The Mechanism Of Soil Microbial Communities Diversity On The Continuous Cropping Of Flue-Cured Tobacco In The Mountainous Areas Of Southwest China

Flue-cured tobacco is an important cash crop in the mountainous areas of Southwest China.However,continuous cropping of flue-cured tobacco is a very serious matter because of the limited amount of cultivated land in this area.Continuous cropping led to the imbalance of soil nutrients,deterioration of physiochemical properties and aggravation of soilborne diseases.Soil microorganisms are important parts of soil ecosystem.They are involved in a variety of life processes in the soil,such as decomposition of organic matter,nutrient cycling,and capacity transfer.Microorganisms also play a key role in soil purification,soil health,and soil ecological stability.Long-term continuous cropping changes the soil ecological environment,and soil microbial communities also change accordingly.However,our knowledge of the changes in soil microbial structure and diversity with the continuous cropping of flue-cured tobacco is limited.The interaction of soil fungal communities under different cropping systems remains unclear.Therefore,the mechanism of soil microbial community structure and diversity in continuous cropping needs to be further expounded.In this study,we presented five different treatments,including four treatments with flue-cured tobacco continuous cropping for 3years,5 years,9 years and 13 years,and flue-cured tobacco-pea rotation was as the control.We used the quantitative polymerase chain reaction and high-throughput sequencing technology to comparatively investigate the soil microbial abundance,diversity,and community composition.And we might to study the relationship between microbial communities,soil properties and the yield of flue-cured tobacco.Therefore,we constructed soil microbial co-occurrence networks,predicted the potential functions of microorganisms,analyzed the variation rules of potential soil pathogens,and identified the response mechanism of soil microbial communities to continuous cropping.The results showed that:1.Long-term continuous cropping of flue-cured tobacco cause soil acidification,with some cases being regarded as extreme.After 9 years and 13 years continuous cropping,the soil pH was 5.87 and 5.72,far from the required level for high-quality flue-cured tobacco production.More over,continuous cropping also affected the soil physicochemical properties.The levels of total nitrogen,total phosphorus and ammonium nitrogen in the soil were enriched continuously as the years of continuous cropping increased.However,the levels of organic matter,nitrate nitrogen and available potassium decreased as the continuous cropping years increased.Furthmore,the content of available phosphorus reached the highest level in continuous cropping within 5 years and thereafter decreased sharply to levels of severe deficit.Soil enzyme activities were also significantly influenced by the continuous cropping of flue-cured tobacco.Urease activity significantly and positively correlated with the increase in continuous cropping years,whereas,in the case of sucrase activity,a negative correlation was found with the increase of continuous cropping years.In addition to this,with the increase in continuous cropping years,catalase activity first decreased,then increased,and then slowly decreased.Continuous cropping inhibited the growth of flue-cured tobacco.Long-term continuous cropping resulted in short stature,slow growth and poor agronomic properties of flue-cured tobacco.After 5 years of continuous cropping,the yield of flue-cured tobacco decreased significantly.The yield of flue-cured tobacco decreased by 51.51% after 13 years of continuous cropping compared with 3 years of continuous cropping.2.Continuous cropping significantly reduced the abundance of soil bacteria.The relative abundance of Sphingomonas and Chujaibacter decreased after continuous cropping,while the abundance of MND1 and Nitrospira increased.Generally speaking,continuous cropping significantly reduced bacterial diversity,and bacterial diversity continuously decreased with an increase in continuous cropping.The organic matter,pH,total phosphorus and ammonium nitrogen in the soil significantly affected bacterial diversity.Bacterial community composition varied significantly under different continuous cropping years.Our results showed that soil pH,available potassium and sucrase were the main factors affecting soil bacterial community structure and composition.Continuous cropping altered the soil bacterial co-occurrence networks.The network formed by continuous cropping was less stable than that formedby crop rotation.The coupling linkages of the bacterial network were mostly negative,suggesting antagonistic or competitive interactions in continuous cropping.In this study,Burkholderia and Holophaga were categorized as keystone taxa in the bacterial network under continuous cropping.3.Long-term continuous cropping of flue-cured tobacco increased the abundance of soil fungal communities,and increased the risk of soil diseases.The abundance of plant pathogenic fungus in the soil,such as Hypocreales and Pleosporales,increased with the number of continuous cropping years.However,the abundance of Sordariales,which can decompose soil toxins,had decreasd.Continuous cropping significantly affected the diversity of fungus in soil.Soil fungal diversity under short-term continuous cropping was lower than that under rotation cropping.However,after continuous cropping for more than 5years,the diversity of the fungal community was higher than that in rotation cropping,and increased as cropping time progressed.The fungal community structure and composition differed under different continuous cropping years.The content of available potassium and available phosphorus in the soil were found to be the main factors affecting fungal community structure and composition.Moreover,the fungal communities were significantly and densely interconnected,formed a complex and stable interaction network in continuous cropping.Meanwhile,Geminibasidium and Cylindrocarpon were categorized as keystone taxa in the fungal network.4.A comparison of the functional predictions of soil bacterial communities from different samples showed that,Carbohydrate metabolism,Nucleotide metabolism,Membrane transport,and Amino acid metabolism were the main functions of bacteria in short-term continuous cropping soil.With the increase in the number of continuous cropping years,bacterial activities such as Cell growth and death,folding,sorting and degradation,Signal transduction,and Cell motility showed an increasing trend.Meanwhile,the Membrane transport,Amino acid metabolism,and Carbohydrate metabolism showed decrease.Functional predictions of the soil fungal communities showed that,the abundance of pathotroph and the pathotroph-saprotroph-symbiotroph fungus were increased by continuous cropping of flue-cured tobacco.Meanwhile,the abundance of Saprotroph-Symbiotroph fungus decreased continuously.Moreovere,Fusarium,Alternaria,and Cylindrocarpon were found to be potentially pathogenic fungus in this study.These fungi highly abundant in continuous cropping soil and have found to increase the occurrence of flue-cured tobacco disease.5.Soil bacterial community had significant positive effect on flue-cured tobacco yield.The diversity of bacterial communities had the greatest contribution to the yield of flue-cured tobacco,followed by the abundance of bacterial communities.Soil fungal communities had a significant negative effect on the yield of flue-cured tobacco.The abundance of fungal communities had the greatest negative effect to the yield of flue-cured tobacco,and the diversity of fungal communities had certain negative effect on the yield of flue-cured tobacco.The decrease of soil pH,organic matter and nitrate nitrogen content in long-term continuous cropping soil caused the decrease of soil bacterial diversity.And the decrease of available phosphorus and available potassium caused the in the increase of the abundance of pathogenic fungi such as Fusarium,Alternaria,and Cylindrocarpon.All of these lead to decine of flue-cured tobacco yield.In conclusion,continuous cropping changed the soil properties and the growth of flue-cured tobacco,affect the community structure and diversity of soil bacteria and fungus,and increase the abundance of potential pathogens.In flue-cured tobacco cultivation,increasing the pH value of soil,increasing the content of organic matter and available nutrients in the soil can increase the bacterial diversity,reduce the abundance of fungi and increase the yield of flue-cured tobacco.