Characteristics Of Aerobic Solid Fermentation Of Lignocellulose For Humic Acids Biosynthesis

Tremendous agricultural straws are often produced in China.However,due to the uncultured treatment technology,these materials are directly burned or discarded.Thus,the utilization efficiency of straw resources is very low,and the utilization methods cause serious environment pollution.Producing organic fertilizer by aerobic fermentation of straws has become the promising method due to many advantages such as simple operation,low cost and no secondary pollution.In this work,rice straw was used as aerobic fermentation substrate to investigate the effects of sludge inoculation loading,particle size and aeration rate on humic acid yield and straw degradation.The optimized technological conditions were obtained,and the changes of microbial community and the differences of KEGG metabolism pathways were analyzed through high-throughput sequencing to reveal the characteristics of humic acid formation.Under the optimized technological conditions,the effects of lactose on humic acid production were studied to reveal the regulation characteristics of external additive.All of this work were listed as following.Firstly,effects of sludge inoculation loadings(5%?10%?15%?20%?25%),particle sizes(10 mm?5 mm?3 mm?0.75 mm?0.25 mm)and aeration rates(0.1 L/min?0.3 L/min?0.5 L/min?0.7 L/min?0.9 L/min)on formation of humic acid,degradation of straw were investigated during aerobic fermentation through single factor experiment.It was found that at sludge inoculation loading 20%,particle size 3 mm and aeration rate 0.3 L/min,the humic acid yield was up to 302.59 mg/g;the degradation rates of lignin,cellulose and hemicellulose were 36.07%,61.38%,17.59% respectively;C/N reduced to 4.64.At the end of the experiment,these fermentation products reached the maturity standard.Then,the microbial community structures of samples at different fermentation stages were analyzed.The result shows that the abundance and diversity of bacterial community increased with the fermentation progress,it indicates that the microorganisms could adapt to the aerobic fermentation environment.Meanwhile,at the genus level,Pseudoxanthomonas,Blautia,Lactobacillus,Phenylobacterium,Pseudomonas,Rhizobium were dominant genera in the aerobic fermentation process.Rhizobium played an important role in the fixation of nitrogen through the whole aerobic fermentation.Pseudoxanthomonas and Bosea could secrete cellulase to hydrolyze cellulose into reducing sugars.Bacteroides could utilize reducing sugars and release low-molecular-weight organic acids.These products were important precursors for formation of humic acid and carbon sources for microbial growth.Also,the differences of KEGG metabolism pathways were analyzed based on the bacterial community.The result demonstrates that the high expression of functional genes in metabolism pathways were amino acid metabolism,energy metabolism,metabolism of cofactors and vitamins,lipid metabolism,carbohydrate metabolism,xenobiotics biodegradation and metabolism.In the late stage of fermentation,xenobiotics biodegradation and metabolism decreased significantly(p<0.01),indicating that macromolecules were polymerized in this stage.Meanwhile,due to the condensation of reducing sugars,amino acids and carboxylic acids to form humic acid,carbohydrate metabolism,amino acid metabolism and lipid metabolism were significantly reduced(p<0.01).Finally,the influence of different contents of lactose on humic acid formation and maturity degree were investigated.The result shows that the humic acid yield increased to 320.69 mg/g with an increase in lactose content to 20%,which was 41.93% higher than control group;TOC and germination index were 8.44% and 226.69%,respectively,which were 33.82% and 51.55% higher than that of the control group.After 29 days the fermentation substrates reached maturity standard completely.