Phosphate additives promote humic acid carbon and nitrogen skeleton formation by regulating precursors and composting bacterial communities.

[Display omitted] • Superphosphate (CP) and 'Mg(OH) 2 &H 3 PO 4 ′ increased NH 4 +-N by 75 % and 44 % respectively. • CP increased nitrogen ratio of humic acid to total by 49% compared to the control. • Phosphate additives improved the humification degree and the polymerization of humus. • Preserved NH 4 +-N due to phosphate additives was converted to nitrogenous humic acid. • Clustered bacterial positive relations in CP help to convert precursor to humic acid. This study aimed to compare the effect of different phosphate additives including superphosphate (CP) and MP [Mg(OH) 2 + H 3 PO 4 ] on nitrogen conversion, humus fractions formation and bacterial community in food waste compost. The results showed the ratio of humic acid nitrogen in total nitrogen (HA-N/TN) in CP increased by 49 %. Ammonium nitrogen accumulation was increased by 75 % (CP) and 44 % (MP). Spectroscopic techniques proved that phosphate addition facilitated the formation of complex structures in HA. CP enhanced the dominance of Saccharomonospora , while Thermobifida and Bacillus were improved in MP. Structural equation modeling and network analysis demonstrated that ammonium nitrogen can be converted to HA-N and has positive effects on bacterial composition, reducing sugars and amino acids, especially in CP with more clustered network and synergic bacterial interactions. Therefore, the addition of phosphate provides a new idea to regulate the retained nitrogen toward humification in composting. [ABSTRACT FROM AUTHOR]