Assessing and predicting phosphorus phytoavailability from sludge incineration ashes

Abstract

Incineration of municipal sludge and agri-food by-products generates large quantities of ash that can be used in agriculture as phosphorus fertilizer. The fertilizing potential of sludge incineration ash (SIA) from 12 cities in Canada and the United States was tested in a greenhouse experiment against a synthetic fertilizer (TSP: triple superphosphate), a natural fertilizer (RP: rock phosphate), and a control without any P fertilizer. Two soil types were used: clay and sandy loam. A reliable a priori indicator of SIA P bioavailability was determined using the random forest method. SIA application increased ryegrass P uptake. The SIA relative P effectiveness (RPE), compared to the TSP, varied from 5.1% to 46.2% depending on the sludge origin and P solubility. SIA RPE was greater than RP for the clay soil but similar for the sandy loam soil. The neutral ammonium citrate (NAC) extraction, sometimes inappropriately used to characterize P availability of sludge and by-products, explained only 53% of the RPE variation. The random forest analysis showed that the oxalate extraction (Al, P, and Fe) is a better indicator (R² = 0.94) of relative availability of SIA than the NAC P solubility (R² = 0.86), and that Al content is the factor that influences most SIA P solubility. Based on our findings, we recommend the use of the Al, Fe, and P oxalate extraction to predict the SIA P availability, instead of the widely used NAC method which extracts only P.

Keywords: Apparent phosphorus recovery; Oxalate extraction; Phosphorus fertilizer; Phosphorus recycling; Random forest; Relative phosphorus effectiveness.