Phosphorus Plant Availability of Biochars Derived From Contrasting Sewage Sludges to Ryegrass

ABSTRACT Background Although phosphorus (P) is an essential element for human life, it is currently sourced unsustainably primarily from non‐renewable phosphate rock sources. Sewage sludge pyrolysis is a method that concentrates nutrients in sludge, a large renewable P source, to produce a sustainable phosphorus fertilizer. However, plant P availability from sewage sludge biochars may vary widely depending on P speciation and soil properties. Aim Our study aimed to assess the fertilizer value of sludge and derived biochar materials with different P speciation. Methods Biochars were produced at 400°C and 600°C from two types of sewage sludge, one iron precipitated (FeSS) and one biologically treated (BioSS), and applied to two types of soil (loamy and sandy) in pot trials with ryegrass. The P fertilizer value was assessed by plant P uptake at four harvests and subsequent comparison with a mineral fertilizer (triple superphosphate [TSP]) treatment, as well as sequential extraction of the P pools in soil samples after the final harvest. Results P uptake by ryegrass was similar in both soils (TSP > BioSS > FeSS > Bio600 = Bio400 > Fe600 = Fe400 > Control), although generally lower in the sandy soil. P uptake from sewage sludges was 10%–12% higher for FeSS and 18%–25% higher for BioSS compared to their biochars. For the BioSS, this was reflected in sequential soil extractions following the final harvest, as water and bicarbonate extractable P was higher for this treatment in both soils. No significant difference in P uptake was found between sewage sludge biochars of different origins or temperatures. However, P uptake from biochar produced from the BioSS was slightly higher in both soils, possibly due to more polymeric phosphate species in the material. Conclusions The results emphasize the negative impact of using chemical phosphorus removal during wastewater treatment on the P plant availability of sludge. After pyrolysis, the P fertilizer effect of the materials decreased significantly, with differences between biochars from differently treated sewage sludges being minimized.