Degradation Of Lignocellulose From Corn Straw During Aerobic Composting

Aerobic composting is a process involving fermentation and metabolism of aerobic microorganisms to make biomass waste no longer perishable and smelly,and to further generate fertile and stable humus,which is then degraded into stable soil conditioner compatible with plant growth.More specifically,the annual output crop straw in China is about 1 billion tons.It is often used as a conditioner in the composting process to regulate the C/N ratio and moisture content.However,straw is rich in lignin,cellulose,and hemicellulose,and its transformation and humification have become the key rate-limiting steps of the aerobic composting process.In this study,lignocellulose-degrading bacteria were screened,isolated and purified in the mesophilic and thermophilic stages of compost.The physiological,biochemical and enzyme producing characteristics of the strain were preliminarily investigated.Then,different combinations of microbial agents were constructed and inoculated in compost.To explore the feasibility of lignocellulose degradation.Meanwhile,the effects of physical and chemical properties and microbial community structure on compost at different stages were analyzed.On this basis,simulating the mesophilic and thermophilic stages of composting,and the main products and pathways of degrading lignin and cellulose were further identified.Finally,the effects of adding rare earth elements which were exogenous substances on lignocellulose degradation and compost humification were studied.The results of this study can not only enrich the bacterial resources of lignocellulose-degrading bacteria,but also provide theoretical support for the biotransformation and degradation of straw-derived lignocellulose during aerobic composting,and have important academic significance and practical value to promote the development of lignocellulosic organic solid waste composting industry and accelerate the engineering application process of composting.The main research contents and results were as follows:(1)Screening and characterization of lignocellulose-degrading bacteria.Various microorganisms were screened at different stages during actual aerobic composting,and their degradation functionality and enzyme production ability were identified and verified.The bacterial strains of ZX5,ZX6,GX2,GX5,and GX9 were found to be related to lignocellulose degradation.Among them,ZX5(B.licheniformis)and GX5(B.thermoruber)degraded lignocellulose by secreting enzymes,have the most comprehensive degradation function.Include laccase(Lac),manganese peroxidase(Mnp),lignin peroxidase(Lip),the enzyme activities of lip produced by ZX5 and GX5 were 662.40 U/L and 629.36 U/L,respectively.The analysis results of Fourier Transform Infrared Spectroscopy(FTIR)showed that before and after biodegradation,the celluloseβ-polysaccharide and C-O bonds on the surface of corn straw were cleaved.Meanwhile,compared with ZX5,the methyl and methylene structures in lignin were reduced after GX5 treatment.After treatment with GX5,ZX5,and GX5+ZX5,the degradation rates of lignin were 57.38%,52.46%and 39.34%,and the degradation rates of cellulose were 80.26%,76.72%and 63.58%,respectively.The degradation rates were in descending order,it is GX5>ZX5>ZX5+GX5.GX5 had a better effect on lignocellulose biodegradation.(2)Rescreening and effects of lignocellulose-degrading bacteria in composting.GX5 and ZX5 were mixed with GX2,GX9 and ZX6,respectively,and apply them to compost to study their effects on the process and effect of aerobic composting.Compared with strain ZX5,the results showed that inoculation withing GX5(B.thermoruber)could prolong the high temperature stage of the compost,and increased activity of related enzymes.The contents of cellulase(CMCase),Lip,Mnp and Lac in the bulk were 4.55,1.93,0.94,and 0.20 U/g,respectively.The degradation rates of lignin and cellulose were 17.11%and 36.70%.The results of the microbial community diversity analysis showed that Bacillus and Thermobifida were the dominant bacteria in the compost,which were significantly correlated with CMCase,Lac,Mnp,Lip,and temperature.Metabolic function analysis showed that when a biological agent contains GX5,the abundance of human disease-related functional genes in compost was lower,and improved the microbial community structure.The results showed that the strain could prolong the thermophilic stage of the compost and effectively kill pathogenic microorganisms,which was beneficial to the harmless of the compost.(3)Characterization and pathway analysis of the degradation of lignin by strain GX5.The degradation products of lignin changed at different temperatures.The biodegradation of lignin G and H monomers mainly proceeded via theβ-ketoadipate pathway at 37°C.At 55°C,the degradation product of lignin S monomer was mainly a benzoic acid substance,indicating that the lignin was degraded via the benzoic acid pathway.At 55°C,B.thermoruber GX5 was capable of secreting Lip,Lac,and Mnp.These degradation enzymes adsorbed onto the surface of lignin,which increased the roughness of lignin,decreased the particle size,increased the specific surface area,and increased the number of active sites of the lignin-degrading enzymes.All of these changes benefited lignin degradation.The lignin degradation rate reached 81.97%on the 7th d.Combined with Atomic Force Microscope(AFM),Nuclear Magnetic Resonance Spectroscopy(NMR),FTIR,and Ultra Performance Liquid Chromatography-Quadrupole Time of Flight Mass Spectrometer(UHPLC-Q-TOF/MS)was used to analyze the degradation process of lignin.Indicate that the monolignol sinapyl alcohol was primarily degraded from a benzoic acid substance,the lignin was degraded via the benzoic acid pathway.Thus,B.thermoruber GX5 promoted the stabilization and utilization of lignin waste.It can be used as a potential candidate strain for lignin degradation in compost and promote the stabilization and utilization of lignin waste,and this strain has a broad application prospects.(4)Characterization and pathway analysis of the degradation of cellulose by strain GX5.After B.thermoruber GX5 treatment for 7 days,the cellulose degradation rate reached 59.77%,which was related to the production of cellulose degradation enzyme activity.Scanning Electron Microscope(SEM)was used to observe the surface morphology of cellulose before and after degradation,as the processing time increases,the internal structure became rough,loose,and porous,destroying the original cellulose complex structure.FTIR results showed that the intermolecular hydrogen bonds of cellulose were weakened,the carbon chain structure was destroyed,and the characteristic peaks of cellulose were weakened to different degrees.X-ray diffractometer(XRD)results showed that the crystallinity of cellulose increased after degradation.The crystallinity of cellulose increased from 42.15%to 67.80%to the control group,indicating that the amorphous part of cellulose precipitated,the lattice structure was destroyed,the intensity of the corresponding diffraction peak was reduced,and the cellulose structure was changed.The degradation products and degradation pathways of cellulose were analyzed by UHPLC-Q-TOF/MS,showed that the degradation products of cellulose were oligosaccharides and polysaccharides with low molecular weight,which are precursors of humus formation.(5)Effect of exogenous rare earth elements on degradation of lignocellulose by GX5.When the GX5 with rare earth element lanthanum chloride was inoculated to the compost,and the temperature increased to 70.3~oC on the third day of composting and remained above 60~oC for3 days.The maximum temperature of the compost is higher than that without lanthanum chloride,it is showed that lanthanum chloride was beneficial to the degradation of organic compounds in the compost by strain GX5.The degradation rates of hemicellulose,cellulose,and lignin at thermophilic stage were 28.40%,18.40%,and 10.70%,respectively.At the end of composting,the organic matter degradation rate was 19.94%,the electric conductivity(EC)value was generally stable at 2～3 m S/cm,germination index(GI)was 221.80%.Three-Dimensional Excitation Emission Matrix Spectra analysis(3D-EEM)showed that at the mesophilic phase of compost,the content of aromatic protein and soluble microbial metabolite was 75.45%,and the content of fulvic acid and region humic acid was 24.55%.At the end of the composting,the content of aromatic proteins and humic acids was 46.00%and 54.00%.The humification of the compost increased obviously.Microorganisms can promote the metabolic process and humification in compost containing lanthanum chloride,not only accelerates the degradation and transformation of lignocellulose,but also improves the humification of compost.The quality of compost products complied with the standards of organic fertilizers in China(NY/T 525-2021).