosolids-amended soils: Relationship to soluble and desorbable phosphorus

Phosphorus has been identified as a major factor involved in decreasing wat er quality through its role in eutrophication, and there is now a focus on controlling nonpoint agricultural P sources. This work was conducted to ide ntify how biosolids applications under current regulations have affected th e forms and release potential of P in agricultural soils. We collected samp les from eight farms with a history of biosolids amendments, selecting fiel ds that had setback areas (where biosolids applications were not permitted) to allow comparison of amended and unamended soils. We analyzed these soil s for P fractions (soluble P, Al-P, Fe-P, reductant soluble P, and Ca-P; th eir sum equals total P), sequentially desorbable P (Fe-strip), oxalate P, A l and Fe, Mehlich-1 P, and the degree of P saturation. Our results show tha t following a N-based biosolids nutrient management plan can significantly increase total P (from 403 to 738 mg kg(-1)) and initially desorbable P (fr om 32 to 61 mg kg(-1)). The main soil components associated with P retentio n (Al-ox and Fe-ox) also tended to be increased by biosolids amendment and this may help mitigate P release. Biosolids amendment significantly increas ed 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 biosol ids sites, but with increasing extractions, the release converged towards t hat from the setback areas. Mehlich-1 P and P-ox were good predictors of de sorbable P release, as measured by one and five sequential extractions with Fe-strips. Desorbable P, by both one and five Fe-strip extractions, was mo re closely correlated with Al-P than Fe-P, especially in setback areas, ind icating that Al-P is probably the most important source of desorbable P ind ependent of biosolids amendment. This work indicates the importance of cons idering P availability at agricultural biosolids application sites and of m aintaining setback areas near water bodies, where no biosolids may be appli ed, to reduce the risk of P losses.