| Forest litter decomposition is a necessary way to return nutrients to soil.Microorganisms play an important role in the decomposition of lignocellulose and nitrogenous organic matter.There are some problems in coniferous pure plantation,such as the slow decomposition of litter,the low nutrient turnover rate,the decline of soil fertility and the decline of forest productivity.Cultivating coniferous and broad-leaved mixed plantation is the development trend of plantation in the future,which can accelerate litter decomposition and promote the release of carbon and nitrogen nutrients.In this study,Larix kaempferi plantation,Sassafras tzumu plantation,Larix kaempferi and Sassafras mixed plantation in the north subtropical subalpine area were selected as the research objects.Metagenome sequencing,chromatography,nutrient content and enzyme activity were used to study the characteristics of nutrient release,enzyme activity change,microbial community structure and organic carbon and nitrogen metabolism during litter decomposition in three types of plantations.Then,strains with lignocellulose degradation ability were isolated and screened,and their litter degradation ability and genomic characteristics were studied.The aim of this study is to reveal the microbial mechanism of coniferous and broad-leaved litter accelerated decomposition,and to provide theoretical support for reasonable cultivation of mixed plantation and maintenance of long-term productivity.The main conclusions of this study were as follows:(1)Coniferous and broad-leaved mixed plantation could promote litter decomposition,nutrient release,and contribute to soil fertility restoration of plantation land.Compared with the pure Larix plantation,Larix and Sassafras mixed plantation increased the growth of diameter at breast height(DBH)(+0.9cm),height(+1.2m),total carbon(+48.5g/kg,+10.7%),total nireogen(+2.4g/kg,17.1%)content,and the nutrient release rate of Larix and Sassafras mixed litter was higher.The mixed litter increased the abundance of Dothideomycetes(+13.9%)and their carbohydrate and amino acid metabolism related functional genes,and decreased the abundance of Acidobacteriia(-4.6%),Solibacteres(-1.3%),and their carbohydrate and amino acid metabolism related functional genes.Redundance analysis(RDA)showed that the p H value and total carbon content of litter were the main factors leading to the differences of microbial community composition and metabolic pathway among different types of litter.(2)The mixed litter of Larix and Sassafras changed the composition of microbial community and lignocellulose degradation genes,and promoted the degradation of lignocellulose.Compared with the pure Larix litter,the degradation rate of cellulose(+22.16%),hemicellulose(+13.59%),lignin(+4.54%)and the enzyme activities in the mixed litter were significantly increased.The relative abundance of ?-proteobacteria(33.4%)and Dothideomycetes(24.5%)in mixed litter were significantly higher than that of Larix litter(27.4%;10.6%).The composition of bacterial cellulose,hemicellulose and fungal lignin degradation genes were significantly affected by litter types.Mantel analysis showed that the content of lignocellulose in litter was significantly related to the composition of bacterial and fungal communities and enzyme activity(R=0.18-0.35),and fungi were the main decomposers of lignocellulose.The gene families of 26 enzymes in lignocellulosic degradation pathway were identified.Mixed litter consistently increased the abundance of bacterial and fungal glycoside hydrolase(GH3)gene families in cellulose and hemicellulose degradation pathway;selectively increased the abundance of bacterial peroxidase(AA2)and fungal oxidase(AA3,AA5 and AA7)gene families in lignin degradation pathway.(3)Bacterial community played a leading role in nitrogen metabolism during litter decomposition.Mixed litter could promote the production of ammonium by increasing the abundance of functional genes of bacterial organic nitrogen degradation and fungal nitrate assimilation.The mixture of conifer and broadleaf significantly increased the p H value(+0.7),nitrate content(+143mg/kg,50.5%)and enzyme activity of organic nitrogen degradation,and accelerated the release of nitrogen.The mixed litter significantly raised the relative abundance of Sphingomonas(+5.8%),Janthinobacterium(+1.9%)and the potential of nitrogen metabolism.The abundance of microbial organic nitrogen degradation and assimilation nitrate reduction functional genes were significantly affected by litter types.The abundance of bacterial genes related to ammonium and fungal genes related to nitrite in mixed litter were significantly higher than that in Larix litter.The co-occurrence network analyses showed that the genera diversity of microbial community in mixed litter were higher than that of single type litter,the coexistence modes were simple and stable,the negative correlation were less.The p H value and nitrate content of litter were the determinants of microbial nitrogen metabolism potential.(4)Six strains of lignocellulosic decomposing bacteria were LP-2(Trichoderma),LP-5(Alternaria),LP-8(Pseudomonas),LP-9(Delft),LP-10(Streptomyces)and SP-3(new strain),which were the main decomposer of lignocellulose.They had high litter degradation abilities and enzyme activities.In addition to the four microbial groups with traditional lignocellulose degradation functions,the study also found that the strains LP-9(Delft)and SP-3(new strain)also had strong lignocellulose degradation ablities.The results of genome sequencing showed that the six strains contained abundant lignocellulose degradation genes,among which GH3 and AA7 were the key genes in common.Through microcosmic incubation experiments,it was found that the strains responded differently to litter types during decomposition.Compared with Larix litter,mixed litter could significantly improve the strains LP-5(Dothideomycetes,Alternaria),LP-9(?-Proteobacteria,Delft)and SP-3(new strain)litter degradation rate(+6.1%,+7.6%,+3.8%),cellulase and lignin peroxidase activities and the expressions of key genes GH3(?-glucosidase)and AA7(glucooligosaccharide oxidase)for lignocellulose degradation.However,the litter degradation rate(-2.8%)and cellulase activity of the strain LP-10 were lower in mixed litter.In summary,the differences in nutrient release rate among three types of litter were mainly determined by p H,nitrate and total carbon content.The mixing of Larix and Sassafras altered the composition of microbial community and lignocellulose degradation genes in litter,promoted the degradation of lignocellulose by the dominant microbial groups ?-Proteobacteria and Dothideomycetes,raised the nitrogen metabolism potential of Sphingomonas and Janthinobacterium,and accelerated the decomposition and nutrient return of litter.From two levels of microbial community and strain,this study revealed the microbial mechanism of mixed plantation to accelerate litter decomposition,which provided a theoretical basis for the rational cultivation of coniferous and broad-leaved mixed plantation. |
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