Ammonia assimilation coupled with rapid humification increases recalcitrant nitrogen reservoirs during bioaugmented mechanical composting

The bioaugmented mechanical composting (BMC) of kitchen waste with rapid heating, dehydration, and humification is promising for nitrogen conservation. The combination of ammonia assimilation and rapid humification in BMC may be a distinct nitrogen conservation pathway, converting NH4+-N to recalcitrant organic nitrogen reservoirs. However, questions remain regarding the existence of this pathway and its enhancing mechanism. Thus, nitrogen transformation during a BMC process and a conventional pile composting (CPC) process were compared in this study. Results indicated a significant increase (68.53–75.56%) in the proportion of initial total nitrogen in raw materials preserved in recalcitrant humins in BMC compared to CPC. The significantly higher activity of glutamate dehydrogenase (64.65 ± 7.10 U·mg−1) and glutamine synthase (29.92 ± 3.37 U·mg−1) associated with ammonia assimilation in BMC facilitated the conversion of NH4+ from ammonification into amino acids at the thermophilic phase. These amino acids were then rapidly polymerised with other humic precursors and gradually condensed into heterocyclic and quaternary nitrogen in humins. Further analysis of the bacterial communities and their potential functions revealed that Firmicutes (e.g., Bacillus) and Proteobacteria (e.g., Acinetobacter), which are capable of mineralisation, were enriched by rapid heating and dehydration in BMC. The resulting accumulated NADPH, α-ketoglutaric acid, and humic precursors activated thermophiles associated with ammonia assimilation (e.g., Ureibacillus, Kroppenstedtia, Virgibacillus, and Tepidimicrobium) and humification (e.g., Sacchsromonospora and Bacilli), accelerating nitrogen transformation. This study provides a theoretical foundation for the development of novel nitrogen conservation strategies of coupling ammonia assimilation with rapid humification through enhanced mineralisation during composting. [Display omitted] •A distinctive N conservation pathway was found during BMC.•BMC preserved NH4+-N as recalcitrant HM-N rather than the widely studied NO3−-N.•Total N in KW preserved as HM-N in products was increased by 68.53–75.56% in BMC.•BMC conserved N by coupling ammonia assimilation with rapid humification.•Active mineralisation enhanced ammonia assimilation and humification in BMC.