Cucumber Rhizosphere Research Articles

Cucumber Rhizosphere Microbial Community Response to Biocontrol Agent Bacillus subtilis B068150

Gram-positive bacteria Bacillus subtilis B068150 has been used as a biocontrol agent against the pathogen Fusarium oxysporum cucumerinum. Cucumber was grown in three soils with strain B068150 inoculated in a greenhouse for 90 days, and the colonization ability of strain B068150 in cucumber rhizosphere and non-rhizosphere soils was determined. Changes in total bacteria and fungi community composition and structures using denaturing gradient gel electrophoresis (DGGE) and sequencing were determined. Colony counts showed that B068150 colonization in the rhizosphere was significantly higher (p < 0.001) than in non-rhizosphere soils. Based on our data, the introduction of B. bacillus B068150 did not change the diversity of microbial communities significantly in the rhizosphere of three soils. Our data showed that population density of B068150 in clay soil had a significant negative correlation on bacterial diversity in cucumber rhizosphere in comparison to loam and sandy soils, suggesting that the impact of B068150 might be soil specific.

Bacillus cucumis sp. nov. isolated from the rhizosphere of cucumber (Cucumis sativus).

A facultative anaerobic, Gram-positive staining, endospore-forming bacterium, isolated from the rhizosphere of cucumber (Cucumis sativus), was taxonomically investigated. Based on 16S rRNA gene sequence similarity comparisons, strain AP-6T clustered together with other species of the genus Bacillus and showed highest similarities with Bacillus drentensis LMG 21831T (99.1 %), Bacillus vireti LMG 21834T (98.7 %) and Bacillus soli LMG 21838T (98.5 %). The 16S rRNA gene sequence similarity to the sequences of the type strains of other species of the genus Bacillus was 98.5 % or less. Chemotaxonomic features supported the grouping of the strain in the genus Bacillus; for example, the major fatty acids were anteiso-C15 : 0, iso-C15 : 0 and C16 : 0, the polar lipid profile contained the major components diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine and an unidentified glycolipid, the major quinone was menaquinone MK-7 and the major compound in the polyamine pattern was spermidine. Additionally, DNA-DNA hybridization with B. drentensis LMG 21831T, B. vireti LMG 21834T and B. soli LMG 21838T resulted in relatedness values that were clearly below 70 %. Physiological and biochemical test results were also different from those of the most closely related species. As a consequence, AP-6T represents a novel species of the genus Bacillus, for which the name Bacillus cucumis sp. nov. is proposed, with AP-6T ( = CIP 110974T = CCM 8651T) as the type strain.

Analysis and cloning of the synthetic pathway of the phytohormone indole-3-acetic acid in the plant-beneficial Bacillus amyloliquefaciens SQR9.

BackgroundThe plant growth-promoting rhizobacteria (PGPR) strain Bacillus amyloliquefaciens SQR9, isolated from the cucumber rhizosphere, protects the host plant from pathogen invasion and promotes plant growth through efficient root colonization. The phytohormone indole-3-acetic acid (IAA) has been suggested to contribute to the plant-growth-promoting effect of Bacillus strains. The possible IAA synthetic pathways in B. amyloliquefaciens SQR9 were investigated in this study, using a combination of chemical and genetic analysis.ResultsGene candidates involved in tryptophan-dependent IAA synthesis were identified through tryptophan response transcriptional analysis, and inactivation of genes ysnE, dhaS, yclC, and yhcX in SQR9 led to 86, 77, 55, and 24 % reductions of the IAA production, respectively. The genes patB (encoding a conserved hypothetical protein predicted to be an aminotransferase), yclC (encoding a UbiD family decarboxylase), and dhaS (encoding indole 3-acetaldehyde dehydrogenase), which were proposed to constitute the indole-3-pyruvic acid (IPyA) pathway for IAA biosynthesis, were separately expressed in SQR9 or co-expressed as an entire IAA synthesis pathway cluster in SQR9 and B. subtilis 168, all these recombinants showed increased IAA production. These results suggested that gene products of dhaS, patB, yclB, yclC, yhcX and ysnE were involved in IAA biosynthesis. Genes patB, yclC and dhaS constitute a potential complete IPyA pathway of IAA biosynthesis in SQR9.ConclusionsIn conclusion, biosynthesis of IAA in B. amyloliquefaciens SQR9 occurs through multiple pathways.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-015-0323-4) contains supplementary material, which is available to authorized users.

Salt tolerant Methylobacterium mesophilicum showed viable colonization abilities in the plant rhizosphere

The source of infection has always been considered as an important factor in epidemiology and mostly linked to environmental source such as surface water, soil, plants and also animals. The activity of the opportunistic pathogens associated with plant root, their adaptation and survival under hostile environmental condition is poorly understood. In this study the salt tolerance ability of Methylobacterium mesophilicum and its colonization in the root and shoot of plants under severe drought and salt stress conditions were investigated. The colonization of plant by M. mesophilicum was investigated in a gnotobiotic sand system, and their survival in pots with saline soil. Bacterial strain was found to colonize rhizosphere of cucumber, tomato and paprika grown under normal and salt stress condition and reached up to 6.4×104 and 2.6×104CFU/g root. The strain was resistant to Gentamicin, Ampicillin, Amoxicillin plus Clavulanic acid, Cefotaxime, neomycin, penicillin and was also tolerant to salinity stress (up to 6% NaCl). These abilities play important roles in enabling persistent colonization of the plant surface by M. mesophilicum strains. In conclusion, this study provides background information on the behaviour of opportunistic pathogen M. mesophilicum on plants and their survival in harsh environmental conditions.

ISOLATION AND MOLECULAR CHARACTERIZATION OF EGYPTIAN TRICHODERMA AND ASSESSMENT OF THEIR ANTAGONISTIC POTENTIAL AGAINST RHIZOCTONIA SOLANI

Morphological and molecular characterization of antagonistic ability of Trichoderma species was studied. Soil dilution plate method was used to isolate trichoderma from rhizosphere of bean, cowpea, cucumber, wheat and faba bean plants. Based on morphological and cultural characteristics, the Trichoderma isolates were identified as T. harzianum (10 isolates), T. koningii (8 isolates), and T. viride (2 isolates). A portion of rDNA, 560-600 bp was amplified from six biocontrol isolates using ITS1 and ITS 4 primers, and was sequenced and aligned against ex-type strain sequences from TrichoBlast and established Trichoderma taxonomy. Molecular phylogenetic analysis were performed based on nucleotide sequences in order to examine these isolates among 15 accession numbers of Trichoderma spp. found in GenBank. The results indicate that the FUE3, FUE5, FUE6, FUE9 and FUE18 Trichoderma isolates are closely related to Trichoderma koningii, while FUE15 isolate is closely related to Trichoderma harzianim .This result was in accordance with the result obtained from morphological and cultural characteristics. Production of volatile inhibitors and mycoparasitism were investigated using in vitro and in vivo tests in dual culture PDA medium and infected soils. The percent inhibitory effect against growth of Rhizoctonia solani was calculated, T. koningii FUE3 showed the greatest antagonistic effect to the pathogen (57.77%) in vitro experiment whereas T. koningii FUE6 and FUE18 were gave the highest reduction 96% of disease incidence caused by R. solani in greenhouse conditions.

Bacillus methylotrophicus isolated from the cucumber rhizosphere degrades ferulic acid in soil and affects antioxidant and rhizosphere enzyme activities

Ferulic acid (FA) accumulates in soil and inhibits plant growth. We examined whether the FA-degrading bacterium Bacillus methylotrophicus degrades FA in soil. B. methylotrophicus strain CSY-F1 was isolated and applied to unplanted soil and soil planted with Cucumis sativus (cucumber). We analyzed the effects of B. methylotrophicus on rhizosphere enzyme activities and antioxidant enzyme activities in cucumber and CSY-F1 exposed to FA. CSY-F1 degraded FA in culture medium and in soil, giving rise to 4-vinyl guaiacol, vanillin, vanillic acid, and protocatechuic acid. When cucumber seedlings were grown in soil treated with FA, the activities of some soil enzymes decreased, and the malonaldehyde content in cucumber leaves increased. The addition of CSY-F1 to FA-treated soil increased the activities of these soil enzymes, decreased the FA concentration in the soil, and elevated the activities of some antioxidant enzymes in seedlings. Moreover, the levels of superoxide radical, hydrogen peroxide, and malonaldehyde were reduced in cucumber leaves. FA treatment increased the activities of antioxidant enzymes, including superoxide dismutase, catalase, and monodehydroascorbate, in CSY-F1. Bacillus methylotrophicus CSY-F1 has potential applications as an FA-degrading agent in soil, as it mitigates FA stress in cucumber seedlings by activating several antioxidant and soil enzymes.

Erratum to: Contribution of indole-3-acetic acid in the plant growth promotion by the rhizospheric strain Bacillus amyloliquefaciens SQR9

Bacillus amyloliquefaciens SQR9, isolated from the rhizosphere of cucumber, can control Fusarium wilt of cucumber and directly stimulate plant growth. To evaluate its potential agricultural use, the plant growth promotion of B. amyloliquefaciens SQR9 was evaluated, and the relative mechanisms, especially the production of the phytohormone indole-3-acetic acid (IAA), were investigated. The related plant-growth-promoting factors were genetically and chemically analyzed, and a mutant library was constructed for selecting strains with different IAA production. B. amyloliquefaciens SQR9 showed a growth-promoting activity in greenhouse experiments. Plant-growth-promoting factors like extracellular phytase, volatile components including acetoin, 2,3-butanediol, and phytohormone IAA were detected in B. amyloliquefaciens SQR9 cultures grown under laboratory conditions. Three IAA production mutant strains showed variation in plant-growth-promoting effect. IAA production in B. amyloliquefaciens SQR9 was related to its plant-growth-promoting effect, but IAA alone could not account for the overall observed plant-growth-promoting effect. The promoted plant growth by the rhizospheric strain B. amyloliquefaciens SQR9 can be attributed to multiple factors, including production of phytohormones, volatile compounds, and extracellular enzymes. Therefore, the strain B. amyloliquefaciens SQR9 may be used as a plant-growth-promoting agent to increase crop yield.

Biodegradation of p-hydroxybenzoic acid in soil by Pseudomonas putida CSY-P1 isolated from cucumber rhizosphere soil

Aims Accumulation of p-hydroxybenzonic acid (PHBA) in soil will cause plant stress. Our aims were to characterize PHBA-degrading Pseudomonas putida CSY-P1 and to assess its role on alleviating PHBA stress in plants.

Bacterial Diversity in the Rhizosphere of Cucumbers Grown in Soils Covering a Wide Range of Cucumber Cropping Histories and Environmental Conditions

Rhizosphere microorganisms in soils are important for plant growth. However, the importance of rhizosphere microorganisms is still underestimated since many microorganisms associated with plant roots cannot be cultured and since the microbial diversity in the rhizosphere can be influenced by several factors, such as the cropping history, biogeography, and agricultural practice. Here, we characterized the rhizosphere bacterial diversity of cucumber plants grown in soils covering a wide range of cucumber cropping histories and environmental conditions by using pyrosequencing of bacterial 16S rRNA genes. We also tested the effects of compost addition and/or bacterial inoculation on the bacterial diversity in the rhizosphere. We identified an average of approximately 8,883 reads per sample, corresponding to around 4,993 molecular operational taxonomic units per sample. The Proteobacteria was the most abundant phylum in almost all soils. The abundances of the phyla Bacteroidetes, Actinobacteria, Firmicutes, Acidobacteria, and Verrucomicrobia varied among the samples, and together with Proteobacteria, these phyla were the six most abundant phyla in almost all analyzed samples. Analyzing all the sample libraries together, the predominant genera found were Flavobacterium, Ohtaekwangia, Opitutus, Gp6, Steroidobacter, and Acidovorax. Overall, compost and microbial amendments increased shoot biomass when compared to untreated soils. However, compost addition decreased the bacterial α-diversity in most soils (but for three soils compost increased diversity), and no statistical effect of microbial amendment on the bacterial α-diversity was found. Moreover, soil amendments did not significantly influence the bacterial β-diversity. Soil organic content appeared more important than compost and microbial amendments in shaping the structure of bacterial communities in the rhizosphere of cucumber.

Syringic acid inhibited cucumber seedling growth and changed rhizosphere microbial communities

Phenolic compounds enter soil as a result of root exudation and plant residue decomposition, but their impacts on soil microbial communities are poorly understood. In this experiment, effects of syringic acid on cucumber rhizosphere microbial communities were evaluated. Rhizosphere bacterial and fungal community structures and abundances were analyzed with PCR-denaturing gradient gel electrophoresis (DGGE) and quantitative PCR, respectively. Results showed that syringic acid inhibited cucumber seedling growth at concentrations of 0.05 to 0.2 μmol/g soil, and increased rhizosphere soil dehydrogenase activity, microbial biomass carbon content, bacterial 16S rRNA gene and fungal ITS rRNA gene densities, and decreased the bacteria-to-fungi ratio at concentrations of 0.02 to 0.2 μmol/g soil. Syringic acid also changed rhizosphere bacterial and fungal community structures: it decreased the richness, evenness, and diversity indices of rhizosphere bacterial community but had no significant influences on that of fungal community, indicating that syringic acid had different influence on bacterial and fungal communities. Taken together, these results showed that syringic acid inhibited cucumber growth and altered rhizosphere microbial communities, suggesting that syringic acid plays some role in the communication between plants and soil microorganisms.

Enhanced rhizosphere colonization of beneficialBacillus amyloliquefaciensSQR9 by pathogen infection

Root exudates play important roles in root-soil microorganism interactions and can mediate tripartite interactions of beneficial microorganisms-plant-pathogen in the rhizosphere. However, the roles of organic acid components in this process have not been well studied. In this study the colonization of a plant growth-promoting rhizobacterium, Bacillus amyloliquefaciens SQR9, on cucumber root infected by Fusarium oxysporum f. sp. cucumerinum J. H. Owen (FOC) was investigated. Chemotaxis and biofilm formation response of SQR9 to root exudates and their organic acid components were analysed. Infection of FOC on cucumber had a positive effect (3.30-fold increase) on the root colonization of SQR9 compared with controls. Root secretion of citric acid (2.3±0.2μM) and fumaric acid (5.7±0.5μM) was enhanced in FOC-infected cucumber plants. Bacillus amyloliquefaciens SQR9 exhibited enhanced chemotaxis to root exudates of FOC-infected cucumber seedlings. Further experiments demonstrated that citric acid acts as a chemoattractant and fumaric acid as a stimulator of biofilm formation in this process. These results suggest that root exudates mediate the interaction of cucumber root and rhizosphere strain B.amyloliquefaciens SQR9 and enhance its root colonization.

Enhanced control of cucumber wilt disease by Bacillus amyloliquefaciens SQR9 by altering the regulation of Its DegU phosphorylation.

Bacillus amyloliquefaciens strain SQR9, isolated from the cucumber rhizosphere, suppresses the growth of Fusarium oxysporum in the cucumber rhizosphere and protects the host plant from pathogen invasion through efficient root colonization. In the Gram-positive bacterium Bacillus, the response regulator DegU regulates genetic competence, swarming motility, biofilm formation, complex colony architecture, and protease production. In this study, we report that stepwise phosphorylation of DegU in B. amyloliquefaciens SQR9 can influence biocontrol activity by coordinating multicellular behavior and regulating the synthesis of antibiotics. Results from in vitro and in situ experiments and quantitative PCR (qPCR) studies demonstrate the following: (i) that the lowest level of phosphorylated DegU (DegU∼P) (the degQ mutation) impairs complex colony architecture, biofilm formation, colonization activities, and biocontrol efficiency of Fusarium wilt disease but increases the production of macrolactin and bacillaene, and (ii) that increasing the level of DegU∼P by degQ and degSU overexpression significantly improves complex colony architecture, biofilm formation, colonization activities, production of the antibiotics bacillomycin D and difficidin, and efficiency of biocontrol of Fusarium wilt disease. The results offer a new strategy to enhance the biocontrol efficacy of Bacillus amyloliquefaciens SQR9.

Changes in rhizosphere soil microbial communities in a continuously monocropped cucumber (Cucumis sativus L.) system

Continuous cropping is not considered to be a long-term sustainable agricultural practice. How soil microbial properties change under continuous cropping is not yet fully understood. Dynamics of cucumber rhizosphere soil microbial communities were monitored in a continuous cropping system, in which cucumber was monocropped in pots filled with field soils for nine successive croppings under greenhouse conditions. Results showed that rhizosphere soil dehydrogenase activity, microbial biomass carbon (MBC) content and active bacterial community abundance were significantly affected by the number of croppings (P < 0.05). Soil dehydrogenase activity was the lowest in the seventh cropping and increased in the ninth cropping. Soil MBC content and active bacterial community abundance increased in the third cropping and tended to decrease with croppings from the third to the ninth cropping. PCR-denaturing gradient gel electrophoresis (DGGE) analysis of bacterial 16S rRNA gene fragments demonstrated that continuous cropping caused shifts in bacterial community structures at both DNA and RNA levels, and the active bacterial communities were more sensitive to continuous cropping. Overall, our results revealed that soil microbial communities changed during continuous cropping of cucumber, which may be both a cause and a reflection of the poor plant growth performance.

Impact of biocontrol agents Pseudomonas fluorescens 2P24 and CPF10 on the bacterial community in the cucumber rhizosphere

Pseudomonas fluorescens 2P24 and CPF10 are two biocontrol agents that were isolated from suppressive soils of wheat take-all disease in Shandong province, China. The main objective of this study was to assess how these strains impact the bacterial community in cucumber rhizosphere in the field over 56 days. Classical microbiological culturing and molecular tools based on denaturing gradient gel electrophoresis (DGGE) and terminal-restriction fragment length polymorphism (T-RFLP) were used in this study. The survival of P. fluorescens 2P24 and CPF10 in cucumber rhizosphere was monitored by using selective plating. The results showed that the concentration of P. fluorescens 2P24 and CPF10 decreased from ca. 107 CFU/g to ca.103 CFU/g in soil after inoculation. The compared data of T-RFLP and DGGE showed temporary changes in the bacterial community in the soil that was treated with the biocontrol agents. From the 7th to 42nd days after inoculation, the alterations of several specific bacterial groups (Cyanobacterium, Beta-proteobacterium, Staphylococcus, and Bacillus) were detected in the results of DGGE with cloning and sequencing. However, on the 56th day after inoculation, there was no prominent alteration of bacterial community. In general, effects of these two biocontrol agents on indigenous bacteria in cucumber rhizosphere were transient and generally similar.

Effects of different plant root exudates and their organic acid components on chemotaxis, biofilm formation and colonization by beneficial rhizosphere-associated bacterial strains

It is necessary to understand the roles of root exudates involved in plant-microbe interactions to inform practical application of beneficial rhizosphere microbial strains. Colonization of Bacillus amyloliquefaciens SQR9 (isolated from cucumber rhizosphere) and Bacillus subtilis N11 (isolated from banana rhizosphere) of their original host was found to be more effective as compared to the colonization of the non-host plant. Organic acids in the root exudates of the two plants were identified by High performance liquid chromatography (HPLC). The chemotactic response and effects on biofilm formation were assessed for SQR9 and N11 in response to cucumber and banana root exudates, as well as their organic acids components. Citric acid detected exclusively in cucumber exudates could both attract SQR9 and induce its biofilm formation, whereas only chemotactic response but not biofilm formation was induced in N11. Fumaric acid that was only detected in banana root exudates revealed both significant roles on chemotaxis and biofilm formation of N11, while showing only effects on biofilm formation but not chemotaxis of SQR9. The relationship between PGPR strain and root exudates components of its original host might contribute to preferential colonization. This study advances a clearer understanding of the mechanisms relevant to application of PGPR strains in agricultural production.

Trichoderma harzianum T-E5 significantly affects cucumber root exudates and fungal community in the cucumber rhizosphere

The Fusarium wilt in cucumbers, caused by the pathogenic fungus Fusarium oxysporum f. sp. cucumerinum, is a serious and destructive disease worldwide. An effort was made to explore the role of Trichoderma harzianum T-E5 in reducing the incidence of Fusarium wilt. Three treatments (Control, T1, and T2) were established in the greenhouse experiment. The effects of T-E5 on the composition of root exudates and fungal community in the cucumber rhizosphere were measured. Compared with the control, the application of a bio-organic fertilizer (BIO) enriched with T-E5 was found to decrease the incidence of Fusarium wilt notably and promote the growth of cucumber plants. Based on real-time PCR, the population of F. oxysporum in the control without T-E5 increased from 103 to 104 ITS copiesg−1 soil, whereas the population decreased from 103 to 102 ITS copiesg−1 soil in the T1 and T2 treatment groups when T-E5 was included. Significant difference in fungal community was also found among the treatment groups. HPLC analysis showed that the detected levels of phenolic compounds in control were significantly higher than the levels in the samples subjected to T1 and T2 treatments. The root exudates from the control group significantly increased the numbers of germinating spores of the pathogen compared with those from the samples treated with T1 and T2. In conclusion, the modification of root exudates and the fungal community by the application of BIO might account for the effective suppression of Fusarium wilt disease in cucumbers.

The Effect of Biocontrol Bacteria on Rhizosphere Bacterial Communities Analyzed by Plating and PCR-DGGE

Bacillus subtilis B579, which was isolated from rhizosphere of cucumber, exhibited an excellent biocontrol activity on soil-born pathogens under greenhouse conditions. It could colonize in rhizosphere of cucumber with large number of populations after inoculated in plant growth season. To reveal the effect of high level colonization of B. subtilis B579 on rhizobacteria community structure, cultivation-based analysis coupled with denaturing gradient gel electrophoresis (DGGE) analysis were used to profile the changes of rhizobacteria community structure sampling at 1week interval. Cultivation-based and DGGE fingerprinting analysis showed significant plant-dependent and seasonal shifts in rhizobacteria populations. Only minimal and transient effects were observed at 4-9weeks after sowing in samples of B579 treatment, without the pathogen inoculation and showed the best plant growth potential. Sequencing of dominant bands excised from the gel revealed that Streptomyces sp. was the dominate species in soils before and after sowing. Burkholderia sp. was the dominate species in bulk soil, while Bacillus sp. was dominated in rhizosphere within the growth season. Arthrobacter ramosus and Nocardioides sp. were identified as the specific species in samples treated by B579 at the maturity and flowering stages of cucumber.

Induced systemic resistance in cucumber and Arabidopsis thaliana by the combination of Trichoderma harzianum Tr6 and Pseudomonas sp. Ps14

Trichoderma species and fluorescent Pseudomonas spp. have been reported to induce systemic resistance in plants. In this study the effectiveness of a combination of these biological control agents on the efficacy of induced resistance was investigated in cucumber and the model plant Arabidopsis thaliana. Trichoderma harzianum Tr6, and Pseudomonas sp. Ps14, both isolated from the rhizosphere of cucumber, were tested as a single application and in combination for their abilities to elicit induced resistance in cucumber against Fusarium oxysporum f. sp. radicis cucumerinum and in A. thaliana against Botrytis cinerea. The combination of Tr6 and Ps14 induced a significantly higher level of resistance in cucumber, which was associated with the primed expression of a set of defense-related genes upon challenge with Fusarium. In Arabidopsis both Ps14 and Tr6 triggered ISR against B. cinerea but their combination did not show enhanced effects. In the induced systemic resistance-defective Arabidopsis mutant myb72, none of the treatments protected against B. cinerea, whereas in the SA-impaired mutant sid2 all treatments were effective. Taken together, these results indicate that in Arabidopsis Ps14 and Tr6 activate the same signaling pathway and thus have no enhanced effect in combination. The enhanced protection in cucumber by the combination is most likely due to activation of different signaling pathways by the two biocontrol agents.

Artificially applied vanillic acid changed soil microbial communities in the rhizosphere of cucumber (Cucumis sativusL.)

Zhou, X. and Wu, F. 2013. Artificially applied vanillic acid changed soil microbial communities in the rhizosphere of cucumber (Cucumis sativus L.). Can. J. Soil Sci. 93: 13–21. Phenolic acids (PAs) have been implicated as autotoxins of cucumber (Cucumis sativus L.), but doubts about the importance of these compounds also exist due to their low concentrations in the field. The physiological alterations caused by PAs are concentration dependent, and the range of bioactivity is between 0.1 and 1 mM. Here, vanillic acid (VA) (0.02, 0.05, 0.1, 0.2 µmol g−1 soil) was applied into the soil every other day for five times. The effects of VA on C. sativus seedling growth and rhizosphere soil microbial communities were evaluated. Soil bacterial and fungal community structures and sizes were analyzed with polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) and real-time PCR methods, respectively. Vanillic acid significantly inhibited C. sativus seedling growth at concentrations ≥ 0.05 µmol...

Soil microbial communities associated with the rhizosphere of cucumber under different summer cover crops and residue management: A 4-year field experiment

Continuous cropping has led to the predominant decline of soil productivity and crop yields in Chinese vegetable production systems. Recently, the use of summer cover crops to restore degraded soil during vegetable production has attracted attention in China. A 4-year field experiment on a greenhouse cucumber double-cropping system was therefore conducted to understand how different summer cover crops affect soil microbial communities associated with the crop rhizosphere. The treatments included sweet corn with residue removal after harvest (SR), sweet corn with residue incorporation after harvest (SI), common bean with residue removal after harvest (CR), common bean with residue incorporation after harvest (CI), Garland chrysanthemum and edible amaranth as summer cover crops (GR), and bare fallow during the summer period (Control). Soil microbial communities were evaluated with a combination of sole-carbon-source utilization capacities, the population of microorganisms and enzymes. Generally, sole-carbon utilization capabilities and enzyme activities were higher in the summer cover crop-related soils compared to control soil in each cropping season. Multivariate analysis indicated that microbial communities differed among the cropping seasons, and significantly separated summer cover crop treatments from the control. Treatments SR, SI and GR with relatively high cucumber productivity generally showed relatively high urease activity and relatively low Fusarium oxysporum population. We concluded that summer cover crops generally had a greater effect on soil microbial communities associated with crop rhizosphere compared to the traditional bare fallow. Soil microorganisms associated with crop rhizosphere played an important role during the phytoremediation processes of summer cover crops.