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

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 (Al(ox) and Fe(ox)) 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 P(ox) 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.