Humic acid biosynthesis and bacterial community evolution during aerobic composting of rice straw

In this study, the effects of inoculum ratio, substrate particle size and aeration rate on humic acid (HA) biosynthesis during aerobic composting of rice straw were investigated, respectively. The contents of total organic carbon, total nitrogen and HA, as well as lignocellulose degradation in the composting were evaluated, respectively. It is found that the maximal HA yield of 356.9 g kg^-1 was obtained at an inoculum ratio of 20%, a substrate particle size of 0.83 mm and an aeration rate of 0.3 L·kg^-1 DM min^-1 in the process of composting. The changes of microbial communities and metabolic functions at different stages of the composting were also analyzed through high-throughput sequencing. The result demonstrates that Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria were the dominant phyla and their relative abundance significantly varied over time (p < 0.05), and Rhizobium, Phenylobacterium, Pseudoxanthomonas and Paenibacillus were positively related to HA content in the compost. Furthermore, the metabolic function profiles of bacterial community indicate that these functional genes in carbohydrate metabolism and amino acid metabolism were involved in lignocellulose biodegradation and HA biosynthesis. This work may be conducive to explore new regulation strategy to improve bioconversion efficiency of agricultural residues to applicable biofertilizers. KEY POINTS: • Temperature, pH, TOC, TN and C/N caused a great influence on humic acids synthesis • The succession of the microbial community during the composting were evaluated • The metabolisms of carbohydrate and amino acids were involved in HA synthesis.