棉花长期连作对新疆土壤细菌群落结构的影响

新疆部分棉区发生连作障碍后与其他地区有所不同,能自发恢复并长期保持高产、稳产,为了查明该类棉田土壤细菌群落结构在连作障碍发生及自发恢复整个过程中的演替规律。以未开垦土地作为对照,利用16S rRNA-PCR-DGGE(polymerase chain reactiondensity gradient gel electrophoresis)法对比研究了新疆阿克苏棉区分别连作1、3、5、10、15和20a棉田1—30 cm深度土壤细菌群落结构组成。结果表明未开垦土地细菌多样性指数丰富度最高,多样性和均匀度指数最低。随着棉花连作年限的延长,土壤细菌群落丰富度指数不断下降,而多样性和均匀度指数逐渐增大。当连作年限继续延长至10a后各指数出现恢复或趋于达到一个新的相对稳定状态。聚类分析显示7个样品分别聚为3簇,其中连作3a的样品差异最大,相似度仅有44%,而连作10a后的样品和对照较为相似。主成分分析也有类似的结果。对比回收的部分序列显示,序列间相似性在88%以上,分属于Microbacterium、Uncultured Chloroflexi bacterium、TM7 Phylum sp.Canine、Flavobacteria等4个不同菌属。分析认为棉花长期连作对该地区土壤细菌群落结构组成影响很大,但随着连作年限延长至5a以后,细菌群落结构组成能自发趋于稳定和回升。此外,对比棉田细菌群落结构整体变化规律和棉花产量的增减及病虫害发生规律发现,在棉花长期连作过程中两者有很强的关联性。

Analysis of the bacterial communities in continuous cotton fields of Xinjiang Province

Long-term continuous cropping of cotton has caused dramatic soil-borne diseases in many places, leading to substantial agricultural losses. However, in some areas of Xingjang Province, the obstacles caused by continuous cotton cropping can spontaneously restore and maintain high yields of cotton for many years. To analyze the variable spectrum of soil bacterial communities and changes in the community structure in these spontaneously restored fields during cropping, soils at depths from 1 to 30 cm were sampled from cotton fields with a history of 0, 1, 3, 5, 10, 15 or 20 years of cotton cropping in the Akesu region of Xinjiang Province. The bacterial communities in these samples were studied using 16S rRNA-based polymerase chain reaction-density gradient gel electrophoresis (PCR-DGGE) with samples from uncultivated land as a control. Bacterial community diversity indices including the Shannon-Wiener diversity (H), Abundance index (S) and Evenness index (E_H) were compared among these samples. Samples from uncultivated land had relatively high levels of the richness indices but both the Diversity and Evenness Indices were at lower levels. With increasing years of cotton cropping, both the bacterial Diversity and Evenness Indices increased, whereas the richness indices showed a general decrease. However, after 10 years of continuous cropping all these indices were restored to their original values or reached a relative stable level. Cluster analysis of DGGE fragments indicated that the seven samples were clustered into three branches: fragments from samples under successional cropping for 0, 10 and 20 years formed one small branch with a similarity of approximately 50%; fragments from successional cropping for 1, 5 and 15 years formed another branch with a similarity of 53%; and the last branch comprised fragments from successional cropping for 3 years with a similarity of 44%. Principal component analysis (PCA) showed that all of the samples were statistically correlated with the major component and fluctuated on the right of the major component between the positive and negative axes of the second principal component. Both cluster analysis and PCA results suggested that, compared to those from original uncropped fields, the bacterial community structure showed the most variation in samples from the field of 3-year cropping, whereas similar patterns of bacterial community structure were found between samples from fields of 10 years of cotton cropping and those from uncropped fields. Nineteen clones were sequenced from each band and among them one sequence was selected and submitted to GenBank (accessory no. JN572545-JN572563). By aligning with the GenBank database, all sequences from DGGE were classified into four groups: Microbacterium, Uncultured Chloroflexi bacterium, TM7 Phylum sp. canine, and Flavobacteria. Further analysis demonstrated that the isolated V3 sequences showed a homology of 88%-100% to known sequences in GenBank and 47% of the sequences belonged to bacteria which were not cultured. No microbial data were correlated with soil-borne plant diseases of cotton. The study demonstrated that the age of cotton fields had significant effects on soil bacterial diversity. Continuous cotton cropping exerted significant influences on the community structure of soil bacteria in Xinjiang Province, with an initial suppression effect on bacterial diversity. However, the bacterial community reached a stabilized or even increased level compared with its original state after 5 years of continuous cropping. In addition, correlations between variations in the bacterial community structure at a depth of 1-30 cm and the yield of cotton and pest disease attacks were also found in this study.