| Continuous cropping obstacle is a major challenge for high-quality tobacco production,which is of great harm to the stability and improvement of tobacco yield and quality.Tobacco continuous cropping slows the growth and development of tobacco plants,deteriorates agronomic and physiological indexes,reduces the yield and quality,and causes serious diseases and insect pests,resulting in huge losses to tobacco production and farmers.To explore the microecological characteristics of continuous cropping tobacco roots and find effective green restoration measures,this study used soil basic property detection,microbial diversity detection and metabonomics to analyze the plants and soil of continuous cropping tobacco for 1-4 years by counting the growth state of tobacco plants,measuring the basic properties of soil,researching the structure and change trends of soil microbial community,analyzing the composition of soil metabolites,and exploring the microecological characteristics of continuous cropping tobacco root system.Based on the research results,combined with the previous test results and the research status at home and abroad,two continuous cropping obstacle restoration measures,Bacillus subtilis dipping root and tobacco-upland rice rotation,were proposed,and the restoration capabilities of the measures were evaluated.The main research results are as follows:1.Effect of continuous cropping on tobacco growthThe growth state of tobacco was obviously affected by the continuous cropping years.It was found that the relationship among tobacco plant height,effective leaf number,maximum leaf length and chlorophyll content in four years was 2 years>1 year>3 years>4 years,while the maximum leaf width,stem circumference and net photosynthetic rate continued to decrease from the first year to the fourth year.The disease index increased year by year with the continuous cropping years,especially the soil-borne diseases.The disease index of tobacco black shank,bacterial wilt and root black rot began to explode in the third year of continuous cropping,and the exploding rate tended to increase.Although the disease index of TMV,CMV and PVY was increasing year by year,it had not reached a significant level.2.Effect of continuous cropping on basic properties of tobacco root-soilContinuous cropping affected the stability of basic properties of tobacco root soil.The influence of cropping years on macroelements was relatively weaker than that of trace elements.The contents of four metal trace elements,available boron and available sulfur decreased with the increase of continuous cropping years,while the available silicon content increased firstly and then decreased.The activity of peroxidase,urease,dehydrogenase and polyphenol oxidase increased with the increase of continuous cropping years,except for the activity of sucrase.3.Effect of continuous cropping on the microbial community structure of tobacco root-soilContinuous cropping of tobacco significantly affected the microbial community structure and diversity in root-soil.With the increase of continuous cropping years,the total richness of bacteria in the soil showed an upward trend,and continuous cropping within a short time increased the uniformity and diversity of bacteria in the soil.With the increase of continuous cropping years,the structure of the soil bacterial community became more and more similar and convergent.After continuous cropping of tobacco,the richness and diversity of soil fungal species showed a trend of first decreasing and then increasing.The original soil fungal community structure had a process of destruction and then remodeling,and the influence of continuous cropping of tobacco on root-soil fungi persisted.Continuous cropping of tobacco had different effects on beneficial and pathogenic microorganisms.With the increase of continuous cropping years,the relative abundance of Bacillus first increased and then decreased.The relative abundance of Colletotrichum gradually increased,while that of Fusarium first decreased and then increased.The relative abundance of Arbuscular Mycorrhizal Fungi first increased and then decreased at the class level.At the genus level,the relative abundance of Septoglomus,Paraglomus and Rhizophagus all showed an increase-decrease trend.4.Effects of continuous cropping on soil metabolitesContinuous cropping of tobacco had a significant effect on soil metabolites.8,367 and3,555 metabolites were identified in the positive and negative ion modes of tobacco continuous cropping soil,respectively.Orthogonal Partial Least-Squares-Discrimination Analysis(OPLS-DA)had built an excellent evaluation model(R~2Y were all greater than 0.99and Q~2Y were all greater than 0.97).Using this model,5,875,7,932 and 8,059 differential metabolites were identified in the comparative combinations of Cc?2 vs Cc?1,Cc?3 vs Cc?1and Cc?4 vs Cc?1,respectively.With the increase of continuous cropping years,a total of 43metabolites were significantly enriched into three metabolic pathways,including two plant hormones and two autotoxins.5.Microecological characteristics of continuous cropping tobacco root systemRedundancy analysis showed that the microbial community structure and metabolite composition of continuous cropping tobacco were affected by different environmental factors.The influence of soil enzyme activity and soil physical and chemical properties on microbial community structure was greater than that on metabolites composition.Eleven factors that had a great influence on the community structure of soil bacteria and fungi and their correlation with community structure were the same.The effect of trace elements in soil on microbial community structure was weaker than that of macroelements.Soil enzyme activity was positively correlated with soil microbial community structure,while a total of seven factors,including plant height,available potassium content,available nitrogen content,available phosphorus content,available iron content,available sulfur content and soil organic matter content were negatively correlated.The influence of soil mass elements on bacterial community structure was less than that of trace elements.Ascomycota significantly affected soil bacterial community structure and soil metabolite composition,while Firmicutes had the greatest influence on soil fungal community structure,and all of them were Bacillus.There were 5 soil fungal amplicons,2 soil bacterial amplicons and 1 soil physical and chemical property index(available zinc content)which had a great influence on soil metabolite composition in a single year.6.Measurement of application effect of exogenous B.subtilisExogenous treatment promoted the growth of tobacco,effectively reduced the incidence of tobacco diseases,improved the basic properties of soil,optimized the structure of the soil microbial community,and increase the output value of tobacco leaves.In the pot experiment,three exogenous application treatments all promoted the improvement of tobacco agronomic traits,among which the single application of B.subtilis had the best effect.Applicating B.subtilis alone or combined with agricultural EM solution had the best effect on reducing the disease index of tobacco black shank.The plants using B.subtilis alone could utilize more available sulfur and available manganese.When B.subtilis was applied alone,the activities of peroxidase,dehydrogenase and polyphenol oxidase in soil were improved.The soil urease activity was the highest when combined with B.subtilis and agricultural EM solution.Exogenous application of B.subtilis made the soil bacterial community structure evolves and gradually become simplified.The species richness of fungi did not change significantly,but the diversity decreased significantly,which led to the change in the uniformity of the fungal community structure,and the community began to evolve in the direction of simplification,which further reduced the diversity of the fungal community.The results of the exogenous application of B.subtilis infield were the same as those of the pot experiment in agronomic characteristics,disease incidence and soil physical and chemical properties of tobacco.In the field experiment,the application of B.subtilis significantly reduced the activity of peroxidase,but increased the activities of dehydrogenase and polyphenol oxidase.Exogenous application of B.subtilis inhibited the growth and reproduction of Actinobacteria and MND1 in field soil,and promoted the colonization of Sphingomonas.At the same time,the species diversity of the bacterial community was increased,and the microbial community structure was directionally evolved to the structure with B.subtilis as the core.It could also accelerate the colonization of Mortierellomycota,slow down the rising trend of relative abundance of Basidiomycota caused by continuous cropping,and make the fungal community structure keep the soil fungal community stable at a low continuous cropping years level,which was different from the continuous cropping fungal community structure.Exogenous application of B.subtilis could significantly improve the plot yield,average procurement price,plot output value and per are output value.7.Measurement of tobacco rotation management abilityRotation could effectively improve the changes of basic soil properties caused by tobacco continuous cropping,and monocotyledonous crops are suitable choices for rotation with tobacco.Compared with tobacco continuous planting for two years,the rotation of monocotyledons and tobacco could increase soil p H more effectively.At the same time,upland rice-tobacco rotation significantly increased exchangeable calcium content and available silicon content,while tobacco-millet rotation could significantly increase the contents of available iron and available copper in the soil.Rotation treatment significantly affected soil enzyme activities,and each enzyme activity was significantly affected by crop species.The crops of rotation significantly affected soil microbial diversity.Upland rice rotation planting was beneficial to increase soil bacterial richness,but other rotation combinations were not conducive to increasing soil bacterial abundance.With the change of rotation crops,rhizosphere soil also screened or recruited different bacteria to colonize.The richness and diversity of fungal species in tobacco-upland rice rotation soil were the highest,which indicated that the microbial community structure in tobacco-upland rice rotation soil was relatively good.Combining the basic properties of soil and the microbial community structure of the soil,it could be concluded that upland rice was suitable for rotation with tobacco,which could improve the basic properties of soil and optimize the microbial community structure.This study comprehensively analyzed the microecological characteristics of continuous cropping tobacco roots,and pertinently put forward and measured the control ability of exogenous use of B.subtilis and tobacco rotation.These results provided a theoretical and applied basis for analyzing and overcoming the tobacco continuous cropping obstacles,which was conducive to optimizing the land use patterns in tobacco-planting areas,improving the economic benefits per unit area and ensuring the green,ecological and sustainable production of tobacco leaves. |
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