Effects Of Continuous Cultivation Of Flue-Cured Tobacco On Soil Microbial Carbon Source Utilization And Community Structure Mediated By Organic Carbon

As a crop with very serious continuous crop barrier,long-term continuous crop leads to the deterioration of microbial community structure.Organic carbon,as an important component of soil nutrients,is both an important nutrient barrier factor in crop disorder and an important nutrient source for soil microorganisms.Therefore,this study was conducted to compare the active organic carbon fraction,organic carbon structure,soil fertility properties,carbon cycle-related enzyme activities,microbial carbon utilization capacity and microbial community structure under different crop years(1a,3a,5a,10a)and crop rotation(NCC)in tobacco soils.The main results of the study are as follows: The organic carbon fraction and organic carbon structure in the soils of successive crops of tobacco,and the driving mechanism of the changes in microbial community structure and soil enzyme activity.The main findings are as follows:(1)The labile organic carbon(LOC)and Light fraction organic carbon(LFOC)in the soil showed a trend of increasing and then decreasing with the increase of continuous crop time,reaching the maximum value in 1a-3a and decreasing to the minimum in 10 a,with a decrease of 17.4%～40.0% compared with the control(p(27)0.05);the particulate organic carbon(POC)and microbial biomass carbon(MBC)decreased with the continuous crop years.POC and MBC decreased with increasing years of continuous crop and reached the lowest level at 10 a,which also led to the decreasing CPMI.In addition,the polysaccharides showed a trend of increasing and then decreasing with the increase of crop duration,and the long-term polysaccharides decreased significantly by 18.32%(p(27)0.05)compared with NCC after 10 a of continuous crop,while the ratio of hydrophobic to hydrophilic carbon and the ratio of aliphatic to aromatic carbon in the organic carbon structure decreased,which increased the biological activity of the organic carbon structure and stabilized the organic carbon structure.The reduction in the ratio of hydrophobic to hydrophilic carbon and the ratio of fatty to aromatic carbon in the organic carbon structure increased the biological activity and stabilized the organic carbon structure.The continuous crop of roasted tobacco significantly reduced the supply of basic nutrients to the soil,resulting in soil acidification.p H decreased by 2.99 and 2.38 at 10 a,respectively,compared with the control,and the alkaline nitrogen,available Fe,available Mn and available Zn decreased continuously with the increase of continuous crop years.Meanwhile,organic matter,effective phosphorus and fast-acting accumulated under short-term continuous crop(≤3a)and depleted under medium and long-term continuous crop(≥5a).The results of redundancy analysis showed that the environmental factors with the greatest influence on the active organic carbon fraction and organic carbon structure in the long term and in the long term were Mn in the active state(49.2% explained)and p H(31.1% explained),respectively.(2)Continuous cropping of roasted tobacco led to a decrease in the utilization capacity of microbial carbon sources in the soil.The Shannon index and the Mc Intosh index of soil microorganisms decreased.The Mc Intosh index decreased by 24.8%(p<0.05)after 10 a of continuous crop at maturity compared to the control.Continuous crop led to a decrease in the activity of amino acid and carbohydrate-based carbon source-associated utilization microorganisms in the soil,which decreased by10.8%～24.2%(p(27)0.05)compared to the NCC after 10 a of continuous crop,and caused an enrichment of phenolic carbon source-associated utilization microorganisms.The results of principal component analysis showed that the main factors leading to changes in the carbon source utilization capacity of soil microorganisms under continuous crop conditions were a decrease in the utilization of amino acids and carbon hydrate carbon sources.The results of redundancy analysis showed that the most significant organic carbon characteristics affecting microbial carbon source utilization efficiency during the peak and maturity periods were polysaccharides(68.5% explained)and aromatic compounds(76.3% explained),respectively.(3)Cultivation of roasted tobacco continuously led to a decrease in soil bacterial and fungal Ace index and Shannoneven index,causing an increase in Simpson index.Nitrospirota and Gemmatimonadota increased by 21.8%～122.9% and 4.3%～166.1%(p(27)0.05),respectively,in bacteria and Leotiomycetes in fungi under NCC treatment compared with each crop year.The relative abundance of Aspergillaceae pathogenic bacteria increased by 54.5%～2752.4% after 5a of continuous crop compared with the rest of the treatments(p(27)0.05).Redundancy analysis showed that particulate organic carbon(POC)and easily oxidizable organic carbon(LOC)were the main drivers of soil bacterial community changes during the vigorous period(65.9% and 63.7%explained,respectively),while aliphatic and aromatic compounds were the main drivers during the maturity period(29.5% and 28.7% explained,respectively).The largest environmental factors affecting fungal community structure in the vigorous stage were microbial mass carbon and polysaccharides(18.2% and 18.0% explained,respectively),while alkyl carbon and aromatic compounds(23.1% and 17.5%explained,respectively)in the mature stage.Long-term continuous cropping reduced the functional activity of soil bacteria related to cellulose hydrolysis and nitrogen fixation and led to an increase in the relative abundance of saprophytic and phytopathogenic nutritional fungi in the soil.In summary,long-term continuous crop in roasted tobacco(5a-10a)reduced the active organic carbon fraction and made the organic carbon structure less stable,which in turn adversely affected the microbial carbon utilization capacity and microbial community composition.In production practice,attention should be paid to timely replenishment of organic materials to increase soil carbon stability by improving organic carbon sequestration rate,so as to regulate the structure and composition of soil microbial communities and transform them from pathogenic to nutritional types to alleviate continuous crop barriers in roasted tobacco.