Seasonal effect of three desert halophytes on soil microbial functional diversity

The objective of this study was to evaluate the effect of some plant ecophysiological adaptations on soil microbial functional diversity in a Negev Desert ecosystem. Soil samples from the upper 0–10 cm layer were collected at the study site under three species of halophyte shrubs, Zygophyllum dumosum, Hammada scoparia, and Reaumuria negevensis. These halophytes represent the most typical cover of the Negev Desert and each of them develops complex strategies that enable greater adaptation and hence, survival. The microhabitat of the shrubs showed differences in trends and magnitude of organic matter content, electrical conductivity, total soluble nitrogen, microbial functional diversity, and C compound utilization. The trends are assumed to be driven by various mechanisms of shrub adaptation in order to be able to survive the harsh desert environment. This study provides evidence that ecophysiological strategies developed by halophytes force microbial communities (from the point of view of activity, composition, and substrate utilization) to adapt to a beneficial plant-microorganism relationship.     

Vertical Distribution of Microbial Community Functionality under the Canopies of <Emphasis Type="Italic">Zygophyllum dumosum</Emphasis> and <Emphasis Type="Italic">Hammada scoparia</Emphasis> in the Negev Desert,Israel

Patchy desert shrubs magnify the horizontal heterogeneities of carbon source and nutrient availability in an arid ecosystem, significantly affecting the abundance and activity of the soil microbial community. Since each shrub species develops special ecophysiological adaptations to the extreme harsh desert environments, previous studies elucidated that the effects of perennial shrubs on microbial diversity are unequal. The aim of the present study, conducted in the Negev Desert, Israel, was to illustrate the vertical changes of soil microbial community functionality in the root zone of perennial shrubs. Soil samples were collected from the 0–50 cm depth at 10-cm intervals under the canopy of Zygophyllum dumosum, Hammada scoparia, and from the open spaces between them, in the wet and dry seasons. Soil moisture and organic matter exhibited a significant (P < 0.001) plant and depth dependence. The mean basal respiration rates and microbial biomass in soils collected beneath perennial shrubs were relatively higher than the control during the wet season, however, a contrasting trend was observed at some soil depths during the dry season. Relatively high abundance and activity of aromatic and carboxylic acid utilizers were observed in the vicinity of perennial shrubs, and the values recorded during the dry season were generally higher than the corresponding values during the wet season. In addition, a “mirror effect” in vertical changes of the community-level physiological profile was observed between Z. dumosum and H. scoparia. This study demonstrated the stratification of the functional aspects in soils under the canopy of perennial shrubs, thus indicating that the scattered distribution of vegetation not only causes horizontal heterogeneities of the microbial community in an arid system, but also that the ecophysiological adaptations developed by xerophytes regulate the abundance and saprotrophic functionality of microorganisms in the root zone.     

Soil Microbial Diversity in the Vicinity of a Negev Desert Shrub��Reaumuria negevensis

The Negev Desert is characterized by low soil-water availability and organic matter content, as well as important factors significantly influencing soil biological activity. In order to overcome the xeric environment, plant and soil biota have evolutionarily developed, over time, ecophysiological abilities that help them fulfill their biological role and function. Microorganisms are known as a major part of the ecosystem's total biomass and play an important role in decomposition processes and the nutrient cycle. Perennial shrubs have been found to play an important role as organic matter suppliers and as a physical barrier prolonging biological activity of microbial communities. Soil samples were collected monthly, from November 2006 to November 2007, from a 0 to 10-cm depth under the canopies of Reaumuria negevensis and from open areas (control) in order to evaluate abiotic components and microbial variables on a temporal basis. H' values, evenness, and β diversity (S?rensen's similarity) were determined by a molecular method based on sequencing. Water availability, organic matter content, and total soluble nitrogen were higher in soil samples collected in the vicinity of R. negevensis than in samples collected in open areas. Our study also indicated that, in spite of the similarity between H' values of soil samples collected in the vicinity of R. negevensis and the open area, a low percentage of similarity was found between the soil bacterial populations. These results support the hypothesis that distribution of resources in the environment under R. negevensis shrubs varies in space and time and also influences soil microbial diversity and the abiotic environmental role.     

Are Biological Effects of Desert Shrubs More Important than Physical Effects on Soil Microorganisms?

Vegetation cover plays a major role in providing organic matter and in acting as a physical barrier, with both together contributing to the formation of “fertile islands,” which play an active role in prolonging biological activity in desert ecosystems. By undertaking this study, a long-term research, we designed an experiment to separate the two components—the physical and biotic parts of the perennial plants—and to identify the factor that contributes the most to the ecosystem. The study site was located in the northern Negev Desert, Israel, where 50 Hammada scoparia shrubs and 50 artificial plants were randomly marked. Soil samples were collected monthly over 3 years of research at three locations: under the canopy of H. scoparia shrubs, in the vicinity of the artificial plants, and between the shrubs (control). The contribution to microbial activity was measured by evaluation of the microbial community functions in soil. The functional aspects of the microbial community that were measured were CO₂ evolution, microbial biomass, microbial functional diversity, and the physiological profile of the community. The results of this study are presented in two ways: (1) according to the three locations/treatments; and (2) according to the phenological situation of the vegetation (annual and perennial plants) in the research field: the growing phase, the drying process, and the absence of annual plants. The only parameters that were found to affect microbial activity were the contribution of the organic matter of perennial shrubs and the growth of vegetation (annual and perennial) during the growing seasons. The physical component was found to have no effect on soil microbial functional diversity, which elucidates the important contribution of the desert shrub in enhancing biological multiplicity and activity.     

Seasonal and Spatial Variation in Nematode Communities in a Negev Desert Ecosystem

Seasonal and spatial variation in soil nematode communities was investigated in a field study conducted on a loessial plain in the northern Negev Desert, Israel. Soil samples from 0- to 50-cm depths were collected seasonally during 2001 under the canopy of Atriplex halimus and Hammada scoparia, and between shrubs (control). Total population abundance ranged from 8 to 887 individuals per 100 g soil, represented by 32 genera from 16 families. Significant temporal and vegetation effects were elucidated using most ecological indices applicable to express nematode community composition. Of those indices computed, only the Shannon index, the modified maturity index, and genus dominance distinguished differences in vertical distribution.     

荒漠植物性状权衡策略及功能多样性研究进展

荒漠植物是干旱区具有独特功能性状与资源权衡表征的地带性植物。植物功能性状及其多样性格局与资源权衡策略对群落结构优化和生态系统功能改善起着关键作用。该综述主要从荒漠植物组织、器官功能性状特征、功能性状权衡策略、功能多样性组分及测度3个方面梳理了荒漠植物性状权衡策略与功能多样性研究的进展脉络：1）荒漠植物独特的根、茎、叶功能性状特征揭露了植被对环境变化的响应以及对生态系统功能的影响，基于植物功能性状的研究有助于解决许多生态学的关键性问题；2）作为植物功能性状之间存在的最普遍的联系，权衡策略是经过自然筛选后形成的性状组合，关键性状已经被发掘并创造性的提出了"经济谱"概念。荒漠植物研究过程中，应分析其根、茎、叶的特征属性筛选关键性状，着眼于关键性状间及整株植物性状间的权衡策略；3）功能多样性是影响生态系统运行和发挥作用的生物多样性的重要组成部分，荒漠植物功能多样性能预测和指示群落中物种对于荒漠生态系统功能发挥和过程变化的影响。功能多样性的组分可以从不同角度反映群落的生态位占据状况和资源利用程度，指数的选择要体现在群落内部物种的功能特征之间的差异程度，同时要考虑这些物种自身在群落内的优势程度。本研究为未来荒漠植物功能性状及多样性研究梳理了一些新的研究方向和内容，期望为荒漠植物生理生态学研究的选题和发展提供一些新的思路。     

Vertical Distribution of a Soil Microbial Community as Affected by Plant Ecophysiological Adaptation in a Desert System

The spatial and temporal patterns of resource distribution in a desert system have been shown to influence a number of soil biota components and processes. The pattern of possible different resources supplied by two typical halophyte shrubs with different ecophysiological adaptations, Atriplex halimus and Hammada scoparia, was found to be the trigger in the present study on the microbial community temporal and spatial (vertical) scale. The uniqueness of A. halimus, a saltbush plant, is in the creation of ‘islands of salinity’ in its rhizosphere system and the relatively high percentage of organic matter beneath its canopy, whereas the H. scoparia shrub has a leafless stem with a relatively lower amount of organic matter accumulation beneath its canopy. Soil samples from 0 to 50 cm depths were collected at 10-cm intervals during each of the four seasons. The vertical distribution of the microbial community was measured in the vicinity of the above plant shrubs and in the open spaces between them. The results obtained from the field study demonstrated that plant ecophysiological adaptation played an important role in the temporal and spatial distribution of abiotic conditions and microbial community levels such as microbial biomass, CO₂ evolution, and colony-forming units (CFUs) of both bacteria and fungi. However, we did not find consistency in the positive effect of plant cover on the microbial community. The shrub effect was observed to have a major outcome on microbial variables not only on a temporal but also on a spatial (vertical) basis. This contributed to the understanding of the controversy regarding the difference between the shrubs.     

外来物种刺槐对土壤微生物功能多样性的影响

以延河流域不同植被区内人工刺槐(Robinia pseudoacacia)群落和乡土植物群落的土壤微生物为研究对象,利用Biolog微平板技术对土壤微生物功能多样性进行测定,分析人工引种刺槐在不同环境梯度(3个植被区)下对土壤微生物功能多样性的影响。结果表明:刺槐对不同环境梯度下的土壤微生物的影响明显不同。从草原区到森林区,刺槐林之间土壤微生物群落的平均颜色变化率(AWCD)和土壤微生物功能多样性指数均没有显著变化;但与乡土植物群落比较,草原区、森林草原区和森林区土壤AWCD分别表现为刺槐乡土植物、刺槐乡土植物、刺槐乡土植物;在草原区和森林草原区刺槐林土壤微生物群落的群落丰富度指数(S)、Shannon-Wiener指数(H)、Simpson指数(D)、Mc Intosh指数(U)均大于乡土植物,森林区刺槐林群落丰富度指数(S)、Shannon指数(H)、Simpson指数(D)均小于乡土植物;刺槐林和乡土植物群落下土壤微生物碳源利用存在差异,主要体现在对糖类、氨基酸类的利用上。PCA分析显示主成分1贡献较大的碳源有24种,在主成分分离中起主要贡献作用的是糖类、氨基酸类和羧酸类。土壤碳氮含量能影响土壤微生物功能多样性指数,土壤含水量和温湿度能够影响碳源的利用类型。刺槐对土壤微生物功能多样性的影响存在区域差异,在评价刺槐对土壤生态过程与功能的影响时必须要考虑这种空间差异性。     

Ecophysiological response and morphological adjustment of two Central Asian desert shrubs towards variation in summer precipitation

As part of global climate change, variation in precipitation in arid ecosystems is leading to plant adaptation in water-use strategies; significant interspecific differences in response will change the plant composition of desert communities. This integrated study on the ecophysiological and individual morphological scale investigated the response, acclimation and adaptation of two desert shrubs, with different water-use strategies, to variations in water conditions. The experiments were carried out on two native dominant desert shrubs, Tamarix ramosissima and Haloxylon ammodendron, under three precipitation treatments (natural, double and no precipitation, respectively), in their original habitats on the southern periphery of Gurbantonggut Desert, Central Asia, during the growing season in 2005. Changes in photosynthesis, transpiration, leaf water potential, water-use efficiency, above-ground biomass accumulation and root distribution of the two species were examined and compared under the contrasting precipitation treatments. There were significant interspecific differences in water-use strategy and maintenance of photosynthesis under variation in precipitation. For the phreatophyte T. ramosissima, physiological activity and biomass accumulation rely on the stable groundwater, which shields it from fluctuation in the water status of the upper soil layers caused by precipitation. For the non-phreatophyte H. ammodendron, efficient morphological adjustment, combined with strong stomatal control, contributes to its acclimation to variation in precipitation. On account of its positive responses to increased precipitation, H. ammodendron is predicted to succeed in interspecific competition in a future, moister habitat.     

Soil Microbial Diversity in the Vicinity of Desert Shrubs

Water and nutrient availability are the major limiting factors of biological activity in arid and semiarid ecosystems. Therefore, perennial plants have developed different ecophysiological adaptations to cope with harsh conditions. The chemical profile of the root exudates varies among plant species and this can induce variability in associated microbial populations. We examined the influence of two shrubs species, Artemisia sieberi and Noaea mucronata, on soil microbial diversity. Soil samples were collected monthly, from December 2006 to November 2007, near canopies of both shrubs (0–10-cm depth). Samples were used for abiotic tests and determination of soil bacterial diversity. No significant differences were found in the abiotic variables (soil moisture, total organic matter, and total soluble nitrogen (TSN)) between soil samples collected from under the two shrubs during the study period. No obvious differences in the Shannon–Weaver index, evenness values, or total phylogenetic distances were found for the soil microbial communities. However, detailed denaturing gradient gel electrophoresis (DGGE) clustering as well as taxonomic diversity analyses indicated clear shifts in the soil microbial community composition. These shifts were governed by seasonal variability in water availability and, significantly, by plant species type.     

五大连池新期火山熔岩台地不同植被类型土壤微生物群落的功能多样性

土壤微生物群落是陆地生态系统的重要生物活性成分,其结构和功能多样性直接影响到系统的碳、氮等生态过程,微生物群落功能多样性与地上植被类型变化密切相关,开展植被类型对土壤微生物群落功能多样性的影响研究具有重要意义。以五大连池新期火山熔岩台地苔藓、草本、灌丛、矮曲林、针阔混交林5种典型植被类型为对象,利用BIOLOG微孔板法研究不同演替阶段植被类型土壤微生物群落功能多样性特征。结果表明：不同植被类型土壤微生物群落功能多样性存在显著差异。平均颜色变化率（AWCD）随培养时间延长而逐渐增加,大小顺序为：苔藓 > 针阔混交林 > 矮曲林 > 草本 > 灌丛。灌丛土壤微生物多样性指数与其他植被类型间差异显著。主成分分析结果表明,主成分1和主成分2分别能解释变量方差的56.24%和29.59%,不同植被类型下土壤微生物的碳源利用格局差异主要是由氨基酸类和带磷基糖类引起,二者合计解释总变异量的47.51%。冗余分析表明,速效磷、铵态氮、C：N和pH对微生物功能多样性具有显著的影响,羧酸类、氨基酸类、酯类和胺类的降解更易受到环境因素的影响。研究结果为进一步探讨植被类型与土壤微生物之间在植被演替过程中的关系提供参考。     

Responses of different herb life-history groups to a dominant shrub species along a dune stabilization gradient

Shrubs play a pivotal role in reversing desertification and in promoting ecological rehabilitation in severe environments. However, how the interactions between shrubs and their understorey species change during restoration remains unclear. Here, we report the results of an observational study conducted in the Mu Us Desert of northern China. This study explored how dune stabilization and the size of individual shrubs affect shrub–herb interactions. In particular, we aimed to determine how different life-history groups and performance indicators (e.g., biomass and richness) of subordinate species respond to shrub–herb interactions during dune stabilization. The shrub Artemisia ordosica had positive effects on understorey species in this dune system. The ability of the shrub to promote the growth of understorey species increased with dune stabilization, but decreased from small to large shrubs. This effect was due to an increase in the relative abundance and biomass of perennials and their higher sensitivity to the positive effects of the shrubs. In contrast, the ability of shrubs to improve the richness of understorey species decreased with dune stabilization, but increased from small to large shrubs. This effect occurred because perennials suffered high strain during recruitment in disturbed open patches of the semi-fixed dunes, particularly below small shrubs. Our results support the theory claiming that communities are sets of hidden interaction groups that have contrasting responses (neutral for annuals, but facilitative for perennials) to dominant neighbors in a single community, depending on their functional strategies. Furthermore, our study highlights the high efficiency of A. ordosica in facilitating ecological restoration of dry and disturbed sandy communities.     

镜泊湖岩溶台地不同植被类型土壤微生物群落特征

为了探讨不同演替阶段植被类型土壤微生物群落特征,分别选取镜泊湖岩溶台地草本、矮灌木、高灌木、小乔木与灌木混生(简称混生)群落、落叶阔叶林及针阔混交林6种典型植被类型,进行植物群落调查和对土壤微生物生物量、群落结构和多样性指标、土壤物理化学性质的测定。结果表明：从土壤微生物量、土壤微生物群落组成、土壤微生物代谢动力学过程和代谢功能多样性的角度来看,各种植被类型土壤微生物群落具有明显的差异。演替前期的草本群落土壤微生物量碳氮、细菌生物量、真菌生物量,代谢活性及丰富度指数均最低,但Shannon-Wiener多样性指数和均匀度指数显著(P<0.05)高于其他植被类型。矮灌木土壤微生物群落组成显著受植被类型的影响。高灌木群落和混生(小乔木与灌木混生)群落具有极强的相似性, 但在碳源利用类型上两者表现出一定的差异。落叶阔叶林代谢活性最高,碳源利用能力最强,能利用BIOLOG微孔板中的所有31种碳源,这与其具有较高的微生物量碳氮和细菌生物量一致,其代谢功能丰富度最高。演替后期的针阔混交林下的土壤pH最低,真菌比例升高,在碳源丰富的条件下具有极强的竞争优势(仅次于落叶阔叶林),但在碳源贫瘠的条件下其利用碳源能力较弱(仅高于草本)。植被可能主要通过土壤全磷和有机质影响土壤微生物代谢功能多样性。     

Microscopic and biomolecular complementary approaches to characterize bioweathering processes at petroglyph sites from the Negev Desert,Israel

Throughout the Negev Desert highlands, thousands of ancient petroglyphs sites are susceptible to deterioration processes that may result in the loss of this unique rock art. Therefore, the overarching goal of the current study was to characterize the composition, diversity and effects of microbial colonization of the rocks to find ways of protecting these unique treasures. The spatial organization of the microbial colonizers and their relationships with the lithic substrate were analysed using scanning electron microscopy. This approach revealed extensive epilithic and endolithic colonization and close microbial–mineral interactions. Shotgun sequencing analysis revealed various taxa from the archaea, bacteria and some eukaryotes. Metagenomic coding sequences (CDS) of these microbial lithobionts exhibited specific metabolic pathways involved in the rock elements' cycles and uptake processes. Thus, our results provide evidence for the potential participation of the microorganisms colonizing these rocks during different solubilization and mineralization processes. These damaging actions may contribute to the deterioration of this extraordinary rock art and thus threaten this valuable heritage. Shotgun metagenomic sequencing, in conjunction with the in situ scanning electron microscopy study, can thus be considered an effective strategy to understand the complexity of the weathering processes occurring at petroglyph sites and other cultural heritage assets.     

Soil microbial community shifts explain habitat heterogeneity in two Haloxylon species from a nutrient perspective

Haloxylon ammodendron and Haloxylon persicum (as sister taxa) are dominant shrubs in the Gurbantunggut Desert. The former grows in inter-dune lowlands while the latter in sand dunes. However, little information is available regarding the possible role of soil microorganisms in the habitat heterogeneity in the two Haloxylon species from a nutrient perspective. Rhizosphere is the interface of plant–microbe–soil interactions and fertile islands usually occur around the roots of desert shrubs. Given this, we applied quantitative real-time PCR combined with MiSeq amplicon sequencing to compare their rhizosphere effects on microbial abundance and community structures at three soil depths (0–20, 20–40, and 40–60 cm). The rhizosphere effects on microbial activity (respiration) and soil properties had also been estimated. The rhizospheres of both shrubs exerted significant positive effects on microbial activity and abundance (e.g., eukarya, bacteria, and nitrogen-fixing microbes). The rhizosphere effect of H. ammodendron on microbial activity and abundance of bacteria and nitrogen-fixing microbes was greater than that of H. persicum. However, the fertile island effect of H. ammodendron was weaker than that of H. persicum. Moreover, there existed distinct differences in microbial community structure between the two rhizosphere soils. Soil available nitrogen, especially nitrate nitrogen, was shown to be a driver of microbial community differentiation among rhizosphere and non-rhizosphere soils in the desert. In general, the rhizosphere of H. ammodendron recruited more copiotrophs (e.g., Firmicutes, Bacteroidetes, and Proteobacteria), nitrogen-fixing microbes and ammonia-oxidizing bacteria, and with stronger microbial activities. This helps it maintain a competitive advantage in relatively nutrient-rich lowlands. Haloxylon persicum relied more on fungi, actinomycetes, archaea (including ammonia-oxidizing archaea), and eukarya, with higher nutrient use efficiency, which help it adapt to the harsher dune crests. This study provides insights into the microbial mechanisms of habitat heterogeneity in two Haloxylon species in the poor desert soil.     

青藏高原高寒草甸不同海拔梯度下土壤微生物群落碳代谢多样性

土壤微生物群落功能多样性对维持生态系统功能和稳定性具有非常重要的意义。为探究青藏高原高寒草甸不同海拔梯度下土壤微生物碳源利用差异以及影响机制,运用Biolog微平板技术,研究了西藏当雄县草原站4300—5100 m的6个不同海拔梯度下土壤微生物群落碳源代谢多样性。研究结果表明:(1)不同海拔下高寒草甸土壤微生物碳源的利用程度均随培养时间的延长而升高;微生物代谢活性和群落多样性指数均随海拔升高呈现先上升后下降的单峰变化趋势,整体表现4800 m4950 m4400 m4650 m5100 m4300 m;(2)主成分分析表明不同海拔显著影响了土壤微生物群落碳源代谢多样性,其中碳水化合物类、氨基酸类和胺类碳源是各海拔土壤微生物的偏好碳源;碳水化合物类、羧酸类、氨基酸类和胺类碳源的利用强度受海拔影响较大;(3)分类变异分析表明,土壤、植物和气候因素是影响不同海拔碳源利用变异的主要影响因子,可解释不同海拔的碳源利用差异的79.0%;排除环境因子之间的多重及交互作用,偏曼特尔检验表明土壤含水量、植被丰富度和年均降水量是影响不同海拔微生物碳源利用多样性的最重要的环境因子。综上,研究表明青藏高寒草甸不同海拔土壤微生物碳源代谢多样性呈现显著的海拔差异趋势,其海拔差异主要受到土壤含水量、植被丰富度和年均降水量的影响。     

Microbial colonization of the salt deposits in the driest place of the Atacama Desert (Chile)

The Atacama Desert (Chile), one of the most arid places on Earth, shows hostile conditions for the development of epilithic microbial communities. In this study, we report the association of cyanobacteria (Chroococcidiopsis sp.) and bacteria belonging to Actinobacteria and Beta-Gammaproteobacteria and Firmicutes phyla inhabiting the near surface of salt (halite) deposits of the Salar Grande Basin, Atacama Desert (Chile). The halite deposits were investigated by using optical, confocal and field emission scanning electron microscopes, whereas culture-independent molecular techniques, 16S rDNA clone library, alongside RFLP analysis and 16S rRNA gene sequencing were applied to investigate the bacterial diversity. These microbial communities are an example of life that has adapted to extreme environmental conditions caused by dryness, high irradiation, and metal concentrations. Their adaptation is, therefore, important in the investigation of the environmental conditions that might be expected for life outside of Earth.     

·荒漠草地植物多样性对草食动物采食的响应机制

草食动物采食对草地植物多样性和生态系统功能的影响机制是放牧生态学研究的核心问题。该研究以内蒙古锡林郭勒盟苏尼特右旗荒漠草原的长期放牧控制实验为平台, 从既有草地植物多样性和动物偏食性两个层面系统地研究了荒漠草地植物多样性对草食动物采食的响应机制。结果显示: 1)荒漠草地植物对草食动物采食呈现4种响应模式: 放牧“隐没种”、放牧“敏感种”、放牧“无感种”、“绝对优势种”; 2)在群落尺度上, 物种多样性指数随放牧强度增加而减少, 与不放牧小区相比, 重度放牧(HG)与适度放牧(MG)小区植物多样性均下降, 且这一规律同样适用于功能群多样性, 灌木半灌木这一功能群内物种多样性对放牧干扰较敏感; 3)在草地既有植物的基础上, 以不放牧小区为参考系, 草食动物对植物功能群偏食性的排序为: 一二年生草本(AB) >多年生杂类草(PF) >灌木半灌木(SS) >多年生禾草(PG), 且偏食性物种主要分布于AB和PF中; 4)植物多样性与动物偏食性基本呈显著负相关关系(p < 0.05)。     

Seasonal Changes in Free-Living Amoeba Species in the Root Canopy of Zygophyllum dumosum in the Negev Desert, Israel

The influence of seasonality and Zygophyllum dumosum root canopy on the species diversity of free-living amoebae at two soil depths (0–10 and 10–20 cm) was studied in a Negev Desert ecosystem in Israel. Free-living amoebae were extracted and identified after cultivation in non-nutritive agar plates. A total of 90 amoeba species were identified in the soil during the study period, with the most common genera present being Hartmannella, Platyamoeba, Vahlkampfia, Acanthamoeba, and Echinamoeba. Differences between the control soil and the soil under Z. dumosum were found mainly during the dry seasons, when 97% similarity was found between the two soil layers, which could be due to the effect of the shrub on the soil microenvironment. The amoeba community exhibited more species diversity in spring (reaching a value of 34 species) than in the winter (18 species) or summer and autumn (20 species), since the community has a time lag for becoming stabilized after the dry summer and autumn. This is one of the first studies on the amoeba population in the Negev Desert and elucidates the importance and the need for taking trophic and functional groups into consideration in order to understand biomineralization processes.     

Microbial co-operation in the rhizosphere

Soil microbial populations are immersed in a framework of interactions known to affect plant fitness and soil quality. They are involved in fundamental activities that ensure the stability and productivity of both agricultural systems and natural ecosystems. Strategic and applied research has demonstrated that certain co-operative microbial activities can be exploited, as a low-input biotechnology, to help sustainable, environmentally-friendly, agro-technological practices. Much research is addressed at improving understanding of the diversity, dynamics, and significance of rhizosphere microbial populations and their co-operative activities. An analysis of the co-operative microbial activities known to affect plant development is the general aim of this review. In particular, this article summarizes and discusses significant aspects of this general topic, including (i) the analysis of the key activities carried out by the diverse trophic and functional groups of micro-organisms involved in co-operative rhizosphere interactions; (ii) a critical discussion of the direct microbe-microbe interactions which results in processes benefiting sustainable agro-ecosystem development; and (iii) beneficial microbial interactions involving arbuscular mycorrhiza, the omnipresent fungus-plant beneficial symbiosis. The trends of this thematic area will be outlined, from molecular biology and ecophysiological issues to the biotechnological developments for integrated management, to indicate where research is needed in the future.