Effects of micro-nano bubble water addition on maturation degree and microbial community during aerobic composting.

In order to explore the effects of micro-nano bubble water (MNBW) on compost maturation and the microbial community in cow manure and straw during aerobic composting, we conducted composting tests using tap water with 12 mg/L (O12), 15 mg/L (O15), 18 mg/L (O18), and 21 mg/L (O21) dissolved oxygen in MNBW, as well as tap water with 9 mg/L dissolved oxygen as a control (CK). The results showed that O21 increased the maximum compost temperature to 64 °C, which was higher than the other treatments. All treatments met the harmless standards for compost. The seed germination index (GI) was largest under O21 and 15.1% higher than that under CK, and the non-toxic compost degree was higher. Redundancy analysis showed that the temperature, C/N, pH, and GI were important factors that affected the microbial community composition. The temperature, C/N, and pH were significantly positively correlated with Firmicutes and Actinobacteria (p < 0.05). Firmicutes was the dominant phylum in the mesophilic stage (2-6 days) and it accounted for a large proportion under O21, where the strong thermophilic metabolism increased the production of heat and prolonged the high temperature period. The bacterial genus Ammoniibacillus in Firmicutes accounted for a large proportion under O21 and it accelerated the decomposition of substrates. Therefore, the addition of MNBW changed the microbial community to affect the maturation of the compost, and the quality of the compost was higher under O21.

Effects of Bacillus subtilis on carbon components and microbial functional metabolism during cow manure-straw composting.

This study is the first to investigate the changes in the composting process and carbon conversion in a cow manure-straw compost matrix with Bacillus subtilis added at four different levels (0, 0.5%, 1%, and 2% w/w compost), and to explain the mechanism responsible for carbon conversion through microbial functional metabolism. Inoculation with Bacillus subtilis at 2% had the best effect on fermentation among all treatments, but it inhibited the synthesis of total organic carbon and humus. Bacillus subtilis at 0.5% reduced mineralization in the cooling and maturity stages of composting, and enhanced the humification of carbon. The total organic carbon and humic sequence contents were significantly higher with Bacillus subtilis at 0.5% (12.5% and 20.2%, respectively) than Bacillus subtilis at 2% (P < 0.05). Redundancy analysis demonstrated that the pH and microbial functional metabolism were closely related to carbon sequestration during composting.

Changes in antibiotic resistance genes and mobile genetic elements during cattle manure composting after inoculation with Bacillus subtilis.

This study explored the effects of Bacillus subtilis at four levels (0, 0.5%, 1%, and 2% w/w compost) on the variations in ARGs, mobile genetic elements (MGEs), and the bacterial community during composting. The composting process had a greater impact on ARGs than Bacillus subtilis. The main ARG detected was sul1. The addition of Bacillus subtilis at 0.5% reduced the relative abundances of ARGs, MGEs, and human pathogenic bacteria (by 2-3 logs) in the mature products. Network and redundancy analyses suggested that intI1, Firmicutes, and pH were mainly responsible for the changes in ARGs, thus controlling these factors might help to inhibit the spread of ARGs.