The plant availability of phosphorus from thermo-chemically treated sewage sludge ashes as studied by 33P labeling techniques

Phosphorus resources have to be managed sustainably and therefore the recycling of P from waste streams is essential. A thermo-chemical recycling process has been developed to produce a P fertilizer from sewage sludge ash (SSA) but its plant availability is unknown. Two SSA products prepared with either CaCl2 (SSACa) or MgCl2 (SSAMg) as chemical reactant during the thermal treatment were mixed with three soils previously labeled with P-33. Reference treatments with water-soluble P added at equal amounts of total P were included. The transfer of P from SSACa and SSAMg to Lolium multiflorum or P pools of sequentially extracted soil-fertilizer incubations were quantified. The shoot P uptake from SSAMg was higher than from SSACa. For SSAMg the relative effectiveness compared to a water-soluble P fertilizer was 88 % on an acidic and 71 % on a neutral soil but only 4 % on an alkaline soil. The proportion of P derived from the fertilizer in the plant and in the first two extraction pools of soil-fertilizer incubations were strongly correlated, suggesting that it is sufficient to conduct an incubation study to obtain robust information on plant P availability. We conclude that under acidic to neutral conditions SSAMg presents an appropriate alternative to conventional P fertilizers and the dissolution of P from SSAMg seems to be governed by protons and cations in the soil solution.