Objectives To reveal the impact of thermophilic compound bacteria on the composting of chicken manure and sheep manure, and explore the microbial mechanism in composting process.

Methods The compost was composed of 75% sheep manure and 25% chicken ferment mattress. The initial material C/N was 28, the compost amount was 1.2 t, the height was 70–90 cm. The open windrow composting technique was used in this experiment. The treatment group added 0.1% of B. fordii FJAT-51578 and U. thermosphaericus FJAT-51579 fermentation broth, while the control group added 1% commercially available Bacillus subtilis. The composting period was from September 18 to October 14, 2021, and the temperature was tested once every two days. Turn the composting materials every two days for the first 15 days of composting, and every 5 days in the later period. The water content of compost was kept 50%–60% until the end of the high temperature period. Compost samples were collected at the 1st, 9th and 26th days after composting, the changes in the physicochemical characteristics including N and P content, nitrification index, and germination index of seed were monitored. Microbial succession was analyzed by 16S rDNA sequencing, and the main environmental factors were explained by RDA (redundancy analysis). Further, the microbial mechanism of available N and available P metabolism in composting was predicted by PICRUSt (Phylogenetic Investigation of Communities by Reconstruction of Unobserved States).

Results The application of microbial agent enhanced the composting rate. The addition of thermophilic compound bacteria minimized initial lag time, reduced the nitrification index and C/N, increased germination index, which promoted compost maturity. In addition, the contents of available N and P were 11.8% and 7.7% higher than those in commercially agents group, respectively. Moreover, higher relative content of Saccharomonospora, Streptomyces and Thermostaphylospora was observed in the treatment group. The RDA analysis showed that, pH and C/N were the main factors affecting the diversity of microbial community. Available N was positively correlated with Bacillus and Saccharomonospora. Available P was positively correlated with Thermobifide, lanifilum, and Actinomadura. PICRUSt analysis showed that Kyoto Encyclopedia of Genes and Genomes orthology (KEGG orthology, KO) related to nitrogen and phosphorus cycle increased with the progress of composting. Further, numbers of KOs associated with nitrification, ammonification, ammonia assimilation, dissimilatory nitrate reduction and nitrate assimilation were higher in treatment group. In the phosphorus cycle, treatment group had higher contents of KOs involved in inorganic phosphate solubilizing, acid phosphatase and alkaline phosphatase.

Conclusions The addition of thermophilic bacteria accelerates and prolongs the thermophilic phase, reduces C/N, increases contents of available N and available P in the compost of livestock manure. Among them, composting indicators including C/N, nitrification index, and GI index reached maturity in the middle stage of composting. The addition of thermophilic bacteria promotes the expression level of KO related to nitrogen and phosphorus metabolism and increases the abundance and diversity of thermophilic bacteria in composting. Available N is positively correlated with the abundance of Bacillus and Saccharomonospora, while available P is positively correlated with Thermobifide, lanifilum, and Actinomadura. Therefore, the addition of thermophilic compound bacteria promotes the maturity of composting and increases the content of available N and P in compost.