Are stabilized biosolids a eutrophication risk?

Regular application of phosphorus (P)-rich biosolids to limited land areas poses a potential eutrophication risk because of rapid soil P accumulation and increased transport of P in land runoff to receiving waterbodies. To assess the effect of biosolid-P on eutrophication risk, we tested the effect of a range of stabilized biosolids on soil test P (STP) accumulation (measured by Olsen-P), and resulting increases in the potential release of soluble P to runoff (measured by soil water-extractable P (WEP)), for five major UK soil types in a 90-day incubation study. Rates of Olsen-P increase (range 1-18% of total P applied) were greatest for lime-stabilized biosolids and least for Fe-rich and thermally dried biosolids. Increases in Olsen-P were not always accompanied by increases in WEP. Biosolids which contained large amounts of P-binding elements (e.g. Ca and Fe) actually reduced WEP concentrations on poorly buffered soils. There was less effect of biosolid type on highly buffered soils, which continued to release low amounts of soluble P even at high Olsen-P concentrations (up to 57 mg/kg). Soil WEP concentrations tended to increase more rapidly above ca. 20% soil P saturation, when the P sorption index was >6, and for biosolids with a molar Fe: P ratio of >1.2, but these potential thresholds require further verification under field conditions. Our results suggest the eutrophication risk associated with biosolids application can be overestimated, and they support the more widespread use of this valuable and renewable nutrient resource on some UK soils.