Abstract:
Abstract: In recent years, along with the rapid development of modern large-scale and intensive agriculture in China, the output of agricultural wastes shows a rapid increasing trend. Some objective reasons (the lagged development of resource utilization technology and equipment, the attributes of property rights of agricultural wastes, the difficult government regulatory, and so on) lead to the extensive cyclic utilization and even direct combustion or arbitrary discard of agricultural wastes, which has caused serious harms to the rural ecological environment, agricultural production environment and farmers' living environment in China. The preparation of biogas through anaerobic fermentation is a high-engineering technology for the energy-oriented use of agricultural wastes, and the quantity, species and activity of anaerobic microorganisms for anaerobic fermentation, as well as the fermented substance are the key factors affecting the efficiency of biogas production. C/N, an important character of the fermented substance, can directly affect the metabolism of microorganisms, and also can affect the methane yield through influencing the transformation of carbon in organic matters. In order to understand the effects of gradient organic loadings on the anaerobic digestion characteristics and microbial community structure of greatly different C/N-containing agricultural wastes, the author investigated the performance of methane production and the changes in microbial community structure of anaerobically-digested swine manure, Flammulina velutipes fungal bag, rice straw, and sugarcane top in continuous stirred tank reactor (CSTR) under gradient organic loading rates (OLRs) 1.11, 1.67, 2.22 and 2.78 g/(L?d), being counted as volatile solid. The results showed that the daily methane yield of the above four substrates all increased with the increase in OLR, but their methane productive rate and microbial community structure revealed different variation trends due to different C/N ratios of substrates. Among four substrates, swine manure with C/N of 13:1 had the highest daily methane yield, and its bacterial and archaea community structure was similar to that in the control group; whereas due to the increase in OLR and the enhancement of ammonia inhibition, the diversity index of bacteria and archaea increased firstly and then decreased, and the methane productive rate decreased gradually when the OLR reached 1.67 g/(L?d). Because Flammulina velutipes fungal bag contained a suitable C/N (about 23.7:1) for anaerobic digestion, it had a relatively stable methane productive rate, and its diversity index of bacteria and archaea increased with the increase in OLR, but its microbial community structure was obviously different from that in the control group. Rice straw and sugarcane top had a higher carbon content, with C/N as 66:1 and 121:1 respectively, and their community structure of bacteria and archaea changed obviously in the anaerobic digestion process. When OLR reached 2.78 g/(L?d), due to the acid accumulation and VFA/TIC above 0.8, the methane productive rate of rice straw decreased obviously, and its diversity index of bacteria and archaea also reduced. However, the methane productive rate of sugarcane top did not decrease obviously in the course of anaerobic digestion, because its VFA/TIC was less than 0.8. Furthermore, the substrate with different C/N had effects on the formation of specific dominant microbial species, and it was found that: Methanolobus zinderi was the specific dominant archaea species in the digested swine manure; Proteiniphilum acetatigenes and Acetivibrio cellulolyticus were the specific dominant bacterial species in the digested Flammulina velutipes fungal bag and sugarcane top, respectively; Methanospirillum hungatei was the specific dominant archaea species in the digested rice straw.