Microbial diversity and dominant microbial communities in wheat straw aerobic composting

Wheat is one of the main food crops in the Controlled Ecological Life Support System (CELSS). Wheat planting can lead to the accumulation of solid waste, such as wheat straw. Aerobic composting can be used to ferment wheat straw into organic fertilizers, thereby realizing renewable source recycling in CELSS. However, only a few studies were focused on the microbial diversity of wheat straw in the process of aerobic composting. In this study, an experiment was designed to identify the dominant microbial communities in different typical fermentation phases of wheat straw in aerobic composting. A forced aerobic composting device was adopted, including a plastic barrel, temperature control, and ventilation system. The wheat straw and food wastes were used as the main materials and nutrient additives. Three microbial agents (named QD, DH, and VT) were selected in aerobic composting. The temperature, organic content, C/N ratio, and microbial diversity were measured during the experiment. Three typical fermentation stages were divided to collect the samples for microbial diversity, according to the temperature variations during the composting process. High-throughput sequencing was used to characterize the microbial diversity in various fermentation phases. The results showed that the processes with QD, DH, and VT addition all experienced heating, thermophilic, and cooling phases, where the peak temperature reached 58.2, 54.7, and 53.7 ℃, while the high-temperature period remained 9, 6, and 6 d, respectively. The organic contents were reduced by 18.87%, 24.48%, and 22.08%, respectively. The C/N ratio decreased from the initial 30:1 to less than 12:1, which reached the standard of reactor decay. The properties of the reactor were obviously changed under inoculating three kinds of agents. Correspondingly, the agents determined the community structure of microorganisms in the reactor, and the degradation effects on the materials, particularly representing by the peak temperature, high-temperature duration, and degradation rate of organic content. The microbial diversity and dominant communities were different in each typical fermentation phase. Pediococcus, Lactobacillus, and Aspergillus were mainly detected in the heating phase. Specifically, Pediococcus degraded lignocellulose in corn straw silage fermentation, and then converted lignocellulose into monosaccharides. Lactobacillus was commonly found in various yoghurt fermentation processes. Aspergillus was secreting cellulase at degrading cellulose. The dominant microorganisms were Bacillus, Thermomyces, and Rhizomucor in the thermophilic phase. Bacillus was widely distributed in fecal compost, further degrading organic matter and lignocellulosic. Thermomyces and Rhizomucor were highly reactive to the degradation of cellulose and xylan. The dominant microorganism evolved into Chloroflexaceae, Chelatococcus, Thermomyces, and Mycothermus during the cooling phase. Chelatococcus shared the function of degrading penicillin. Chelativorans was a kind of aerobic denitrifying bacteria, further degrading EDTA to some extent. Mycothermus had a certain ability to degrade hemicellulose. There were rich metabolic pathways of microbial communities in the process of composting. The metabolic functions were closely related to sugars, oils, and lignocellulose. This experiment can provide the theoretical basis to screen and cultivate efficient bacterial strains for wheat straw composting. Some solutions also contributed to the resourceful treatment of biomass solid waste (particularly wheat straw) in CELSS.