Give priority to abiotic factor of phosphate additives for pig manure composting to reduce heavy metal risk rather than bacterial contribution

• Labile phosphate additives had obviously diverse effects on the metals’ species. • K 2 HPO 4 benefit the stabilization of As, Mn and Zn and improve bacterial diversity. • C f and E r of heavy metals especially Cu and Mn in P, S and K were higher than CK. • Key was pH not bacteria for metal speciation and risk under phosphate addition. • Alkaline phosphate additives are suggested in practical applications. Phosphate additives especially superphosphate can reduce nitrogen loss, and increase phosphorus availability in composting. This study investigated the changes of different heavy metals fractions and their relationship with bacterial community and abiotic factors during pig manure composting with adding equimolar H 3 PO 4 , H 2 SO 4 and K 2 HPO 4 . Results showed that both acidic and alkaline labile phosphate increased the potential ecological risk of heavy metals compared to control, but K 2 HPO 4 decreased the accumulation of exchangeable Zn and Mn by 12% and 15% than that with H 3 PO 4 and H 2 SO 4 addition. Network analysis showed that K 2 HPO 4 enhanced the proportion of negative links in bacterial species with heavy metals, but H 3 PO 4 decreased the stability of bacterial network. Redundancy analysis demonstrated that pH was the key factor on metal speciation and risk with phosphate additives than bacterial role. The study presented theoretical basis for additive selection in controlling composting nitrogen fixation and environmental risk.