Ammonium stress promotes the conversion to organic nitrogen and reduces nitrogen loss based on restructuring of bacterial communities during sludge composting

[Display omitted] •Elevated ammonium levels promote the formation of organic nitrogen.•High ammonium nitrogen reduces nitrogen losses during sludge composting.•Transformation of nitrogen fractions by the bacterial community was altered.•Core bacterial communities promoted the conversion of NH4+-N to AN and HUN. This study aimed to clarify the conversion relationship between organic and inorganic nitrogen. The NH4Cl was used to enhance the inorganic nitrogen content. The key role of bacterial conversion of ammonium to organic nitrogen under ammonium stress was explored. Studies had shown that ammonium stress increased the amide nitrogen and bioavailable nitrogen content by 36.95% and 32.25%, respectively. Network and regression analyses showed that the microbial community structure was restructured by high ammonium and more bacteria were involved in the conversion of inorganic nitrogen to organic nitrogen(i.e., amide nitrogen, unknown nitrogen). Variation partition analysis and structural equation model showed that the bacterial community was the main contributor to organic nitrogen production(up to 67.4%), which reduced the nitrogen loss by 6.03%. These findings shed light on the poorly understood interaction between inorganic and organic nitrogen by clarifying the role of core bacterial communities in nitrogen conversion.