Decomposition Characteristics Of Crop Straws And Their Effects On Soil Microorganisms And Phytophthora Nicotianae In Paddy Tobacco Fields

Tobacco is one of the important economic crops.Tobacco-rice and tobacco-rape rotation are often used in Southern China.The biomass of crop straws is huge in paddy tobacco fields,which is an important organic matter resource.In recent years,direct returning to the field has become the main way of comprehensive utilization.Tobacco black shank caused by Phytophthora nicotianae is a common destructive soil borne disease in China.It is difficult to prevent and control.It is an important topic to study the characteristics of straw decomposition and carbon and nitrogen mineralization in tobacco planting soil,and analyze the occurrence of diseases under the condition of straw returning to the field.Straws are decomposed under the action of microorganisms.The metabolites of decomposition process can affect Phytophthora nicotianae directly.At the same time,the changes of soil microbial community caused by straw decomposition will also have an indirect effect on the survival and infection of Phytophthora nicotianae.Aiming at the common tobacco rotation systems in Southern China,this paper focused on the decomposition characteristics of crop straws and their effects on soil microorganisms and Phytophthora nicotianae,starting from the decomposition characteristics and material structure changes of three crop straws（tobacco,rape and rice）,investigating the transformation of carbon and nitrogen in straw returning field and its effect on the growth of flue-cured tobacco,exploring the responses and interaction of soil microbial diversity and Phytophthora nicotianae to straw decomposition,so as to reveal the relationship among straw,soil and Phytophthora nicotianae and the effect of straw returning on the occurrence of tobacco black shank.The main results are as follows:（1）The decomposition characteristics of three crop straws were different.Under the two soil moisture conditions,the accumulative decomposition rate of three straws was55.69%～69.39%,which showed as rice straw>rape straw>tobacco straw and dry-wet alternate treatment>flooding treatment;the nutrient release rate of straw in each treatment was K>P>N,and the release rates of N,P and K were 60.65%～70.18%,75.33%～84.81%and 93.96%～97.34%respectively in 180 days.FTIR analysis showed that the content of aromatic compounds in tobacco straw was the highest;carbohydrates,amide compounds and aliphatic compounds in straws were gradually decomposed to form carbonate,ammonium salt,nitrate and carboxylate,and Si was released to form Si O₂and silicate in rice straw.TG-DTG analysis showed that hemicellulose decreased or disappeared,relative content of lignin and other difficult decomposable components in the straw increased at the end of decomposition;there were significant differences in fixed carbon and volatile content of the three straws,fixed carbon content of rice straw was the highest and volatile content was the lowest,rape straw was the opposite.（2）The addition of three straws had a significant effect on soil carbon and nitrogen mineralization.The contents of ammonium nitrogen and nitrate nitrogen in different straw treatments were all significantly higher than those in the control,adding the three kinds of straw in two ways significantly increased the nitrification rate and net nitrogen mineralization rate,the net mineralized nitrogen in straw mixed treatments was 3.51～4.53times of CK treatment in 180 days,which was significantly higher than that in straw mulching treatments.The average net nitrogen mineralization rate showed the trend of rice straw>rape straw>tobacco straw.However,straw mulching was more conducive to the mineralization of soil organic carbon,and the release rate and cumulative release amount of CO₂were rape straw>tobacco straw>rice straw.During the whole cultivation period,the mineralization intensity of soil organic carbon was 3.94%～5.87%.The addition of straw significantly increased the contents of total organic carbon and humic substances in soil.The PQ value of rice straw mixed treatment was 43.95%at 180 days,showing the highest soil humification degree.The results of pot experiment showed that straw returning could improve the enzyme activity,organic matter and available nutrient content of tobacco growing soil.Low amount of straw addition can significantly improve the root activity and promote the growth of flue-cured tobacco,the root activity of DS treatments reached147.67～153.97μg·g^-1·h^-1,1.26～1.31 times of the control.And the growth and development of flue-cured tobacco were inhibited by high amount of straw addition.（3）The addition of straw had a significant effect on the microbial diversity of tobacco growing soil.The bacterial community structure showed a dynamic change,compared with the control,the difference of the bacterial diversity level gradually reduced with time,while the fungal diversity level was decreased in the 30 days.The relative abundance of microorganisms with cellulose decomposition ability and nutrient transformation function increased significantly.The relative abundance of Bacteroides and Firmicutes,Pseudomonas,Cellvibrio,and Devosia increased.And the effects of three kinds of straw on bacterial community structure were different.The proportion of Bacteroides and Flavobacterium in rape straw treatments was 16.02%and 4.14%respectively,which was significantly higher than other treatments.Tobacco straw treatments significantly increased the relative abundance of Firmicutes and Herbinix.While rice straw treatments significantly affected Fibrobacteres,Aeromonas and Cellvibrio.Meanwhile,the total relative abundance of Ascomycota and Basidiomycota increased with the addition of straw,and the proportion of Apiotrichum,Cercophora,Scutellinia and Volutella increased significantly.（4）Straw decomposition had significant effects on the growth and infection of Phytophthora nicotianae.The results showed that the three kinds of straw decomposition solution inhibited the hypha growth,sporangia production and zoospore germination of Phytophthora nicotianae,and the extracts decomposed for 30 days had the strongest effects.The inhibition rate was positively correlated with the concentration of the decomposition solution.The overall performance was as follows:rice straw decomposition solution>tobacco straw decomposition solution>rape straw decomposition solution,and their effective medium concentrations were 0.82 g·L^-1,1.40 g·L^-1and 2.08 g·L^-1,respectively.The results of high throughput sequencing showed that there were differences in the microbial community structure of three kinds of solution,and the bacterial diversity of rice straw decomposition solution was the lowest.HPLC-MS/MS analysis showed that there are a lot of allelopathic chemicals in straw decomposition solution.The relative contents of benzoic acid,terpenoids and phenols in rice straw decomposition solution are the highest,while the alkaloids in tobacco straw decomposition solution are 4.91～6.01 times of the other two decomposition solutions.And the results of pot experiment showed that straw addition can affected the infection of Phytophthora nicotianae,the incidence rate of tobacco black shank in DS treatments was the lowest,and the disease index decreased by 74.55%compared with the control.In conclusion,the decomposition characteristics of three kinds of crop straws are different.In the tobacco rotation system reasonable straw returning can increase the soil nitrogen content and carbon fixation and promote the growth of flue-cured tobacco.The metabolites in the process of straw decomposition not only directly inhibit the growth of Phytophthora nicotianae,but also cause the change of soil microbial community structure,and then affect its infection on tobacco plants.