Assessment of microbial activity and bacterial community composition in the rhizosphere of a copper accumulator and a non-accumulator

Soil microorganisms may play an important role in the uptake of heavy metals from soils. However, assessments of bacterial activity and community composition in the rhizosphere of accumulators have been largely ignored. We studied potential effects of a copper (Cu)-accumulator, Elsholtzia splendens, and a non-Cu-accumulator plant, Trifolium repens, on soil microbial activity and community composition with increasing Cu addition. The results showed that concentrations of Cu in the shoots of E. splendens were 2.1, 2.2 and 2.4 times those of T. repens under the treatment of different Cu concentrations. Soil microbial biomass and phosphatase activity in the rhizosphere of E. splendens were higher than those of T.repens. PCR-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprint analysis revealed that addition of Cu decreased the number of bands in bare soil and soil with T. repens. However, there was a significant increase in the number of bands in soil with E. splendens incorporated with either 200 or 500 mg kg⁻¹ Cu. The abundances of five phylogenetic groups related most closely to -, β-, γ-proteobacteria, Gram-positive bacteria and CFB group, respectively, were determined in the rhizosphere of plants. Some specific clone such as E13 (metal-contaminated soil clone K20-64) was found in the rhizosphere of E. splendens. Results indicated that E. splendens, as a Cu-accumulator, played an important role in governing soil microbial activity and bacterial community composition in the rhizosphere in response to Cu stress.     

棉花根际营养滴灌技术研究初报

研究了棉花根际营养滴灌不同方式和施肥技术。结果表明,与漫灌相比,根际营养滴灌能改善棉田土壤结构和通透性,节水省肥,棉花根系发达,耗水系数低,水分生产效率高,高产高效。土壤容重降低0．1～0．14g／cm3,水分生产效率提高41．2％～52.9％,节水56％,皮棉增产2．3％。75％施肥量根际营养滴灌比全额施肥等量水漫灌增产皮棉14．5％,水分生产效率提高13．6％。     

Microsensor analysis of oxygen and pH in the rice rhizosphere under field and laboratory conditions

 O₂ and pH microsensors were used to analyse the microdistribution of O₂ and pH inside and outside roots of lowland rice (Oryza sativa L.). The roots of 3-week-old transplants had O₂ concentrations of about 20% air saturation at the surface, but due to a high rate of O₂ consumption in the rhizosphere, the oxic region only extended about 0.4 mm into the surrounding soil. Also the fine lateral roots created an oxic zone extending about 0.15 mm into the soil. The O₂ concentration within the roots approached air saturation close to the base, but only about 40–60% of air saturation in a region about 8 cm below the base where lateral rootlets were present. A shift from air to N₂ around the leaves led to a drop of 50% in the O₂ concentration after 12 min at a distance of 8.5 cm from the base. Flowering plants did not export O₂ to the soil from the majority of their roots, but high microbial activity was present in a very thin biofilm covering the root surface. A brown colour around the thin lateral roots indicated some O₂ export from these also during flowering. No oxidized zone was present around the roots at later stages of crop growth. The roots caused only minor minima in pH (<0.2 pH units) in the rhizosphere as compared to the bulk soil. Illumination of the plants had no effect on rhizosphere pH. Received: 28 April 1998     

茶树根际土壤抗酸铝微生物的分离与初步鉴定

将茶树根际土壤微生物提取液接种到含铝5 ～20 mmol/L的培养基上培养并分离获得抗酸铝真菌ALF 1 。该真菌具有抗酸铝能力, 能在含铝20 mmol/L的培养基上正常生长, 并降低培养基铝浓度且使苏木精褪色。经鉴定, ALF 1 真菌为链孢霉属的1 种( Neurosporasp.) 。     

水曲柳落叶松林土壤养分特性的研究

通过对试验地不同林分土壤性质及营养元素的测定,发现水曲柳叶松混交林土壤ｐＨ值,有机质含量与水曲柳纯林无显著差异,土壤有效性Ｃａ,Ｍｇ,Ｚｎ,Ｆｅ均高于土壤养分的生物有效性临界值,不会对树木生产产生限制作用,而土壤有效Ｍｎ含量较低,在不曲柳纯林林地尤为不足,在水朵柳落叶松混交林中这种状况有较大改善,表现出与林木生长相一致的规律,说明土壤有效Ｍｎ的改善是水曲柳落叶松混交林较水曲柳纯林生产力高的主要原因     

三种酚酸类化感物质对花生根际土壤微生物及产量的影响

为进一步探明花生连作障碍的机理,通过外源添加的方法研究了不同浓度肉桂酸、邻苯二甲酸、对羟基苯甲酸及其混合物对花生开花下针期、结荚初期、结荚末期根际土壤微生物及产量的影响。结果表明,与对照相比,各处理均降低了各生育期花生根际土壤细菌和放线菌的数量、微生物量生物量以及呼吸强度,增加了真菌的数量。随浓度增加,化感作用增强,以3种酚酸类化感物质混合物的化感作用最强。随处理时间的延长,对真菌的化感作用增强,对其他指标的化感作用减弱。各处理均显著降低了花生产量（P < 0.01）,处理浓度越高,花生减产幅度越大。最高浓度（初始含量为90mg·kg-1干土）的肉桂酸、邻苯二甲酸、对羟基苯甲酸以及混合物处理使花生每盆荚果产量分别降低了43.2%、32.9%、40.2%和47.3%。这三种酚酸类物质的累积与连作花生根际土壤微生物群落结构的变化相关,是花生连作障碍产生的可能因素之一。     

Trichoderma asperellum is effective for biocontrol of Verticillium wilt in olive caused by the defoliating pathotype of Verticillium dahliae

Verticillium wilt caused by a highly virulent, defoliating (D) pathotype of Verticillium dahliae is threatening olive production in Spain and other Mediterranean countries. This disease must be managed by an integrated strategy, in which biocontrol agents can play an important role. We have investigated the potential of Trichoderma asperellum strains for antagonism against V. dahliae and suppression of Verticillium wilt of olive caused by the D pathotype. First, we tested the antagonistic potential of T. asperellum strains Bt2, Bt3 and T25 against six V. dahliae isolates, four of the D and two of the nondefoliating (ND) pathotypes, in different in vitro assays. All T. asperellum strains overgrew the colonies of all V. dahliae isolates to a similar extent. However, extracellular compounds from strains Bt3 and T25 showed higher anti-V. dahliae activities than those of Bt2 in membrane assays. Also, growth of Bt2 was reduced by ND V. dahliae whereas that of Bt3 and T25 was not affected by V. dahliae-secreted compounds. In planta assays using strains Bt3 and T25, and ’Picual’ olive plants, showed that the two T. asperellum strains significantly reduced the severity of symptoms and the standardized area under the disease progress curve caused by highly virulent D V. dahliae, but not the final disease incidence. Strain T25 significantly increased growth of ‘Picual’ plants and displayed higher ability for colonizing the olive rhizosphere and establishing endophytic infection in olive roots than Bt3.     

Density, structure, and diversity of the cultivable arylsulfatase-producing bacterial community in the rhizosphere of field-grown rape and barley

In this study we investigated the arylsulfatase-producing bacterial community (ARS-BC) in the rhizosphere soils of field-grown rape in comparison with that of barley. For this, the rhizosphere soils from both plant species were sampled four times during plant growth. Soil arylsulfatase (ARS) activity and the density and the structure of the cultivable ARS-BC on M9-Xsulf medium were then determined. ARS activity in rape rhizosphere was greater than in barley rhizosphere and evolved along the phenology in the two rhizosphere soils. In parallel, the average density of ARS-BC in the rape rhizosphere was higher than that in the barley rhizosphere. Moreover, ARS activity is correlated with ARS-BC density both in rape and barley rhizosphere soils. The structure of the ARS-BC in the rape rhizosphere was different from that in the barley rhizosphere. In the rape rhizosphere, the ARS-BC was substantially more structured than in the barley rhizosphere. Among the ARS-BC, Actinobacteria and Pseudomonads were significantly present in the both rhizosphere soils. Actinobacteria predominated in the barley rhizosphere while Pseudomonads were mostly represented under rape. It is possible that the differences in ARS activity observed between rape and barley can be attributed to a different ARS-BC size and/or a different ARS-BC structure under these two plant covers. This impact of rape may be connected to a selective effect of rhizodeposits released by rape roots to the functional bacterial community. Our findings suggest that plant species, via their rhizodeposits, may affect the functional bacterial community and thus influence the dynamic of S in soil.     

Microbial response to exudates in the rhizosphere of young beech trees (Fagus sylvatica L.) after dormancy

Plants act as an important link between atmosphere and soil: CO2 is transformed into carbohydrates by photosynthesis. These assimilates are distributed within the plant and translocated via roots into the rhizosphere and soil microorganisms. In this study, 3 year old European beech trees (Fagus sylvatica L.) were exposed after the chilling period to an enriched ¹³C–CO₂ atmosphere (δ¹³C = 60‰ – 80‰) at the time point when leaves development started. Temporal dynamics of assimilated carbon distribution in different plant parts, as well as into dissolved organic carbon and microbial communities in the rhizosphere and bulk soil have been investigated for a 20 days period. Photosynthetically fixed carbon could be traced into plant tissue, dissolved organic carbon and total microbial biomass, where it was utilized by different microbial communities. Due to carbon allocation into the rhizosphere, nutrient stress decreased; exudates were preferentially used by Gram-negative bacteria and (mycorrhizal) fungi, resulting in an enhanced growth. Other microorganisms, like Gram-positive bacteria and mainly micro eucaryotes benefited from the exudates via food web development. Overall our results indicate a fast turnover of exudates and the development of initial food web structures. Additionally a transport of assimilated carbon into bulk soil by (mycrorhizal) fungi was observed.