Humus formation driven by ammonia-oxidizing bacteria during mixed materials composting.

• Identified effects of ammonia-oxidizing bacteria (AOB) on humus formation. • AOB can increase humus production by releasing NH 3 emission. • AOB has also promoted nitrogen transformation. • AOB has dual functions to reduce nitrogen loss and improve compost quality. The aim of this study was to identify the effects of ammonia-oxidizing bacteria (AOB) inoculation on humus formation. Both nitrogen conversion and humus formation were considered as the main processes, because NH 4 +-N-like compounds not only substrates of nitrification, but also precursors of humus. During composting, the inoculation of AOB indeed increased humus concentration by fixing NH 3 emission as NH 4 +-N, but it has also promoted nitrogen transformation. While the main reason was the changed bacteria community structure caused by inoculating AOB. Moreover, the relationship between bacteria and nitrogen transformation and humus formation has become closer. And bacteria were more likely to synthesize humus. Therefore, it is conjectured that AOB inoculation could not only provide NH 4 +-N for humus formation, but also enhance the anabolism of microorganisms. This suppose has been confirmed by structural equation model in this study. Therefore, AOB inoculation has a driving effect on promoting humus formation. [ABSTRACT FROM AUTHOR]