Material Conversion,Microbial Community Composition And Metabolic Functional Succession During Green Soybean Hull And Maize Straw Composting

As the development of vegetable industry,the proportion of vegetable yields in agricultural production increased rapidly,whereas the amount of vegetable waste increased in the meantime.Vegetable waste has become the fourth largest agricultural waste in China at present,next to rice,corn and wheat straw.Due to its high moisture content and low carbon-nitrogen ratio,vegetable waste is difficult to recycle that causes severe wasting of resources and environmental pollution.Composting is an environmentally friendly,sustainable,and socially acceptable organic waste treatment technology.During composting process,organic waste will be converted into stable,relatively non-toxic,complex macromolecular substances by microorganism.However,during natural vegetable waste composting,the relationship among substance conversion,microbial population,and metabolic function is still unclear.In this study,green soybean hulls were used as raw materials and corn stalk as bulking agents in a 55 days composting experiment.In order to reach 60%moisture content and 25-30:1 carbon-nitrogen ratio of the composting materials,we mixed triplicate green soybean hulls and one copy of maize straw and set three 2 m×1.5 m×1 m composting piles.To ensure enough oxygen for microorganism activities,composting piles were turned every two days.The temperature of composting piles and the surrounding were measured every half-day.Based on the temperature curve,composting materials of 0 d,2 d,4 d,8 d,14 d,22 d,28 d,34 d,40 d and 55 d were used for followed analysis of material conversion,microbial population,and metabolic functions.The main results are as follows:（1）Aerobic compost converts green soybean hulls waste into organic fertilizer effectively.During the composting process,the temperature of the compost increased to more than 60℃fast and lasted for 16 days in the thermophilic phase,after that the temperature of the compost decreased to about ambient temperature and kept stable.After the composting,the pH and EC stabilized at the range of 8.22-8.30 and 2.0-2.2 mS·cm^-1 respectively.The C/N ratio of the compost material decreased from 32.23 to 19.19,and the HA/FA ratio increased from 0.26 to1.99.The GI increased from 28.46%of 0 d to 104.08%of the end of the compost.（2）Composting significantly affects the composition of microbial communities.Theα-diversity results revealed that the number of OTU,Chao1 index,and Shannon index of bacterial increased with the composting process and reached a peak at the end of composting,the fungal OTU number,Chao1 index,and Shannon index showed a pattern of first increase and then decrease.Theβ-diversity results showed that the bacterial and fungal community composition had changed significantly at different stages of compost,and showed different temporal succession patterns.Proteobacteria（39.20%）,Firmicutes（28.55%）,Bacteroidetes（12.93%）,Actinobacteria（9.91%）were the dominant bacteria phyla,Ascomycota（61.56%）and Basidiomycota（8.33%）were the dominant fungal phyla.Random forest results showed that the Streptosporangiaceae of bacteria was the biomarker of the thermophilic phase of composting,while Trueperaceae and Pirellulaceae were the biomarkers of the cooling and maturity phase.The Chaetomiaceae of fungi was a biomarker during the thermophilic and cooling phase of composting.（3）Composting significantly affects the metabolic function of microbial communities.Bacterial Tax4Fun analysis showed that the majority of predicted protein sequences annotated with KEGG pathway in compost samples were clustered into metabolism,environmental information processing,genetic information processing,cellular processes and organismal systems.Among them,carbohydrate metabolism and amino acid metabolism were the two main metabolism pathways.FAPROTAX prediction results showed that the four pathogen-related functions,plant pathogens,human pathogens all,human gut,and animal parasites or symbionts,were decrease to less than 0.3%during the composting process.FUNGuild functional predictions showed that OTU1,OTU5,OTU8,and OTU11 were annotated as plant and animal pathogens,with abundance less than 0.01%at the end of composting.OTU2 and OTU6belonged to Thermomyces,OTU32,OTU22,OTU26 and OTU18 belonged to Aspergillus.They were dominant in the thermophilic phase and functioned in lignocellulose degradation.（4）The physicochemical characteristics of compost are closely related to the microbial communities.CCA analysis showed that GI explained 18.4%of changes in bacterial composition,followed by temperature（12.5%）,TN（5.3%）,FA（4.3%）,and pH（4%）.The five physicochemical characteristics were significantly affected the bacterial community composition（p<0.05）.For fungal communities,HA/FA ratio（15.7%）,GI（10.6%）,pH（6.2%）and TOC（4.4%）significantly affected the fungal community composition（p<0.05）.Co-occurrence network analysis showed that there was correlation between OTU and GI,pH and HA/FA ratio in the bacterial network,and OTU in fungal networks were correlated with temperature,pH,TOC,GI,FA,HA,FA/HA ratio,TN,TK and C/N ratio.In summary,aerobic composting can effectively transform green soybean hulls waste into organic fertilizer with significant phase successions of physicochemical characteristics,microbial diversity and community structure.The variations of microbial community structure,especially of fungi,are closely related to the changes of physicochemical characteristics during composting process.Microorganisms can metabolize cellulose,lignin and other components,and inactivate harmful microorganisms.These findings expand understanding of the coupling of material transformation and community succession during the composting process,also afford theoretical foundation for the resource utilization of vegetable waste.