Recycled Products from Municipal Wastewater: Composition and Effects on Phosphorus Mobility in a Sandy Soil

Recycled products from wastewater may contain high concentrations of phosphorus (P) and are thus promising alternative fertilizers. However, to better predict their P fertilizer efficiency and potential for P leaching, investigations on P forms and P mobility in soil are essential. In this study, different recycled products—an untreated sewage sludge ash (SSA), an H 2 SO 4 –digested SSA, four thermochemically treated SSAs (two Mg‐SSAs and two Ca‐SSAs), and struvite—were investigated using a combination of wet chemical methods and P K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy concerning their composition and their effects on P sorption in a sandy soil in comparison to triple superphosphate. Most of the P in the SSAs was associated with Ca in stable P fractions. The lowest P values in labile fractions (H 2 O‐P, NaHCO 3 ‐P) were found for the untreated SSA and struvite. However, the addition of struvite resulted in an immediate increase in the bioavailable P fractions and the degree of P saturation in soil after only 1 d of incubation. This suggests a high P fertilizer potential for struvite but also a risk of P losses. Among the SSAs, the two Mg‐SSAs increased the bioavailable P fractions in soil the most, whereas the lowest values were measured after application of the untreated SSA. Our results demonstrate that chemical analyses of recycled P products may involve the risk of misjudging the fertilizer quality when performed alone, without considering the behavior of these products in soil. Core Ideas Thermochemically treatment increased the P availability of sewage sludge ashes. Struvite mainly contained less soluble P forms but raised high soluble soil P pools. Chemical analyses of recycled P products should be combined with incubation studies.