Linking rhizoplane pH and bacterial density at the microhabitat scale

Ion-selective microelectrodes and a novel micro-sampling technique were used to investigate the relationship in field soil between Brassica napus rhizoplane pH and bacterial density at a spatial scale approximating a microhabitat. Bacterial densities were observed to increase with decreasing pH, rhizoplane pH measurements varied by up to 1 pH unit over a distance of 1 mm and the mean pH of the rhizoplane at the root base varied by more than 1 pH unit between plants. These findings highlight the appropriateness of investigating the interactions between bacterial communities and their environment at the micro-spatial scale and the utility of the micro-sampling method.     

Different populations of bacteria associated with sheathed and bare regions of roots of field-grown maize

It has been previously shown that soil sheaths cling tightly to some portions of all axile roots and cover all but the growing tips of the young roots of field-grown maize. These sheaths overlie immature regions of the roots which have intact epidermal cells with root hairs, and living, thus non-conducting, late metaxylem elements. Loss of the soil sheath in the proximal region coincides with the opening of these large metaxylem vessels. Now, total, and viable counts have been recorded of bacteria associated with the root surface and adhering soil of sheathed and bare regions. These showed some common features, in that populations of similar size were associated with the two root regions in plants beginning to flower. Each population included about the same numbers of bacteria that were viable on each of three selective media (nitrogen-free, Pseudomonas F or MacConkey). However, more spore-formers capable of growth on nitrogen-free media and more fluorescent bacteria were isolated from the sheathed regions. Actinomycetes were absent from sheathed but plentiful on bare regions.The high numbers of diverse types of bacteria associated with both root surfaces can be related to the previously demonstrated similarity in amounts of organic carbon released from each region. The proliferation of actionomycetes on the bare roots and their exclusion from sheathed roots may in part be due to the lower water status of the bare region, which is related to its greater axial conducting capacity. Thus the distribution of the two types of root surface within an individual root system has important implications for the choice of root and rhizosphere sampling techniques and for root bacterization work.     

A Precise Technique for Measurement of Roots and Root Distribution in Soils

A procedure is described in detail which yields directly thecomplete spatial distribution and sizes of roots in soils, andso provides such information as the length and surface areaof root per unit volume, the number of root tips per unit volume,and the proximity of roots to one another. The method involves stabilization of the soil by vacuum impregnationwith Plastrene 47 and examination of the serial surfaces preparedby surface-grinding, using both visible and ultraviolet fluorescencephotography.     

Delivery methods for introducing endophytic bacteria into maize

The effectiveness of fivemethods for delivery of ten endophyticbacteria into maize stem and root tissues wasstudied in greenhouse conditions at EmbrapaMilho Sorgo, Sete Lagoas, MG, Brazil. Thedelivery methods included seed inoculation,soil drench, foliar spray, pruned-root dip andseed inoculation + soil drench. The bacterialendophytes were previously isolated from maizeplants, and reinoculated and recovered aftertreatments from maize, cv BR201. Each of thefive methods led to establishment and recoveryof bacterial endophytes in root tissues, butonly four isolates were recovered by the seedtreatment method. All 10 isolates wererecovered by pruned-root dip and seed treatment+ soil drench. No isolates were recovered instem tissues by the seed treatment method, andin the root by foliar spray method. However,all isolates were recovered in stem tissue bypruned-root dip method. The pruned-root dip wasthe most efficient method to deliver bacterialendophytes into maize. The isolate, BR201, wasrecovered by almost all methods in root andstem tissues. The results demonstrate thatendophytic bacteria can be recovered from maizetissues following inoculation by the differentmethods described, but the delivery depends onthe methods used and the endophytic bacterialisolate.     

Tribology of the root cap in maize (Zea mays) and peas (Pisum sativum)

Frictional resistance to a penetrating body can account for more than 80% of the total resistance to penetration of soil. We measured the frictional resistance between growing root caps of maize and pea and ground and smooth glass surfaces, which was linearly correlated to load, allowing calculation of the coefficient of kinetic friction and adhesion. Coefficients of kinetic friction between the root caps and the ground and smooth glass surfaces were approximately 0.04 and 0.02, respectively, the first measurements of the frictional properties of root tips at rates approaching those of root elongation, and an order of magnitude smaller than those previously reported. Results suggest that roots are well designed for penetrating soil, and encounter only small frictional resistance on the root cap. These data provide important parameters for modelling soil stresses and deformation around growing root tips.     

Oribatid mite communities in the canopy of montane Abies amabilis and Tsuga heterophylla trees on Vancouver Island, British Columbia

To study the oribatid mite community inhabiting microhabitats in the canopy of montane Abies amabilis [(Douglas ex D. Don) Lindl.] and Tsuga heterophylla [(Raf.) Sarg] tree species across five elevational sites, we collected 180 branch tips and 180 foliose/crustose lichen samples over three time periods. Thirty-three species of oribatid mites were identified from the study area. Mite species richness and abundance was significantly affected by microhabitat, and this association was independent of sampling time. At the microhabitat scale, distinct species assemblages were associated with lichen and branch tip habitats, and to a lesser degree, tree species. Conifer specificity was most apparent in the closely related species of Jugatala, where Jugatala tuberosa Ewing was only found on branch tips from A. amabilis and Jugatala sp. was primarily found on branch tips from T. heterophylla. Microhabitat specificity was most pronounced in Dendrozetes sp. where most individuals were found on branch tips and Anachiperia geminus Lindo et al. that occurred primarily on lichens. Principal components analysis of oribatid mite community composition further showed a high degree of association with microhabitat and tree species. Habitat profiles are difficult to discern for many species because tree, microhabitat, and elevation preferences confound distribution patterns. Given the significant tree-microhabitat associations in species composition in this montane canopy study, we suggest that sampling multiple microhabitats across elevations to look for patterns in community structure offers opportunities to explicitly test organizing principles in community ecology.     

A study of root distribution and growth in a Pinus sylvestris L. (Scots pine) plantation in east anglia

Summary The root systems of Scots pine in a plantation were studied by three methods; soil coring, soil monoliths, and a root trench with observation windows were used to estimate root length, root diameters and the initiatio of new root tips. The vertical and horizontal distribution of roots is described and root distribution has been related to distance from the tree and soil heterogeneity. It was found that the initiation of new root tips was not readily relatable to the soil environment and the usefulness of the root window technique for observing new root tips is questioned.     

Relationship between spatial and genetic distance in Agrobacterium spp. in 1 cubic centimeter of soil

The spatial and genetic unit of bacterial population structure is the clone. Surprisingly, very little is known about the spread of a clone (spatial distance between clonally related bacteria) and the relationship between spatial distance and genetic distance, especially at very short scale (microhabitat scale), where cell division takes place. Agrobacterium spp. Biovar 1 was chosen because it is a soil bacterial taxon easy to isolate. A total of 865 microsamples 500 microm in diameter were sampled with spatial coordinates in 1 cm(3) of undisturbed soil. The 55 isolates obtained yielded 42 ribotypes, covering three genomic species based on amplified ribosomal DNA restriction analysis (ARDRA) of the intergenic spacer 16S-23S, seven of which contained two to six isolates. These clonemates (identical ARDRA patterns) could be found in the same microsample or 1 cm apart. The genetic diversity did not change with distance, indicating the same habitat variability across the cube. The mixing of ribotypes, as assessed by the spatial position of clonemates, corresponded to an overlapping of clones. Although the population probably was in a recession stage in the cube (10(3) agrobacteria g(-1)), a high genetic diversity was maintained. In two independent microsamples (500 microm in diameter) at the invasion stage, the average genetic diversity was at the same level as in the cube. Quantification of the microdiversity landscape will help to estimate the probability of encounter between bacteria under realistic natural conditions and to set appropriate sampling strategies for population genetic analysis.     

不同尺度苔藓结皮土壤性状的空间分布特征

苔藓结皮是生物结皮的高级阶段,它在防风固沙、改善土壤水分、养分等方面具有十分重要的生态作用,但前期研究多是对结皮区和无结皮覆盖区土壤性状的比较,对结皮斑块内土壤理化因子的空间分布情况尚不明确。运用经典统计学,地统计学以及克里格插值法探讨了土壤水分、有机质、全氮、全磷、总盐在样方和斑块两个尺度的空间变异特征,旨在阐明不同尺度藓类结皮土壤空间异质性的强度,明确不同尺度土壤各性状合适的取样距离。研究结果表明藓类结皮土壤性状在两种尺度均表现出中等程度的变异,随着取样尺度减小,自相关性在空间异质性中占的比重增加,结构因素的影响增强,随机因素的影响减弱。样方尺度的空间异质性大于斑块尺度异质性,且土壤性状的变程较大,采样时可适当增大取样间距;两种尺度下5个土壤性状中,全磷的变程最小,实际取样时应适当缩小取样距离,总盐变程大,自相关程度低,因而取样间距可适当放大。斑块尺度,藓类结皮土壤性状由边缘向中心递增(磷递减)的现象与斑块的边缘效应有关。     

Effects of the hummock–depression microhabitat on plant communities of alpine marshy meadows in the Yellow River Source Zone,China

黄河源区丘-洼微生境对高寒沼泽草甸植物群落的影响 黄河源区高寒沼泽草甸中有许多不均匀的小丘和洼地,形成了独特的微生境,深刻影响着植物特性和土壤养分含量。通过研究高寒湿地冻融丘和洼地空间异质性对植物群落和土壤性质的影响,可以深入了解微地形水文条件对丘-洼微生境空间波动的影响。本研究在黄河源区高寒沼泽湿地的冻融丘 (淹水和无淹水)和洼地(蓄水和无蓄水)共设置36个样地(1 m × 1 m),采集了45个植物样和225个土壤样, 并采用比较法评价高寒沼泽湿地是否存在“肥岛效应”。研究结果显示,冻融丘的存在增加了微生境的 空间异质性,促进了藏嵩草群落的物种多样性和土壤肥力。淹水和无淹水的冻融丘生境下的植物高度、 盖度、地上生物量、物种丰富度和多样性均显著高于湿地外围的高寒草甸。与高寒草甸相比,高寒沼泽 湿地丘-洼复合体为莎草科植物的生长提供了有利的微生境。另外,湿地丘-洼微生境与周围高寒草甸 在0–50 cm土层之间的比较表明,土壤有机碳和全氮距离地表越近含量越高。在深层次土壤中,丘洼微生 境与高寒草甸土壤养分之间的差距变小。因此,湿地丘-洼微生境形成了一个富饶的“肥沃岛”格局。这些研究结果有助于加深对高寒沼泽草甸生态系统恢复的认识。     

Oligotrophic bacteria from rendzina forest soil

Oligotrophic bacteria were shown to exist abundantly in all layers of a rendzina forest soil throughout the year. Two-hundred-three oligotrophic bacteria were isolated from forest soil (Aoba, Sendai) at different layers (L, F, H and A layers) throughout the year, and their morphological and physiological characteristics were examined. A high proportion (95%) of the isolated oligotrophs were Gram-negative, non-spore forming bacteria. Based on the cell shape, the isolates were divided into four groups: regular rods, curved/spiral bacteria, irregular rods, and buddin and/or prosthecate bacteria. Each group of bacteria is discussed in relation to the physiological characteristics. Notably oligotrophic bacteria of different cell types showed a marked zonal distribution in respect to profile depth.     

New approaches to studying chemical and physical changes in the rhizosphere: an overview

The past decade has seen the rapid development of new techniques that have revealed substantial changes in soil physical and chemical properties in the rhizosphere compared to the bulk soil. This brief overview focuses on some examples of recently developed, innovative techniques now available and indicates the technical developments required for the future. The development of non-invasive imaging allied with computed tomography has begun to allow the study of root systems in situ and the measurement of localized uptake of water. Further development is still required to disaggregate the simultaneous changes in bulk density and water content that may occur as roots occupy new soil volumes, but resolution of 0.1 mm is now feasible in scanning times of less than 1 h thereby allowing dynamic processes to be measured. Changes in surface tension and composition of solutions close to roots, and of pH, can now be measured with a variety of techniques. Temporal and spatial changes of pH can be measured with micro-electrodes and dye indicator/agar gel techniques have allowed quantitative estimates of H⁺ fluxes albeit in artificial systems. Novel micro-sampling techniques are under development to quantify rhizosphere changes. So far these techniques have rarely been applied in soils but innovative sampling and analytical techniques should soon allow such studies. This revised version was published online in June 2006 with corrections to the Cover Date.     

Contrasting below-ground views of an ectomycorrhizal fungal community

Ectomycorrhizal fungal communities have been characterized in a number of ways. Here we compare colonized root-tip and mycelia views of an ectomycorrhizal fungal community. Ectomycorrhizal fungi, both as mycelia and colonized root tips, were identified in soil samples taken from a pine plantation. We determined that for some ectomycorrhizal fungal species multiple root tips from a single soil sample were not independent. Therefore in the comparison of root-tip and mycelia views, we considered species to be present or absent from each soil sample irrespective of the number of root tips colonized by the species. We observed 39 ectomycorrhizal fungal species in total, but 12 were observed exclusively as mycelia and 11 exclusively colonizing root tips. The relative frequencies of 10 species occurring as both mycelia and root tips were not independent of the method of observation. Our results suggest that ectomycorrhizal fungal species differ in their spatial distributions on root tips, and that root-tip and mycelia views of the community are different.     

Relationship between Spatial and Genetic Distance in Agrobacterium spp. in 1 Cubic Centimeter of Soil

The spatial and genetic unit of bacterial population structure is the clone. Surprisingly, very little is known about the spread of a clone (spatial distance between clonally related bacteria) and the relationship between spatial distance and genetic distance, especially at very short scale (microhabitat scale), where cell division takes place. Agrobacterium spp. Biovar 1 was chosen because it is a soil bacterial taxon easy to isolate. A total of 865 microsamples 500 μm in diameter were sampled with spatial coordinates in 1 cm³ of undisturbed soil. The 55 isolates obtained yielded 42 ribotypes, covering three genomic species based on amplified ribosomal DNA restriction analysis (ARDRA) of the intergenic spacer 16S-23S, seven of which contained two to six isolates. These clonemates (identical ARDRA patterns) could be found in the same microsample or 1 cm apart. The genetic diversity did not change with distance, indicating the same habitat variability across the cube. The mixing of ribotypes, as assessed by the spatial position of clonemates, corresponded to an overlapping of clones. Although the population probably was in a recession stage in the cube (10³ agrobacteria g⁻¹), a high genetic diversity was maintained. In two independent microsamples (500 μm in diameter) at the invasion stage, the average genetic diversity was at the same level as in the cube. Quantification of the microdiversity landscape will help to estimate the probability of encounter between bacteria under realistic natural conditions and to set appropriate sampling strategies for population genetic analysis.     

基于土芯法的亚热带常绿阔叶林细根空间变异与取样数量估计

树木细根具有高度空间异质性,确定合理的细根取样策略是林木细根研究的前提。通过在福建省三明米槠天然常绿阔叶林内随机钻取96个土芯,分析细根生物量和形态特征的空间变异特征,并估计各指标所需的取样数量。结果表明:(1)随着径级增加,细根各指标变异系数增大,相应的取样数量增加;(2)随着土壤深度增加,单位面积细根生物量变异程度和相应的取样数量均增加。在置信水平为95%、精度为80%的条件下,直径为0-1 mm和1-2 mm的细根,分别采集16和42个样品可以满足测定单位面积细根生物量,采集17和31个样品可以满足测定单位面积细根长度,采集25和33个样品可以满足测定单位面积细根表面积。Shapiro-Wilk检验表明,除表层土壤0-1 mm细根单位面积生物量符合正态分布外,其余细根生物量和形态指标数据均不符合正态分布。研究结果为亚热带常绿阔叶林细根的合理取样提供了科学依据。     

青稞根腐病对根际土壤微生物及酶活性的影响

选取甘肃省卓尼县青稞种植区为研究地点,调查青稞根腐病的发病情况,并分别采集其健康植株和发病株根际的土壤,对比分析其土壤微生物生物量(碳、氮、磷)、微生物数量(细菌、真菌、放线菌)以及过氧化氢酶、蔗糖酶、脲酶、碱性磷酸酶、纤维素酶5种酶活性。结果发现,研究区10个采样点均有青稞根腐病的发生,发病率在5%—20%之间,不同地点发病率不同。根腐病的发生,会显著影响青稞根际微生物生物量,导致微生物生物量碳、氮、磷的含量发生变化,其中微生物生物量氮和磷含量整体降低,且不同采样点微生物量不同。土壤微生物数量总体呈现细菌放线菌真菌的趋势,但不同微生物对根腐病发病的响应不同,细菌和放线菌数量因根腐病的发生而减少,真菌的数量则增多;不同采样点土壤微生物数量不相同,细菌和真菌呈现区域性特征,放线菌的数量不呈现地域性。根腐病的发生还造成土壤酶活性的改变,其中蔗糖酶、脲酶、磷酸酶的含量因根腐病的发生而降低,而纤维素酶则升高,过氧化氢酶的变化没有规律。总而言之,根腐病的发生会使青稞根际土壤微生物组成发生改变,碳、氮、磷等物质代谢受到抑制,而能量代谢发生紊乱。因此,研究和防治青稞根腐病就必须重视土壤微生物及土壤酶的作用。     

绿洲-荒漠过渡带景观的植被与土壤特征要素的空间异质性分析

采用变异函数的理论和方法,通过空间异质性特征比较,研究了阜康绿洲-荒漠过渡带景观中的植被盖度和土壤环境因子的空间异质性、空间格局及其关系．在此基础上,应用Kriging局部块段插值法,以三维图形显示出各个要素的空间分布格局．结果表明,除土壤全盐量空间相关性较弱外,其他各要素具有中等程度以上的空间相关性,且空间变异主要发生在较小尺度上．其中,植被盖度的空间异质性程度较高,在3km的尺度内具有明显的空间格局．从空间分布格局看,植被盖度、土壤表层含水量和土壤pH值沿样带相邻点差异对比明显,高值区和低值区交错．草本盖度与土壤表层含水量、土壤pH间有相关关系,灌木平均盖度主要依靠深层次地下水维系．灌木和草本盖度的块金值具有明显差异,可能因为二者要求不同的生态条件,因而在同等尺度上具有不同的生态学过程．     

Scanning Electron Microscopy of Rhizobium trifolii Infection Sites on Root Hairs of White Clover

White clover root hairs which were inoculated with Rhizobium trifolii 4S (infectious strain) contained infection threads which were observed by light microscopy and scanning electron microscopy. Three morphological types of root hairs retaining infection threads were recognized. The bacteria were strongly attached between the surfaces of two plant cell walls as follows: between surfaces of a root hair tip curled back on itself, between a protuberance from a root hair and its cell surface, or between two root hair tips clinging together. An anatomical analysis documented the attachment site of the infection thread sheath from the inside of the root hair cell.     

Ultrastructure of the in situ adherence of Mobiluncus to vaginal epithelial cells

From patients with bacterial vaginosis motile, anaerobic, comma-shaped bacteria can be isolated, which have recently been placed into the new genus Mobiluncus. In this study, electron microscopy was used to examine the in situ adherence of these motile curved rods to detached epithelial cells (comma cells) in vaginal fluid from two patients with bacterial vaginosis. Thin sections showed that the curved rods attached both directly to the epithelial cell surface and at various distances from it. It is concluded that after initial attachment these motile bacteria can grow at the epithelial cell surface in sessile microcolonies. Ruthenium red staining demonstrated a coating of precipitated glycocalyx material both on the surface of the curved rods and on their flagella. This may indicate that in situ the adherent curved rods were enclosed in a very hydrated matrix of exopolysaccharides. Conspicuous was the ability of the curved rods to attach to the epithelial cell surface via their cell tips. However, in situ no specialized bacteria cell surface structures were seen that might explain this polar attachment. Electron microscopy of pure cultures demonstrated that both Mobiluncus curtisii subsp. curtisii and Mobiluncus mulieris can produce a glycocalyx in vitro.     

Seasonal variations in the spatial distribution of root tips in teak (Tectonia grandis Linn. f.) plantations in the Varanasi Forest Division,India

Summary Seasonal variations in the spatial distribution of root tips were studied in 19 and 29 year old teak plantations, located on red and alluvial soils respectively. The pattern was essentially similar at both sites, but generally the alluvial soil site exhibited a greater number of root tips. Root tips decreased with increasing distance from the tree base. Through-out most of the year the relative distribution of root tips decreased with depth; the difference between 0–10 and 10–20 cm depths was marginal, but 20–40 cm depth contained distinctly fewer root tips. At all distances a similar seasonal trend was noticed, a mid rainy season peak being followed by a steady decline until the dry summer except for an abrupt rise to a smaller peak in February after the winter rains.The root tip density was positively correlated with the 2 mm root biomass and both showed a similar bimodal annual cycle. Of three environmental variables studied, soil moisture and rainfall were significantly positively correlated with root tip densityl the relationship between soil temperature and root tip density was negative and non-significant. The combined effect of soil moisture and temperature on root tip density, evaluated by a multiple regression model, accounted for 80–95% of the variation in root tip density.