Thermochemical treatments allow production of sewage sludge ash (SSA) rich in P and low in heavy metals, which could be recycled in agriculture. Our objective was to quantify P release from SSA using ion sink assays and to relate these results to P speciation in SSA and plant P uptake. Anion and cation exchange membranes saturated with different counterions (HCO, Na, and H) were used to create a gradient in pH, P, or cation concentration between SSA particles and the surrounding solution. Phosphorus speciation in SSA was assessed using X-ray powder diffraction, and plant P uptake was determined in a pot experiment with an acidic and a neutral soil. Four SSA products were investigated: a SSA thermochemically treated with CaCl or MgCl (SSA Ca/Mg), a SSA blended with KCl, and a SSA blended with KCl and triple superphosphate (TSP) to obtain a marketable 12-20 P-K fertilizer. The H membranes dissolved all P species present in SSA. Combined HCO/Na membranes extracted diffusible P and noncrystalline P from SSA Ca/Mg and stanfieldite from SSA Mg. Blending with KCl hardly changed P release from SSA, whereas blending with TSP masked P release. The amount of P extracted from SSA by combined HCO/Na membranes was correlated to plant P use in the acid soil, whereas the amount of P extracted by HCO membranes alone was correlated to P use in the neutral soil. In conclusion, the ion sink assays delivered information on P release that was related to both SSA mineralogy and P use by plants.
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