Objectives Understanding microbial communities and their physicochemical indexes during composting process is important for the screening of efficient strains to improve the nutritional quality and accelerating the composting process.
Methods Pig manure and maize straw were mixed in a mass ratio of 1 to 6 as the testing manure compost. The high temperature resistant microbial inoculums (Acinetobacter pittii, Bacillus subtilis subsp. Stercoris and Bacillus altitudinis) were inoculated in composting piles as the treatment (MI), and the piles without inoculation as control (CK). The temperatures and pH were monitored, and the compost samples were collected on the 4th, 12th, 24th and 32th day since the starting of the composting, and the bacterial community composition was analyzed using 16s rRNA high-throughput sequencing technology. The collected fresh composting samples were also extracted with water, and a germination test of alfalfa seeds was carried out using the extracts. The contents of total N, P and K were determined at the end of composting.
Results Microbial inoculation led to a 2-day advanced and 2-day prolonged thermophilic stage. The seed germination index (GI) was higher than 80% when soaked in the extract from the 24th and 32th day of MI composting, and the promoting effect on the taproot growth of alfalfa seedlings was significantly greater than that of CK (P < 0.05). As fermentation proceeded, the volume of compost piles decreased gradually. On the 32th day, the content of total P in MI (2.63%) was significantly higher than that in CK (2.28%), while the total K, total N and pH were not significantly different between CK and MI. The degradation rate of total organic carbon (TOC) in MI was greater than that in CK on days of 4–12 and 24–32, and the ratios of C/N were 13.28 and 15.26 (P < 0.05), respectively. The dominant bacterial communities at the phylum level included Proteobacteria, Actinobacteria, Firmiutes and Bacteroides, and the abundance of Firmicutes in CK and MI during the thermophilic phase (on day 24) of composting were 17.4% and 59.8% (P < 0.05), respectively. At the phylum level, Proteobacteria, Firmicutes and Actinobacteria were the dominant bacteria community in the temperature rising, thermophilic and maturity stage, and their relative abundances present in MI were higher than those in CK. Acinetobacterium, Lysinibaculus, Solibaculus, Psedomonas and Flavobacterium were dominant bacteria at the genus level, and the addition of microbial inoculums increased the abundance of Acinetobacter by 21.41% on the 4th day. The Shannon and Observed species indices on the 12th day of composting were increased due to microbial inoculation. Correlation analysis showed that temperature, total N, total P, total K, GI and TOC were significantly correlated with bacterial composition, but pH was not.
Conclusions The microbial inoculums could significantly increase and keep the abundance of dominant bacteria at phylum and genus levels during the composting process, accelerating temperature rising and stable, shortening the time needed for composting, stimulating the decomposition of total carbon and increasing the available P content at the end. Acinetobacter Pittii in the microbial inoculation plays a major role in the temperature rising stage of composting, while Bacillus Subtilis Subsp. Stercoris and Bacillus altitudinis in the thermophilic stage. We suggest that screening of high temperature resistant bacteria should focus on the genus of Bacillus in phylum level of Firmicutes.
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