From waste to amendment: Quality, transformations, and risk of heavy metals in composts from diverse organic wastes

From waste to soil amendment, we profiled Pb, Cr, Ni, Zn, Cu, Mn, Cd, and Fe in matured composts from household organics (D), olive pomace (Gr), green waste (V), poultry manure (F), sewage sludge (AH, AB and BB), and their blends. Priority monitoring is necessary for sludge-based composts as they contain the greatest levels of controlled elements (Zn 282.21-741.99, Pb 16.28-92.09, Ni 5.67-17.43, Cd 1.39-1.49, Cu 38.67-89.63 mg kg−1). F and GF mixes had consistently higher Zn (273-471) and Cu (means 26.25-45.34) levels, as well as the highest Fe (to 1850.31) and Mn (F 100%: 509.73), indicating mineral additions in the feed. Green waste-olive pomace (GV) produced the highest Cr (36.51). In contrast, single-stream Gr, V, and D were low for Zn (10.88-58.57) and Cu (8.30-8.54). Indicative risk priority across matrices was Zn ≳ Cu > Ni > Pb > Cr > Cd, with Mn and Fe reflecting mineral stability rather than contamination. A risk-tiered approach should be implemented, including stringent monitoring and potential dilution/targeted application for sludge- and manure-based composts, routine surveillance for olive-pomace blends (particularly GV for Cr), and increased agronomic reuse for single-stream Gr, V, and D, in accordance with standards and site-specific soil-crop limitations.