Root-induced decrease in metal binding capacity of dissolved organic matters in the rhizosphere: evidences from two convergent studies : S10.01b -1

The parallel understanding of dissolved organic matters (DOM) impact on trace metal speciation in soil and root ability to change DOM concentration and composition in the rhizosphere strongly suggests a substantial alteration of metal binding capacity of DOM in the rhizosphere, with consequent impacts on metal phytoavailability. This hypothesis is investigated in the present communication on the basis of two independent set of experiments. Both experiments used the RHIZOtest experimental set-up, which enables an easy and fast recovery of both plants (i.e. shoots and roots) and rhizosphere, to grown either lettuce (Lactuca sativa) or durum wheat (Triticum turgidum durum) on two different soil samples notably varying in pH, organic matter content and geographical origin (tropical vs. temperate area). Usual chemical properties (i.e. pH, concentration of DOM, major cations/anions and metals) and free copper activity were measured in the rhizosphere solution. Copper speciation was then modelled in the rhizosphere solution with the humic ion-binding model VI (Model VI) by adjusting the metal binding capacity of DOM to fit experimental data. Compared with bulk soil measurements, a large increase in both pH and DOM concentration was observed in durum wheat rhizosphere while these two parameters did not change significantly in lettuce rhizosphere. Alternatively, the fraction of DOM involved in copper binding decreased similarly by 40 % in both durum wheat and lettuce rhizosphere. These very convergent pictures of a decrease in metal binding capacity of DOM in both experiments lead to discuss the hypothetical governing mechanisms and the genericity of this finding. (Texte intégral

Agricultural management and plant selection interactively affect rhizosphere microbial community structure and nitrogen cycling.

BACKGROUND:Rhizosphere microbial communities are key regulators of plant performance, yet few studies have assessed the impact of different management approaches on the rhizosphere microbiomes of major crops. Rhizosphere microbial communities are shaped by interactions between agricultural management and host selection processes, but studies often consider these factors individually rather than in combination. We tested the impacts of management (M) and rhizosphere effects (R) on microbial community structure and co-occurrence networks of maize roots collected from long-term conventionally and organically managed maize-tomato agroecosystems. We also explored the interaction between these factors (M × R) and how it impacts rhizosphere microbial diversity and composition, differential abundance, indicator taxa, co-occurrence network structure, and microbial nitrogen-cycling processes. RESULTS:Host selection processes moderate the influence of agricultural management on rhizosphere microbial communities, although bacteria and fungi respond differently to plant selection and agricultural management. We found that plants recruit management-system-specific taxa and shift N-cycling pathways in the rhizosphere, distinguishing this soil compartment from bulk soil. Rhizosphere microbiomes from conventional and organic systems were more similar in diversity and network structure than communities from their respective bulk soils, and community composition was affected by both M and R effects. In contrast, fungal community composition was affected only by management, and network structure only by plant selection. Quantification of six nitrogen-cycling genes (nifH, amoA [bacterial and archaeal], nirK, nrfA, and nosZ) revealed that only nosZ abundance was affected by management and was higher in the organic system. CONCLUSIONS:Plant selection interacts with conventional and organic management practices to shape rhizosphere microbial community composition, co-occurrence patterns, and at least one nitrogen-cycling process. Reframing research priorities to better understand adaptive plant-microbe feedbacks and include roots as a significant moderating influence of management outcomes could help guide plant-oriented strategies to improve productivity and agroecosystem sustainability

Root-induced alterations of copper speciation in solution in the rhizosphere of crop species

As a prerequisite to establish trace metal phytoavailability, it is essential to determine trace metal speciation in the solution of the rhizosphere, where substantial alterations of physicalchemical properties (e.g. pH, Eh, organic matters) are induced by root activities. Investigations in the past decades were mainly dedicated to studying illustrative cases of how each individual rhizosphere process is able to influence trace metal speciation in solution. On a more integrative perspective, the present study aimed at investigating (i) the diversity of chemical modifications occurring in the solution of the rhizosphere of crop species cultivated on soils exhibiting a very wide range of physical-chemical properties and (ii) their consequent impact on copper (Cu) speciation in solution. Three plant species from three distinct botanical families, i.e. one graminaceous species, red fescue (Festuca rubra), two dicotyledonous species, tomato (Lycopersicon esculentum) and cabbage (Brassica oleracea), and 55 soils exhibiting a wide variety of physico-chemical properties (e.g. pH 4.4-8.2, 1-126 g Corg kg-1 , 6-1077 mg Cutotal kg-1 ) were selected for this study. Plants were grown using the RHIZOtest experimental design. This biotest consists in growing plants for two weeks in hydroponics, then for 8 days in contact with soil. Soils harvested from planted and unplanted devices are considered to be representative of rhizosphere and bulk soil, respectively. The solution of each rhizosphere and bulk soil was extracted with an unbuffered salt solution, and pH, concentrations of major ions and trace elements, dissolved organic matter (DOM), as well as the free Cu2+ activity were measured. The reactivity of DOM towards Cu was also estimated by modeling of Cu speciation, using the Humic Ion-Binding Model VII. Root activities induced variation in pH and in DOM concentration and its reactivity, thereby inducing substantial alterations of Cu speciation in solution. Fescue induced an overall alkalization of the rhizosphere that tended to be stronger as the bulk soil was more acidic. Conversely, tomato and cabbage induced an acidification or alkalisation of the rhizosphere depending on soil type. Surprisingly, the concentration of DOM tended to decrease in the rhizosphere, especially for soils initially exhibiting the highest DOM concentrations. This result could be explained by an increase in microbial activity in the rhizosphere leading to a higher rate of DOM mineralization. The reactivity of DOM varied in a complex pattern the rhizosphere , either increasing or decreasing compared to the bulk soil depending on soil properties and plant species. As a result of the drastic alteration of chemical properties in the solution of the rhizosphere and of Cu uptake by roots, the free Cu2+ activity was changed by up to three orders of magnitude in the rhizosphere compared to the bulk soil. Free Cu2+ activity tended to decrease in the rhizosphere for soil exhibiting the highest free Cu2+ activities in the bulk soil. Conversely, free Cu2+ activity increased up to 1 to 2 orders of magnitude in the rhizosphere for soil exhibiting the lowest free Cu2+ activities. In such soils, the decrease in DOM reactivity could explain the increase in free Cu2+ activity in the rhizosphere. Our results show a consistent picture of how root activities can substantially alter trace metal speciation in the rhizosphere in a wide range of soils and plant species. Among the rhizosphere properties relevant for trace metal biogeochemistry and phytoavailability, the characterization of DOM reactivity should deserve further attention. (Texte intégral

Unlocking the microbiome communities of Banana (Musa spp.) under disease stressed (Fusarium wilt) and non-stressed conditions

We assessed the diversity, structure, and assemblage of bacterial and fungal communities associated with banana plants with and without Fusarium oxysporum f. sp. cubense (Foc) symptoms. A total of 117,814 bacterial and 17,317 fungal operational taxonomy units (OTUs) were identified in the rhizosphere, roots, and corm of the host plant. Results revealed that bacterial and fungal microbiota present in roots and corm primarily emanated from the rhizosphere. The composition of bacterial communities in the rhizosphere, roots, and corm were different, with more diversity observed in the rhizosphere and less in the corm. However, distinct sample types i.e., without (asymptomatic) and with (symptomatic) Fusarium symptoms were the major drivers of the fungal community composition. Considering the high relative abundance among samples, we identified core microbiomes with bacterial and fungal OTUs classified into 20 families and colonizing distinct plant components of banana. Our core microbiome assigned 129 bacterial and 37 fungal genera to known taxa

The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

The rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes, bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial microorganisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health

Soil enzyme activities in the rhizosphere of field-grown sugar beet inoculated with the biocontrol agent Pseudomonas fluorescens F113

The original publication is available at www.springerlink.com . Copyright Springer DOI : 10.1007/s003740050397Pseudomonas fluorescens F113, which produces the antimicrobial compound 2,4-diacetylphloroglucinol is a prospective biocontrol agent. Soil enzyme activities were used to investigate the ecological impact of strain F113 in the rhizosphere of field-grown sugar beet. There were distinct trends in rhizosphere enzyme activities in relation to soil chemistry (studied by electro-ultrafiltration). The activities of enzymes from the phosphorus cycle (acid phosphatase, alkaline phosphatase and phosphodiesterase) and of arylsulphatase were negatively correlated with the amount of readily available P, whereas urease activity was positively correlated with the latter. Significant correlations between electro-ultrafiltration nutrient levels and enzyme activity in the rhizosphere were obtained, highlighting the usefulness of enzyme assays to document variations in soil nutrient cycling. Contrary to previous microcosm studies, which did not investigate plants grown to maturity, the biocontrol inoculant had no effect on enzyme activity or on soil chemistry in the rhizosphere. The results show the importance of homogenous soil microcosm systems, used in previous work, in risk assessment studies, where inherent soil variability is minimised, and where an effect of the pseudomonad on soil enzymology could be detected.Peer reviewe

Assessing the influence of the rhizosphere on soil hydraulic properties using X-ray Computed Tomography and numerical modelling

Understanding the dynamics of water distribution in soil is crucial for enhancing our knowledge of managing soil and water resources. The application of X-ray Computed Tomography (CT) to the plant and soil sciences is now well established. However, few studies have utilised the technique for visualising water in soil pore spaces. Here we utilise this method to visualise the water in soil in situ and in three-dimensions at successive reductive matric potentials in bulk and rhizosphere soil. The measurements are combined with numerical modelling to determine the unsaturated hydraulic conductivity, providing a complete picture of the hydraulic properties of the soil. The technique was performed on soil cores that were sampled adjacent to established roots (rhizosphere soil) and from soil that had not been influenced by roots (bulk soil). A water release curve was obtained for the different soil types using measurements of their pore geometries derived from CT imaging and verified using conventional methods e.g. pressure plates. The water, soil and air phases from the images were segmented and quantified using image analysis. The water release characteristics obtained for the contrasting soils showed clear differences in hydraulic properties between rhizosphere and bulk soil, especially in clay soil. The data suggests that soils influenced by roots (rhizosphere soil) are less porous due to increased aggregation when compared to bulk soil. The information and insights obtained on the hydraulic properties of rhizosphere and bulk soil will enhance our understanding of rhizosphere biophysics and improve current water uptake models

Two Entomophagous Isolated From Sumatera Utara; Potential as Biocontrol Agent Againts Nematode

Two species of nematophagous fungi has been isolated from Sumatera Utara soil, with an aim of harnessing their potential in the biological control of plant parasitic nematodes or animal parasitic nematodes in Indonesia, especially in Sumatera Utara. Soil samples were collected from tobacco plantations, vegetable fields and ornamental plantings in the Berastagi area, and also from livestock in local farms and a dairy farm in Berastagi Area, Karo Regency. Soil also collected from un-cultivated area in Sibolangit National Park, Karo Regency. The pour method described by Larsen et al., (1988) and the sprinkle method described by Jafee et al., (1996) were used to isolate the nematophagous fungi from soil. In this study the Chloramphenicol Water Agar Media has been used as culture media and Ceanorhabditis elegans has been used as bait. Two nematophagous fungi known as insect pathogens (entomophagous) have been isolated and determined as Lecanicillium lecanii and Paecilomyces fumosoroseus

Long-term field metal extraction by pelargonium:phytoextraction efficiency in relation to plant maturity

The long length of periods required for effective soil remediation via phytoextraction constitutes a weak point that reduces its industrial use. However, these calculated periods are mainly based on short-term and/or hydroponic controlled experiments. Moreover, only a few studies concern more than one metal, although soils are scarcely polluted by only one element.In this scientific context, the phytoextraction of metals and metalloids (Pb, Cd, Zn, Cu,and As) by Pelargonium was measured after a long-term field experiment. Both bulk and rhizosphere soils were analyzed in order to determine the mechanisms involved in soil-root transfer. First, a strong increase in lead phytoextraction was observed with plant maturity, significantly reducing the length of the period required for remediation. Rhizosphere Pb, Zn, Cu, Cd, and As accumulation was observed (compared to bulk soil), indicating metal mobilization by the plant, perhaps in relation to root activity. Moreover, metal phytoextraction and translocation were found to be a function of the metals’ nature. These results, taken altogether, suggest that Pelargonium could be used as a multi-metal hyperaccumulator under multi-metal soil contamination conditions, and they also provide an interesting insight for improving field phytoextraction remediation in terms of the length of time required, promoting this biological technique