Community composition of ammonia-oxidizing bacteria in the rhizosphere of intercropped wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.), maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.), and faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.)

Cereal/cereal and cereal/legume intercropping systems are popular in the north, northwest, and southwest of China and often result in yield increases compared to monocropping. Rhizosphere interactions may play a significant role in the yield increases, particularly with respect to nutrient availability. The aim of this study was to investigate the effects of intercropping on N availability and community composition of ammonia-oxidizing bacteria in the rhizosphere of wheat, maize, and faba bean at different growth stages. Denaturing gradient gel electrophoresis (DGGE) based on 16S rRNA genes was used to analyze the community composition of bacterial ammonia oxidizers belonging to β-proteobacteria. The results showed that intercropping with faba bean significantly increased nitrate concentrations in the rhizosphere of wheat and maize at the second sampling time (20 June) compared to monocropping or intercropping between maize and wheat. Intercropping significantly affected the community composition of ammonia-oxidizing bacteria in the rhizosphere compared to monocropping, and the effects were most pronounced in the maize/faba bean and wheat/maize intercropping systems when faba bean and wheat were at anthesis and maize was in seedling stage. In wheat/faba bean intercropping, the effects of intercropping on community composition of ammonia-oxidizing bacteria were less pronounced at the seedling stage of the two species but were significant at anthesis.     

Responses by iron-efficient and inefficient oat cultivars to inoculation with siderophore-producing bacteria in a calcareous soil

Rhizosphere bacteria may enhance plant uptake of Fe by producing siderophores that chelate sparingly soluble Fe³⁺ in calcareous soils. To evaluate the extent to which plants benefit from colonization of the roots by prolific siderophore-producing bacteria, we inoculated two oat cultivars with six strains of bacteria that produced high concentrations of siderophores under Felimiting conditions in vitro. Oat cv Coker 227, an Fe-efficient cultivar, which produces the phytosiderophore avenic acid, and cv TAM 0-312, and Fe-inefficient cultivar, which does not produce the phytosiderophore, were grown in a calcareous soil (Weswood silt loam) on a light bench in the laboratory. Half of the plants were fertilized with a nutrient solution containing 5 mM Fe and half with a nutrient solution containing no Fe. After 6 weeks of growth, we compared colonization of the roots by the inoculant bacteria and the dry weight and Fe content of roots and shoots. Three species of Pseudomonas colonized the roots of both oat cultivars in high numbers (10⁶ cells g^-1 root dry weight), whereas the remaining bacteria colonized the roots in substantially lower numbers (10⁴ cells g^-1 root dry weight). Plants fertilized with 5 mM Fe were larger and supported greater numbers or rhizosphere bacteria per gram of root than plants not supplied with Fe. Comparisons of the Fe content and dry weight of roots and shoots revealed few significant differences between inoculated and uninoculated plants, or among the plants inoculated with the different strains of siderophore-producing bacteria. The differences that were observed revealed no consistent response to inoculation. We conclude that inoculation of the roots of the two oat cultivars with bacteria that produce high concentrations of siderophores in response to an Fe deficiency had little or no effect on Fe acquisition by the plants.     

Gram-negative bacterial flora on the root surface of wheat (Triticum aestivum) grown under different soil conditions

We identified 161 Gram-negative bacterial strains isolated from the root surface of wheat grown under different soil conditions. The strains were divided into seven groups based on major morphological and physiological properties. Taxonomic allocation of the groups was verified by guanine+cytosine contents of DNA. Except for one group, which may be assumed to include bacteria belonging to the genera Flavobacterium and Cytophaga, the various groups were taxonomically united. The distribution of the groups changed with soil improvement. Pseudomonads predominated in unimproved soil, but Flavobacterium and Cytophaga spp. were predominant in the most improved soil. As all the strains were non-fermentative by Hugh and Leifson's test, API 20NE identification was applied. However, many strains were misidentified by this system, especially in the Flavobacterium and Cytophaga spp. group. For ecological studies, the strains were classified to species level by the API 20 NE system and by the results of a combination of guanine+cytosine (mol%) and isoprenoid quinone data. The pattern of distribution of the bacteria on the root surface of wheat varied at species level within one genus depending on soil conditions.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

Relative importance of motility and antibiosis in the rhizoplane competence of a biocontrol agent Pseudomonas fluorescens MelRC2Rif

Mutants defective in motility or antibiotics production were obtained by Tn5 mutagenesis of a biocontrol agent Pseudomonas fluorescens MelRC2Rif (wt). Tomato or melon seeds were co-inoculated with a Tn5 mutant and wt in a 1:1 ratio and then grown in soil for 10 days. There was no change in ratios of Tn5 mutants defective in antibiosis to wt in the process of rhizoplane colonization, suggesting little contribution of in vitro antibiosis to the rhizoplane competence of P. fluorescens MelRC2Rif. Similar results were also obtained when seeds treated with bacteria were planted in soil artificially infested with fungal pathogens. In contrast, ratios of Tn5 mutants defective in motility to wt significantly decreased, suggesting the contribution of motility to the rhizoplane competence of this bacterium. When a non-motile Tn5 mutant and wt were co-inoculated into soil at a matric potential of pF 2.3 (–20 kPa) and plants were then grown, there was no change in the ratio in rhizoplane colonization, suggesting that motility might have a role in the movement along roots but an insignificant role in the movement from bulk soil towards roots. When they were co-inoculated into 0.2% water agar (WA) instead of soil, a remarkable decline in ratios was detected. Thus it was soil structure that hindered the efficiency of motility. Time course enumeration of rhizoplane colonization of tomatoes grown in WA revealed that motility was an important means of movement towards and/or along roots rather than the multiplication on roots. Received: 8 July 1996     

Salt marsh rhizosphere affects microbial biotransformation of the widespread halogenated contaminant tetrabromobisphenol-A (TBBPA)

Estuarine sediments are the repository for a wide range of contaminants. Anthropogenic impacts and variations in the belowground biomass of salt marsh plants potentially select for different sediment microbial communities with different functional capabilities, including the ability to biotransform anthropogenic contaminants. There are large differences in both root morphology and the amount of fine root biomass of Spartina alterniflora and Phragmites australis; Spartina is the species commonly used to replace Phragmites in northeastern US salt marsh restoration projects. Our study compared the effect of these two macrophyte species on sediment microbial communities responsible for the biotransformation of the halogenated flame retardant tetrabromobisphenol A (TBBPA). Sediments were obtained from contaminated and uncontaminated salt marsh field sites in New Jersey. Anaerobic methanogenic sediment microcosms were established and incubated for up to 130 days. TBBPA was reductively dehalogenated resulting in the transient formation of two intermediates, identified as tribromobisphenol A and dibromobisphenol A, and the formation and accumulation of bisphenol A (BPA) as the end product. Spartina sediments from both sites were found to dehalogenate TBBPA more rapidly than the Phragmites or unvegetated sediments, resulting in greater production of BPA. Microbial community diversity as measured by in situ sediment phospholipid fatty acid (PLFA) composition prior to TBBPA exposure, was found to be higher in the uncontaminated sediments; differences in microbial PLFA diversity were not seen in contaminated sediments associated with either the different plant species or unvegetated sediment. The results of this study demonstrate that these two plant species affected sediment microbial community function with respect to dehalogenation capabilities, even though the disturbed and undisturbed sediments varied in microbial community composition.     

Effect of bulk density and soil water tension on denitrification in the rhizosphere of spring wheat (Triticum vulgare

Summary Pot experiments were carried out to study the influence of bulk density (D _b), soil water tension (pF) and presence of plants (spring wheat) on denitrification in a low-humus B_t-horizon of a udalf. Pots of only 5-cm depth were found to be most suitable for the experiments when using the acetylene inhibition method. Almost homogeneous soil compaction between 1.1 and 1.6g soil cm^–3 was achieved by a Proctor tamper. Water tensions were adjusted by means of ceramic plates on which negative pressure was applied. No denitrification was detected in unplanted pots. With planted pots and increasing bulk density denitrification increased more in pots with 14-day-old plants than in pots with 7-day-old plants. With 14-day-old plants N₂O emission pot^–1 increased steadily from 2 mol at D _b 1.1 to 8 mol at D _b 1.6, when soil moisture was adjusted to pF 1.5, although root growth was impaired by higher bulk density. From an experiment with different bulk densities and water tensions it could be deduced that the air-filled porosity ultimately determined the rate of denitrification. When low water tension was applied for a longer period, water tension had an overriding effect on total denitrification. Denitrification intensity, however, i.e. the amount of N₂O g^–1 root fresh weight, was highest when low water tension was accompanied by high bulk density. The results suggest that the increase in denitrification intensity at oxygen stress is partly due to higher root exudation.     

几种有机酸对恒电荷和可变电荷土壤吸附Cu2+的影响

以恒电荷土壤(黄褐土和黄棕壤)和可变电荷土壤(红壤和砖红壤)为供试材料,研究了乙酸、草酸、酒石酸和柠檬酸对土壤吸附重金属铜离子(Cu2 )的影响。结果表明,在相同酒石酸浓度下,土壤对酒石酸的吸附量依次为黄棕壤(2 1 8mmolkg-1) >红壤(15 4mmolkg-1) >砖红壤(9 5mmolkg-1) ,土壤吸附有机酸后负电荷量增加,相同条件下增幅为砖红壤>红壤>黄棕壤;无有机酸配体时,供试土壤对Cu2 的吸附量为黄褐土>黄棕壤>砖红壤>红壤;加入有机酸时,随有机酸浓度增高,土壤对Cu2 的吸附一般表现为“峰”形曲线,峰所对应的有机酸浓度因有机酸类型而异,且随土壤可变电荷性质增强而增高;土壤吸附有机酸后对Cu2 的次级吸附不同于有机酸与铜共存时的竞争吸附,且因土壤性质表现迥异。这些结果意味着在存在有机酸配体的根际环境中,恒电荷土壤与可变电荷土壤对Cu2 的吸附明显不同,并将影响重金属离子在根际的转化与有效性     

重茬病防治剂对大棚重茬草莓根际微生物的影响

采用未处理和溴甲烷熏蒸处理土壤为对照,研究原美草莓重茬病防治剂不同剂量,对大棚重茬草莓根际土壤真菌、放线菌、细菌数量和种类的影响。拟证实原美防治剂的生态安全性和土壤处理效果,确定其有效剂量。试验结果表明:在各不同处理和不同生育期(初花期、盛花期和采果后期),防治剂处理A3(150g/m2)根际土壤真菌、细菌、放线菌数量高于未处理、溴甲烷处理和其它浓度处理,充分提高土壤肥力。草莓产量比溴甲烷处理提高33.6%,比对照提高388.9%。因此认定原美重茬防治剂对草莓生产比溴甲烷处理效果好得多,其有效剂量为150g/m2。同时从不同处理的根际土壤中经分离、纯化得到10个纯真菌菌株,通过形态学特征初步鉴定以疫霉属为优势菌属。     

植物根际解磷细菌与植物生长发育

综述了土壤解磷细菌(PSB)的种类、数量、分布,解磷能力以及对植物生长发育影响,目的是为解磷菌的开发应用和进一步研究提供信息。众多研究表明,土壤中普遍存在能够分解矿质态磷和有机态磷的微生物,而且植物根际土壤的解磷细菌不仅数量多于土体,其种类也明显多于土体土壤。一些接种试验表明,如果条件适宜,解磷细菌可以分解许多难溶性的磷酸盐,为植物提供有效的磷素营养。除分解矿物态的磷酸盐外,解磷菌对植物还具有促生作用,其机制是解磷菌分泌的激素对植物的生长发育产生了影响。文章最后还对今后的研究作了展望。     

怀地黄根际土壤水提物的GC-MS分析

采用GC-MS分析方法,鉴定了怀地黄根际土壤的水提物。结果表明,怀地黄根际土壤含有12种特有的化感物质,它们是邻苯二甲酸正丁酯、邻苯二甲酸异丁酯、邻苯二甲酸二异辛酯、2-甲氧基-5-(1-丙烯基)苯酚、苯丙烯酸、苯甲酸、1-十八醇、1-二十一醇、3-甲氧基-4-羟基苯甲酸、3-甲基-4-羟基苯乙酸、邻苯二甲酸癸基辛酯和邻苯二甲酸异癸基辛酯。