Evaluating composted sewage sludge as a phosphorus fertilizer in wheat grown on soils with different properties

Urbanization generates significant amounts of sewage sludge (SS), with relatively high concentrations of plant nutrients such as phosphorus (P). When biochemically stabilized by composting, SS (composted sewage sludge [CSW]) can serve as a sustainable P source in agriculture, supporting a circular economy and reducing agriculture's dependence on finite phosphate rock. The objective of this study was to assess the ability of CSW to supply P to plants grown on calcareous and noncalcareous soils of contrasting properties. Compost quality was assessed to support the scaling up of this study for agronomic application. Durum wheat (Triticum durum L.) was grown under controlled conditions, and three treatments were applied to the soils: (1) control without P (CON), (2) diammonium phosphate (DAP)-a common synthetic P fertilizer, and (3) CSW, the last two to supply 50 mg kg^-1 of P. The P content was 46% higher in CSW than before composting (24.0 vs. 16.4 g kg^-1). In the pot experiments, total soil P concentrations increased by 11% with DAP and CSW relative to CON. The highest P internal utilization efficiency was obtained with CON and CSW in the Entisol and CON in the Vertisol, while wheat grain yields were highest with DAP and CSW across soils. CSW increased harvest index and one-grain weight in the calcareous soil relative to CON. Moreover, CSW and DAP led to similar P uptake in wheat, significantly higher than CON. Our results highlight the potential of CSW as an effective P source for wheat in soils of southern Europe and others with similar properties.