Hyperthermophilic pretreatment composting significantly accelerates humic substances formation by regulating precursors production and microbial communities

Hyperthermophilic pretreatment composting (HPC) is superior to traditional composting (TC) with enhanced compost maturity and accelerated humic substances (HS) formation. However, the regulators affecting HS formation, which is of great importance in evaluating the compost maturity, are still unclear. This study aimed to examine and compare the effects of HPC and TC on (i) HS formations under varying precursors, (ii) bacterial and fungal dynamics, and (iii) factors affecting HS formations. Results revealed that HS formation was accelerated in the heating, thermophilic and maturity phases for HPC, whereas the synthesis of HS was observed in the maturity phase for TC. Particularly, concentrations of humic acid, polyphenols, amino acids, polysaccharides and reducing sugar were increased in compost by 50, 60, 52, 44 and 92%, respectively after the hyperthermophilic pretreatment. These increased precursors could stimulate the activity of Planococcaceae that possessed a high degradation capacity on D-mannitol in the heating stage. Additionally, the thermophilic microbes Solibacillus and Aspergillus with high degradation capacity on lignocelluloses and lignin, respectively dominated in the thermophilic stage. These microorganisms may promote the formation of precursors and thus accelerated synthesis of HS in HPC. Finally, structural equation model (SEM) showed polyphenol and reducing sugar were the key precursors to directly or indirectly promote HS formation in HPC and the higher temperature rise as well as the higher N content provided advantages over TC in improving HS formation. This study provides the stability to the accelerated humification process in HPC and reveals its potential applicability in improving HS formation.