Inoculation of the redox effector Pseudomonas fluorescens C7R12 strain affects soil redox status at the aggregate scale

In order to investigate the redox condition in soil aggregates, an approach using the inoculation of the soil bacteria Pseudomonas fluorescens C7R12, which affects soil redox, was developed. First, redox modifications were studied at the colony level in pure culture on solid synthetic medium. Then, the effect of strain inoculation on the redox status of soil aggregates was studied under controlled conditions with millimeter size aggregates from a Mollisol. A redox micro-electrode was employed to determine redox intensity (redox potential, E), which ranged from 240G70 to 110G25 mV at the surface of non-inoculated (control) and inoculated aggregates after 3 days. Differential pulse polarography (DPP) was employed to assess redox capacity (amount of electro-active compounds), which was higher (7.17 nA/mV) in inoculated aggregates than in the control samples (3.04 nA/mV). Similarities between redox potential depth profile for P. fluorescens colonies on plates and for some inoculated aggregate profiles suggested a patch colonisation of the surface of the aggregates by the redox effector strain. By comparison with non-inoculated aggregates, the influence of the inoculated strain on the redox status of the aggregates was observed to increase with the amount of some electroactive compounds with potential values from about K110 and C15–40 mV. Lower mean values and the limitation of the redox potential range fluctuation along the profiles from surface to 220 mm depth were also characteristics of the inoculated aggregates

Distribution of a genetically - engineered Escherichia coli population introduced into soil

International audienc

Does disturbance and restoration of alpine grassland soils affect the genetic structure and diversity of bacterial and N2-fixing populations?

Responses of bacterial communities to disturbance and restoration processes were investigated on alpine grassland soil. Bulk soil, rhizosphere soil and two soil separates, i.e. sand-size (2000-200 mm) and silt-size (50-2 mm) were sampled from undisturbed grassland soil to soil under restoration for 1 month, 1 year, 4 years and 13 years after disturbance. Automated ribosomal intergenic spacer analysis (ARISA) and restriction fragment length polymorphism (RFLP) of nifH gene pools were used to assay genetic structure of the bacterial communities and N2-fixing guild. According to the distribution of ARISA band length in bacterial phyla, the dominance of ARISA bands below 400 bp showed that Gram-positive bacteria would be predominant in the studied grassland soil when not disturbed. Disturbance affected the genetic structure of bacterial community and of N2-fixing guild in relation to their location within the selected habitats. Shifts in IGS and nifH profiles of bulk soil metagenome were larger than those observed from sandsize- and silt-size-fractions, accounting for 40–50% of the variance in the profiles. Restoration of the genetic structure of telluric bacteria community and N2-fixing populations was found to be influenced by the spatial heterogeneity of the soil and niche diversification.Particular bacterial genetic structure within distinct habitats were evidenced and must be defined as subdivisions of the meta-community of bulk soil. Scale of soil microbial diversity/stability relationships is discussed with special attention to disconnected bacterial habitat compared with whole soil with multiple niches

Does disturbance and restoration of alpine grassland soils affect the genetic structure and diversity of bacterial and N2-fixing populations?

Responses of bacterial communities to disturbance and restoration processes were investigated on alpine grassland soil. Bulk soil, rhizosphere soil and two soil separates, i.e. sand-size (2000-200 mm) and silt-size (50-2 mm) were sampled from undisturbed grassland soil to soil under restoration for 1 month, 1 year, 4 years and 13 years after disturbance. Automated ribosomal intergenic spacer analysis (ARISA) and restriction fragment length polymorphism (RFLP) of nifH gene pools were used to assay genetic structure of the bacterial communities and N2-fixing guild. According to the distribution of ARISA band length in bacterial phyla, the dominance of ARISA bands below 400 bp showed that Gram-positive bacteria would be predominant in the studied grassland soil when not disturbed. Disturbance affected the genetic structure of bacterial community and of N2-fixing guild in relation to their location within the selected habitats. Shifts in IGS and nifH profiles of bulk soil metagenome were larger than those observed from sandsize- and silt-size-fractions, accounting for 40–50% of the variance in the profiles. Restoration of the genetic structure of telluric bacteria community and N2-fixing populations was found to be influenced by the spatial heterogeneity of the soil and niche diversification.Particular bacterial genetic structure within distinct habitats were evidenced and must be defined as subdivisions of the meta-community of bulk soil. Scale of soil microbial diversity/stability relationships is discussed with special attention to disconnected bacterial habitat compared with whole soil with multiple niches

Impact agronomique et écologique de l'inoculation du maïs par Azospirillum lipoferum CRT1

Les effets de l'inoculation du maïs (Zea mays) par la souche bactérienne Azospirillum lipoferum CRT1 (AZOGREEN®) ont été étudiés au champ et en serre. La production d'hormone (AIA) et la capacité à fixer l'azote atmosphérique sont deux propriétés des bactéries du genre Azospirillum impliquées dans les effets bénéfiques. Cependant, d'autres effets sur la colonisation racinaire par les micro-organismes indigènes pourraient également intervenir. Sur le plan agronomique, un suivi pluri annuel a montré une grande variabilité des effets sur le rendement ; en terme de production de grain ou de fourrage, les conditions climatiques n'ont pas permis de distinguer clairement un effet de l'inoculation. L'effet indéniable est une augmentation de la biomasse racinaire, dont nous avons déterminé les paramètres morphologiques par analyse d'image (WINRHIZO®). Chez les plantes inoculées, la surface racinaire est multipliée par 1,2 au stade 2-3 feuilles, 7 jours après le semis, par 1,6 au stade 9-10 feuilles. Des modifications de la qualité du matériel racinaire (composition, résistance mécanique) ont été observées (J35). Du point de vue de l'écologie des micro-organismes, l'inoculation s'accompagne d'un accroissement spécifique des densités de populations indigènes d'A.lipoferum (autres que l'inoculum) et d'A.brasilense/amazonense. Les changements de structure associés à l'inoculation sont significatifs avec une élévation de la proportion des IGS (A-RISA) de taille supérieures à 500bp dans les stades de recrutement comme J7 et J35. L'évaluation de l'effet sur le potentiel de fixation de l'N est une augmentation du taux de transcrits du gène nifH (PCR quantitative) significativement plus élevée dans la rhizosphère des plantes inoculées à partir de la 4ème semaine après le semis. En conclusion, la modification du système racinaire est l'effet principal de l'inoculation du maïs par A.lipoferum CRT1 cultivé sur un luvisol de la région lyonnaise. Les effets sur les populations microbiennes sont associées à un contact accru entre les surfaces racinaires actives et les bactéries indigènes. L'élévation du taux de transcrits NifH est sensible au moment où, au champ, se fait la fertilisation azotéeLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Response of soil bacteria to Hg (II) in relation to soil characteristics and cell location

International audienc

Root impact on the stability and types of microaggregates in silty soil under maize

We wished to assess the impact of maize roots on the stability, nature and formation of soil organomineral aggregates under field conditions. We sampled coarse-textured soil, cropped with maize for 3 years, and combined in situ study of the root system with evaluation of soil structural stability. We examined root morphology (i) on thin sections of still recognizable root tissues, and (ii) at the interfaces between roots and soil, using transmission electron microscopy (TEM) to differentiate between fine and coarse roots. Soil structural stability was evaluated in relation to a spatial gradient of increasing distance from the roots. For that we distinguished between soil adhering to roots and that not adhering in the rhizosphere, and soil between the rows of this crop, and used two methods of granulodensimetric soil fractionation involving different disaggregation strengths. We isolated and identified the organomineral associations in the soil fractions determining soil stability. We found that the roots at the soil–root interface generated stable aggregates of 2–20 mm size, which were much involved in the structural stability of the soil adhering to roots. We identified by TEM several types of organomineral associations that depended on the nature of the plant or microbial organic matter. The distribution of the different types of aggregates varied according to their positions in relation to the roots. We showed that humification of roots contributes significantly to the development of stable organomineral associations in the soil

SOIL PHYSICO-CHEMICAL CHANGES FOLLOWING APPLICATION OF MUNICIPAL SOLID WASTE LEACHATES TO GRASSLANDS

Concerns about the use of residues from municipal solid waste incinerators (MSWI) in construction materials usually focus on the potential for heavy metals and organic chemicals to leach into drainage waters under the influence of rain.We hypothesised that high level of salts in the MSWI leachates may cause more of a problem, particularly on soil physico-chemical properties. Both bottom ash (BA) and Solidified Air Pollution Control residue (SAPCr) leachates were added to experimental grassland plots. The amounts of Na+ increased by up to 13% in soils supplemented with each leachate. A decrease of the soil total porosity (−14%) was evidence of a subsequent adverse physical effect of this strong salinity. The potential for the grass cover type (species composition or density) to limit this adverse effect was discussed. Laboratory tests allowed us to determine that undiluted SAPCr induced slaking of aggregates accompanied by a strong decrease of aggregate stability, to 49% of control values. Undiluted BA induced dispersion of clays and others fine particles, which are then dislodged and transported into pores, causing blockage and decreasing total porosity. Clay dispersion followed by aggregate collapse occurred when soil solution contaminated by SAPCr was diluted by rainwater. This work stressed the importance of accounting for mineral contaminants, such as salts, when conducting an assessment of waste reuse scenarios

SOIL PHYSICO-CHEMICAL CHANGES FOLLOWING APPLICATION OF MUNICIPAL SOLID WASTE LEACHATES TO GRASSLANDS

Concerns about the use of residues from municipal solid waste incinerators (MSWI) in construction materials usually focus on the potential for heavy metals and organic chemicals to leach into drainage waters under the influence of rain.We hypothesised that high level of salts in the MSWI leachates may cause more of a problem, particularly on soil physico-chemical properties. Both bottom ash (BA) and Solidified Air Pollution Control residue (SAPCr) leachates were added to experimental grassland plots. The amounts of Na+ increased by up to 13% in soils supplemented with each leachate. A decrease of the soil total porosity (−14%) was evidence of a subsequent adverse physical effect of this strong salinity. The potential for the grass cover type (species composition or density) to limit this adverse effect was discussed. Laboratory tests allowed us to determine that undiluted SAPCr induced slaking of aggregates accompanied by a strong decrease of aggregate stability, to 49% of control values. Undiluted BA induced dispersion of clays and others fine particles, which are then dislodged and transported into pores, causing blockage and decreasing total porosity. Clay dispersion followed by aggregate collapse occurred when soil solution contaminated by SAPCr was diluted by rainwater. This work stressed the importance of accounting for mineral contaminants, such as salts, when conducting an assessment of waste reuse scenarios