Soil eukaryotic microorganism succession as affected by continuous cropping of peanut--pathogenic and beneficial fungi were selected

Peanut is an important oil crop worldwide and shows considerable adaptability but growth and yield are negatively affected by continuous cropping. Soil micro-organisms are efficient bio-indicators of soil quality and plant health and are critical to the sustainability of soil-based ecosystem function and to successful plant growth. In this study, 18S rRNA gene clone library analyses were employed to study the succession progress of soil eukaryotic micro-organisms under continuous peanut cultivation. Eight libraries were constructed for peanut over three continuous cropping cycles and its representative growth stages. Cluster analyses indicated that soil micro-eukaryotic assemblages obtained from the same peanut cropping cycle were similar, regardless of growth period. Six eukaryotic groups were found and fungi predominated in all libraries. The fungal populations showed significant dynamic change and overall diversity increased over time under continuous peanut cropping. The abundance and/or diversity of clones affiliated with Eurotiales, Hypocreales, Glomerales, Orbiliales, Mucorales and Tremellales showed an increasing trend with continuous cropping but clones affiliated with Agaricales, Cantharellales, Pezizales and Pyxidiophorales decreased in abundance and/or diversity over time. The current data, along with data from previous studies, demonstrated that the soil microbial community was affected by continuous cropping, in particular, the pathogenic and beneficial fungi that were positively selected over time, which is commonplace in agro-ecosystems. The trend towards an increase in fungal pathogens and simplification of the beneficial fungal community could be important factors contributing to the decline in peanut growth and yield over many years of continuous cropping.     

辣椒连作对土壤细菌群落的影响

【目的】土壤微生物对农业生态系统的长期可持续性至关重要。为探讨不同连作年限对辣椒土壤细菌群落结构和潜在功能的影响。【方法】采用16S rRNA基因高通量测序PICRUSt功能预测相结合的研究方法,对不同连作年限下(1Y、3Y、5Y和10Y)的辣椒土壤细菌微生物群落结构和功能进行分析。【结果】微生物多样性指数和共生网络复杂度随连作年限的延长而降低,同时,连作年限变化对细菌群落组成有显著影响。不同的土壤细菌种群对连作措施的响应程度不一,长期连作增加了变形菌门和拟杆菌门的相对丰度,但降低了绿弯菌门、酸杆菌门、厚壁菌门和髌骨细菌门的相对丰度。PICRUSt功能预测结果表明,延长连作年限改变了土壤细菌整体的氮、磷代谢能力,导致细菌群预测功能基因发生了变化,能量代谢、氨基酸代谢和碳水化合物代谢等重要代谢功能基因减少,而折叠、分类和降解、复制和修复、膜转运、细胞生长与死亡等功能基因丰度明显增加。冗余分析表明,土壤有机质和有效磷是影响细菌群落迁移和功能变化的关键土壤理化因子。【结论】延长辣椒连作年限后,细菌群落结构改变和多样性下降导致土壤微生物群落功能失调可能是造成辣椒连作障碍的原因之一。     

Diversity of prokaryotes associated with soils around coal-fire gas vents in MaNasi county of Xinjiang, China

Bacterial and archaeal diversity in surface soils of three coal-fire vents was investigated by T-RFLP analysis and clone libraries of 16S rRNA genes. Soil analysis showed that underground coal fires significantly influenced soil pH, moisture and NO₃ ^? content but had little effect on other elements, organic matter and available nutrients. Hierarchical cluster analysis showed that bacterial community patterns in the soils were very similar, but abundance varied with geographic distance. A clone library from one soil showed that the bacterial community was mainly composed of Firmicutes, Proteobacteria, Acidobacteria, Bacteroidetes, Planctomycetes, Actinobacteria, and unidentified groups. Of these, Firmicutes was the most abundant, accounting for 71.4 % of the clones, and was mainly represented by the genera Bacillus and Paenibacillus. Archaeal phylotypes were closely related to uncultivated species of the phyla Crenarchaeota (97.9 % of clones) and Thaumarchaeota (2.1 %). About 28 % of archaeal phylotypes were associated with ammonia oxidization, especially phylotypes that were highly related to a novel, ammonia-oxidizing isolate from the phylum Thaumarchaeota. These results suggested that microbial communities in the soils were diverse and might contain a large number of novel cultivable species with the potential to assimilate materials by heterotrophic metabolism at high temperature.     

连作对芝麻根际土壤微生物群落的影响

采用稀释平板计数法研究了不同连作年限处理芝麻根际土壤细菌、真菌、放线菌、芽孢杆菌、尖孢镰刀菌(FO)和青枯劳尔氏菌(RS)数量的变化情况。结果表明,随着连作年限的增加,芝麻根际土壤中细菌和放线菌的数量下降,而真菌的数量则呈上升趋势。新种芝麻地根际土壤芽孢杆菌数量显著高于连作2a处理和连作5a处理,而连作2a处理又显著高于连作5a处理;连作5a芝麻根际土壤尖孢镰刀菌数量显著高于新种地、轮作1a和连作2a等3个处理;轮作1a、连作2a及连作5a等3个处理青枯劳尔氏菌数量显著高于新种地处理。说明连作导致土壤微生物环境恶化,引起根际微生物区系结构发生定向改变。连作2a与轮作1a相比,各菌群(类群)数量差异均不显著。     

Analysis of bacterial communities in rhizosphere soil of continuously cropped healthy and diseased konjac

The bacterial community and diversity in healthy and diseased konjac rhizosphere soils with different ages of continuous cropping were investigated using next-generation sequencing. The results demonstrated that the number of years of continuous cropping significantly altered soil bacterial community and diversity. Soil bacterial Shannon diversity index and Chao 1 index decreased with the increasing cropping years of konjac. After 1 year of cropping, the soil exhibited the highest bacterial relative abundance and diversity. Of the 44 bacterial genera (relative abundance ratio of genera greater than 0.3%), 14 were significantly affected by the duration of continuous cropping and plant status. With increasing continuous cropping, Alicyclobacillus decreased, while Achromobacter, Lactobacillus, Kaistobacter, Rhodoplanes increased after 3 years continuous cropping. Continuous cropping altered the structure and composition of the soil bacterial community, which led to the reduction in the beneficial bacteria and multiplication of harmful bacteria. These results will improve our understanding of soil microbial community regulation and soil health maintenance in konjac farm systems.     

The Effect of Long-Term Continuous Cropping of Black Pepper on Soil Bacterial Communities as Determined by 454 Pyrosequencing

In the present study, 3 replanted black pepper orchards with continuously cropping histories for 10, 21, and 55 years in tropical China, were selected for investigating the effect of monoculture on soil physiochemical properties, enzyme activities, bacterial abundance, and bacterial community structures. Results showed long-term continuous cropping led to a significant decline in soil pH, organic matter contents, enzymatic activities, and resulted in a decrease in soil bacterial abundance. 454 pyrosequencing analysis of 16S rRNA genes revealed that the Acidobacteria and Proteobacteria were the main phyla in the replanted black pepper orchard soils, comprising up to 73.82% of the total sequences; the relative abundances of Bacteroidetes and Firmicutes phyla decreased with long-term continuous cropping; and at genus level, the Pseudomonas abundance significantly depleted after 21 years continuous cropping. In addition, bacterial diversity significantly decreased after 55 years black pepper continuous cropping; obvious variations for community structures across the 3 time-scale replanted black pepper orchards were observed, suggesting monoculture duration was the major determinant for bacterial community structure. Overall, continuous cropping during black pepper cultivation led to a significant decline in soil pH, organic matter contents, enzymatic activities, resulted a decrease in soil bacterial abundance, and altered soil microbial community membership and structure, which in turn resulted in black pepper poor growth in the continuous cropping system.     

p-Coumaric Acid Influenced Cucumber Rhizosphere Soil Microbial Communities and the Growth of Fusarium oxysporum f.sp. cucumerinum Owen

Background

Autotoxicity of cucumber root exudates or decaying residues may be the cause of the soil sickness of cucumber. However, how autotoxins affect soil microbial communities is not yet fully understood.

Methodology/Principal Findings

The aims of this study were to study the effects of an artificially applied autotoxin of cucumber, p-coumaric acid, on cucumber seedling growth, rhizosphere soil microbial communities, and Fusarium oxysporum f.sp. cucumerinum Owen (a soil-borne pathogen of cucumber) growth. Abundance, structure and composition of rhizosphere bacterial and fungal communities were analyzed with real-time PCR, PCR-denaturing gradient gel electrophoresis (DGGE) and clone library methods. Soil dehydrogenase activity and microbial biomass C (MBC) were determined to indicate the activity and size of the soil microflora. Results showed that p-coumaric acid (0.1–1.0 µmol/g soil) decreased cucumber leaf area, and increased soil dehydrogenase activity, MBC and rhizosphere bacterial and fungal community abundances. p-Coumaric acid also changed the structure and composition of rhizosphere bacterial and fungal communities, with increases in the relative abundances of bacterial taxa Firmicutes, Betaproteobacteria, Gammaproteobacteria and fungal taxa Sordariomycete, Zygomycota, and decreases in the relative abundances of bacterial taxa Bacteroidetes, Deltaproteobacteria, Planctomycetes, Verrucomicrobia and fungal taxon Pezizomycete. In addition, p-coumaric acid increased Fusarium oxysporum population densities in soil.

Conclusions/Significance

These results indicate that p-coumaric acid may play a role in the autotoxicity of cucumber via influencing soil microbial communities.     

Elucidating the microbial resuscitation cascade in biological soil crusts following a simulated rain event

Biological soil crusts (biocrusts) are photosynthetic mats formed through an association of prokaryotic and eukaryotic microorganisms with soil particles. Biocrusts are found in virtually any terrestrial ecosystem where vascular plant coverage is abiotically limited, with drylands comprising the primary habitat for them. We studied the dynamics of the active bacterial community in two biocrusts from an arid and a hyperarid region in the Negev Desert, Israel, under light‐oxic and dark‐anoxic incubation conditions after simulated rainfall. We used H₂¹⁸O for hydrating the crusts and analysed the bacterial community in the upper and lower parts of the biocrust using an RNA‐stable isotope probing approach coupled with 454‐pyrosequencing. In both biocrusts, two distinct bacterial communities developed under each incubation condition. The active anaerobic communities were initially dominated by members of the order Bacillales which were later replaced by Clostridiales. The aerobic communities on the other hand were dominated by Sphingobacteriales and several Alphaproteobacteria (Rhizobiales, Rhodobacterales, Rhodospirillales and Rubrobacteriales). Actinomycetales were the dominant bacterial order in the dry crusts but quickly collapsed and accounted for < 1% of the community by the end of the incubation. Our study shows that biocrusts host a diverse community whose members display complex interactions as they resuscitate from dormancy.     

Variation of Bacterial Community Diversity in Rhizosphere Soil of Sole-Cropped versus Intercropped Wheat Field after Harvest

As the major crops in north China, spring crops are usually planted from April through May every spring and harvested in fall. Wheat is also a very common crop traditionally planted in fall or spring and harvested in summer year by year. This continuous cropping system exhibited the disadvantages of reducing the fertility of soil through decreasing microbial diversity. Thus, management of microbial diversity in the rhizosphere plays a vital role in sustainable crop production. In this study, ten common spring crops in north China were chosen sole-cropped and four were chosen intercropped with peanut in wheat fields after harvest. Denaturing gradient gel electrophoresis (DGGE) and DNA sequencing of one 16S rDNA fragment were used to analyze the bacterial diversity and species identification. DGGE profiles showed the bacterial community diversity in rhizosphere soil samples varied among various crops under different cropping systems, more diverse under intercropping system than under sole-cropping. Some intercropping-specific bands in DGGE profiles suggested that several bacterial species were stimulated by intercropping systems specifically. Furthermore, the identification of these dominant and functional bacteria by DNA sequencing indicated that intercropping systems are more beneficial to improve soil fertility. Compared to intercropping systems, we also observed changes in microbial community of rhizosphere soil under sole-crops. The rhizosphere bacterial community structure in spring crops showed a strong crop species-specific pattern. More importantly, Empedobacter brevis, a typical plant pathogen, was only found in the carrot rhizosphere, suggesting carrot should be sown prudently. In conclusion, our study demonstrated that crop species and cropping systems had significant effects on bacterial community diversity in the rhizosphere soils. We strongly suggest sorghum, glutinous millet and buckwheat could be taken into account as intercropping crops with peanut; while hulled oat, mung bean or foxtail millet could be considered for sowing in wheat fields after harvest in North China.     

Prokaryotic diversity in continuous cropping and rotational cropping soybean soil

In spite of the techniques based on the amplification of 16S rRNA genes (16S rDNA) to compare bacterial communities that are now widely in use in microbial ecology, little is known about the composition of the soybean continuous cropping (CC) and rotational cropping (RC) soil microbial community. To address this, we compared the levels of bacterial community diversity in RC and 5-year CC rhizosphere soil samples. We selected 407 clones in RC and 490 clones in CC for restriction fragment length polymorphism analysis. A total of 123 phylotypes were identified among the 16S rDNA clones, while 78 unique and 21 common phylotypes were identified among the CC soil isolates. Analysis of sequences from a subset of the phylotypes showed that at least 11 bacterial divisions were represented in the clone libraries. The phylotype richness, frequency distribution (evenness), and composition of the two clone libraries were investigated using a variety of diversity indices. Although the analysis of diversity indices and LIBSHUFF comparisons revealed that the compared libraries were not significantly different ( P =0.05) between the RC vs. CC soils, some differences could be observed in terms of specific phyla and groups. We concluded that the group variance was not determined immediately by the cropping system's induction, but was a long-term and slow process.     

The accumulation and transport of abscisic acid insoybean (Glycine max L.) under aluminum stress

It is well known that agricultural practices change the physical and chemical characteristics of soil. As a result, microbial populations can also be affected. The aim of this study was to analyze the effect on soil bacterial communities of zero tillage (ZT) under maize monoculture (MM) with crop residue removal (-R) (MM/-R treatment), compared to a ZT system under wheat monoculture (WW) with crop retention (+R) (WW/+R treatment). Phylogenetic analysis was used to characterize soil bacterial communities. Phylogenetic groups found exclusively in MM/-R were Caldilineales, Chromatiales, Oscillatoriales, Legionellales, Nitrosomonadales and unclassified ?-Proteobacteria, while Bacillales, Burkholderiales, Pseudomonadales and Rubrobacteriales were found only in WW/+R. Sequences of bacteria related to fluorescent Pseudomonas sp. were detected only in WW/+R. Acidobacteria, a largely unknown group of bacteria, were the dominant group in both treatments with a relative proportion of 0.703 and 0.517 for MM/+R and WW/-R respectively. It was found that zero tillage with removal of crop residue in soil cultivated with a monoculture of maize strongly reduced microbial diversity (H?=?3.30; D?=?0.9040) compared to soil where crop residue was retained in a wheat zero tillage situation (H?=?4.15; D?=?0.9848).     

Highly Heterogeneous Soil Bacterial Communities around Terra Nova Bay of Northern Victoria Land,Antarctica

Given the diminished role of biotic interactions in soils of continental Antarctica, abiotic factors are believed to play a dominant role in structuring of microbial communities. However, many ice-free regions remain unexplored, and it is unclear which environmental gradients are primarily responsible for the variations among bacterial communities. In this study, we investigated the soil bacterial community around Terra Nova Bay of Victoria Land by pyrosequencing and determined which environmental variables govern the bacterial community structure at the local scale. Six bacterial phyla, Actinobacteria, Proteobacteria, Acidobacteria, Chloroflexi, Cyanobacteria, and Bacteroidetes, were dominant, but their relative abundance varied greatly across locations. Bacterial community structures were affected little by spatial distance, but structured more strongly by site, which was in accordance with the soil physicochemical compositions. At both the phylum and species levels, bacterial community structure was explained primarily by pH and water content, while certain earth elements and trace metals also played important roles in shaping community variation. The higher heterogeneity of the bacterial community structure found at this site indicates how soil bacterial communities have adapted to different compositions of edaphic variables under extreme environmental conditions. Taken together, these findings greatly advance our understanding of the adaption of soil bacterial populations to this harsh environment.     

Diversity and dynamics stability of bacterial community in traditional solid-state fermentation of Qishan vinegar

Qishan vinegar is a typical Chinese fermented cereal product that is prepared using traditional solid-state fermentation (SSF) techniques. The final qualities of the vinegar produced are closely related to the multiple bacteria present during SSF. In the present study, the dynamics of microbial communities and their abundance in Daqu and vinegar Pei were investigated by the combination of high throughput sequencing and quantitative PCR. Results showed that the Enterobacteriales members accounted for 94.7%, 94.6%, and 92.2% of total bacterial sequences in Daqu Q3, Q5, and Q10, respectively. Conversely, Lactobacillales and Rhodospirillales dominated during the acetic acid fermentation (AAF) stage, corresponding to the quantitative PCR results. Lactobacillus, Acetobacter, Weissella, Leuconostoc and Bacillus were the dominant and characteristic bacterial genera of Qishan vinegar during AAF process. Redundancy analysis suggested that Lactobacillales and Rhodospirillales had a positive correlation with humidity and acidity, respectively. These results confirmed that the bacterial community structure could be affected by physiochemical factors, which determined the unique bacterial composition at different fermentation stages and showed batch-to-batch consistency and stability. Therefore, the conformity of bacterial community succession with physiochemical parameters guaranteed the final quality of Qishan vinegar products. This study provided a scientific perspective for the uniformity and stability of Qishan vinegar, and might aid in controlling the manufacturing process.     

Acromyrmex Leaf-Cutting Ants Have Simple Gut Microbiota with Nitrogen-Fixing Potential

Ants and termites have independently evolved obligate fungus-farming mutualisms, but their gardening procedures are fundamentally different, as the termites predigest their plant substrate whereas the ants deposit it directly on the fungus garden. Fungus-growing termites retained diverse gut microbiota, but bacterial gut communities in fungus-growing leaf-cutting ants have not been investigated, so it is unknown whether and how they are specialized on an exclusively fungal diet. Here we characterized the gut bacterial community of Panamanian Acromyrmex species, which are dominated by only four bacterial taxa: Wolbachia, Rhizobiales, and two Entomoplasmatales taxa. We show that the Entomoplasmatales can be both intracellular and extracellular across different gut tissues, Wolbachia is mainly but not exclusively intracellular, and the Rhizobiales species is strictly extracellular and confined to the gut lumen, where it forms biofilms along the hindgut cuticle supported by an adhesive matrix of polysaccharides. Tetracycline diets eliminated the Entomoplasmatales symbionts but hardly affected Wolbachia and only moderately reduced the Rhizobiales, suggesting that the latter are protected by the biofilm matrix. We show that the Rhizobiales symbiont produces bacterial NifH proteins that have been associated with the fixation of nitrogen, suggesting that these compartmentalized hindgut symbionts alleviate nutritional constraints emanating from an exclusive fungus garden diet reared on a substrate of leaves.     

Characterization of an alkane-degrading methanogenic enrichment culture from production water of an oil reservoir after 274 days of incubation

Oil reservoirs represent special habitats for the activity of anaerobic microbial communities in the transformation of organic compounds. To understand the function of microbial communities in oil reservoirs under anaerobic conditions, an alkane-degrading methanogenic enrichment culture was established and analyzed. Results showed that a net 538 ??mol of methane higher than the controls were produced over 274 days of incubation in microcosms amended with alkanes and a decrease in the alkanes profile was also observed. Phylogenetic analysis of 16S rRNA gene sequences retrieved from the enrichment microcosms indicated that the archaeal phylotypes were mostly related to members of the orders Methanobacteriales and Methanosarcinales. The bacterial clone library was composed of sequences affiliated with the Firmicutes, Proteobacteria, Deferribacteres, and Bacteroidetes. However, most of the bacterial clones retrieved from the enrichment cultures showed low similarity to 16S rRNA gene sequences of the cultured members, indicating that the enrichment cultures contained novel bacterial species. Though alkane-degrading methanogenic enrichment consortium has rarely been reported from petroleum reservoirs, our results indicated that oilfield production water harbors a microbial community capable of syntrophic conversion of n-alkanes to methane, which sheds light on the bio-utilization of marginal oil reservoirs for enhanced energy recovery.     

Bacterial,archaeal and eukaryal community structures throughout soil horizons of harvested and naturally disturbed forest stands

Disturbances caused by timber harvesting have critical long‐term effects on the forest soil microbiota and alter fundamental ecosystem services provided by these communities. This study assessed the effects of organic matter removal and soil compaction on microbial community structures in different soil horizons 13 years after timber harvesting at the long‐term soil productivity site at Skulow Lake, British Columbia. A harvested stand was compared with an unmanaged forest stand. Ribosomal intergenic spacer profiles of bacteria, archaea and eukarya indicated significantly different community structures in the upper three soil horizons of the two stands, with differences decreasing with depth. Large‐scale sequencing of the ribosomal intergenic spacers coupled to small‐subunit ribosomal RNA genes allowed taxonomic identification of major microbial phylotypes affected by harvesting or varying among soil horizons. Actinobacteria and Gemmatimonadetes were the predominant phylotypes in the bacterial profiles, with the relative abundance of these groups highest in the unmanaged stand, particularly in the deeper soil horizons. Predominant eukaryal phylotypes were mainly assigned to known mycorrhizal and saprotrophic species of Basidiomycetes and Ascomycetes. Harvesting affected Basidiomycetes to a minor degree but had stronger effects on some Ascomycetes. Archaeal profiles had low diversity with only a few predominant crenarchaeal phylotypes whose abundance appeared to increase with depth. Detection of these effects 13 years after harvesting may indicate a long‐term change in processes mediated by the microbial community with important consequences for forest productivity. These effects warrant more comprehensive investigation of the effects of harvesting on the structure of forest soil microbial communities and the functional consequences.     

三种根系分泌脂肪酸对花生生长和土壤酶活性的影响

为了探讨花生连作后土壤中脂肪酸类物质的累积与花生连作障碍间的关系,为花生连作障碍机理的研究提供新的理论依据,以田间土壤为介质,采用盆栽试验的方法研究了花生根系分泌物中3种长链脂肪酸,即:豆蔻酸、软脂酸和硬脂酸的混合物,对花生植株生长、产量和土壤酶活性的影响。结果表明,当土壤中脂肪酸的初始含量较低时(80 mg/kg土),对花生植株的生长和产量有微弱的促进作用(P>0.05),当土壤中脂肪酸的初始含量较高时(160 mg/kg土和240 mg/kg土),显著抑制了花生植株的生长和产量(P<0.05)。叶片叶绿素含量、根系活力、土壤酶(蔗糖酶、脲酶、磷酸酶)活性在低脂肪酸含量处理下升高,在高脂肪酸含量处理下显著降低(P<0.001)。光合产物、根际有效养分的减少和根系养分吸收能力的降低,可能是导致花生植株生长和产量降低的原因之一。花生连作土壤中豆蔻酸、软脂酸和硬脂酸的累积与花生的连作障碍有着密切关系。     

Palmitic acid mediated change of rhizosphere and alleviation of Fusarium wilt disease in watermelon

Palmitic acid (PA) in root exudates or decaying residues can reduce the incidence of soil-borne diseases and promote the growth of some crop plants. However, the effects of PA on soil-borne pathogens and microbial communities are poorly understood. Here, we investigate the effects of PA on overall watermelon microbial communities and the populations of Fusarium oxysporum f.sp. niveum (Fon). The effects of PA on the mycelial growth and spore production of Fon were tested in vitro, while its effects on Fon, total bacteria and total fungi populations, and microbial communities were evaluated in a pot experiment. The results revealed that all test concentrations of PA inhibited Fon mycelia growth and spore production. The pot experiment showed that 0.5 mM and 1 mM PA reduced Fon but increased total bacteria populations, and 0.5 mM and 1 mM PA 0.5 mM and 1 mM PA promoted the change to a soil type of bacteria soil. Meanwhile, 0.5 mM PA and 1 mM PA altered the community composition of the rhizosphere microorganisms and reduced the relative abundance of two bacterial operational taxonomic units (OTUs) and the two fungal OTUs that were significantly (p < 0.01) related with disease severity and increased that of four bacterial OTUs and the two fungal that were highly significantly (p < 0.01) negatively correlated with the disease severity. These results suggest that application of PA decreased the populations of Fon, changed the rhizosphere microbial composition, reduced the disease severity of Fusarium wilt, and promoted the growth of watermelon.     

Responses of Bacterial Communities in Arable Soils in a Rice-Wheat Cropping System to Different Fertilizer Regimes and Sampling Times

Soil physicochemical properties, soil microbial biomass and bacterial community structures in a rice-wheat cropping system subjected to different fertilizer regimes were investigated in two seasons (June and October). All fertilizer regimes increased the soil microbial biomass carbon and nitrogen. Both fertilizer regime and time had a significant effect on soil physicochemical properties and bacterial community structure. The combined application of inorganic fertilizer and manure organic-inorganic fertilizer significantly enhanced the bacterial diversity in both seasons. The bacterial communities across all samples were dominated by Proteobacteria, Acidobacteria and Chloroflexi at the phylum level. Permutational multivariate analysis confirmed that both fertilizer treatment and season were significant factors in the variation of the composition of the bacterial community. Hierarchical cluster analysis based on Bray-Curtis distances further revealed that bacterial communities were separated primarily by season. The effect of fertilizer treatment is significant (P = 0.005) and accounts for 7.43% of the total variation in bacterial community. Soil nutrients (e.g., available K, total N, total P and organic matter) rather than pH showed significant correlation with the majority of abundant taxa. In conclusion, both fertilizer treatment and seasonal changes affect soil properties, microbial biomass and bacterial community structure. The application of NPK plus manure organic-inorganic fertilizer may be a sound fertilizer practice for sustainable food production.     

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。