Fonctionnement des phosphatases dans les sols tropicaux (influence de la composition organo-minérale sur l'expression de l'activité enzymatique)

La prédiction de l'activité des phosphatases fongiques dans l'amélioration de la nutrition phosphatée doit prendre en compte les facteurs qui influencent leur fonctionnement dans les sols. L'objectif de cette thèse a été d'étudier divers facteurs qui pouvaient influencer l'efficacité des phosphatases de champignons ectomycorhiziens dans les sols. L'adsorption et l'activité à l'état adsorbé des phosphatases de Suillus collinitus et de Hebeloma cylindrosporum ont été étudiées avec plusieurs fractions de différents sols tropicaux. La persistance de l'activité de ces enzymes immobilisées sur les sols a également été étudiée. Ces phosphatases ont montré une diversité d'affinité avec les colloïdes des sols, liée surtout à leur origine et leurs caractéristiques. En outre, aucune relation n'a été établie entre l'adsorption et l'activité catalytique résultante, car il n'y avait généralement pas de perte d'activité à l'état adsorbé. L'une des enzymes qui à montré une dégradation rapide en solution suivant le temps d'incubation, a été protégée par les sols ferrallitiques mais pas par les vertisols. Des essais de purification et de caractérisation ont été faits sur ces échantillons de phosphatases fongiques. Les fractions de phosphatases de S. collinitus purifiées et retenues sur une colonne hydrophobe de chromatographie ont montré une activité en contact avec des argiles fortement supérieure à celle en solution. L'hypothèse d'une dimérisation produite à la surface des argiles a été avancée pour expliquer l'amplification inattendue d'activité catalytique à l'état adsorbé des fractions purifiéesThe role of catalytic activity of fungal phosphatases in the improvement of the phosphorus nutrition cannot be reliably predicted without taking into account the factors which influence their behaviour in the soil. The objective of this thesis was to study various factors which could influence the effectiveness of ectomycorrhizal fungal phosphatases in soils. Adsorption and the activity in the adsorbed state of phosphatases produced by Suillus collinitus and Hebeloma cylindrosporum were studied in contact with several fractions of various tropical soils. The persistence of the activity of these enzymes immobilized on the soils was also studied. These phosphatases showed a diversity of affinity for soil colloids, due to their origin and their characteristics. Moreover, no relation was found between adsorption and the resulting catalytic activity; there was generally no loss of activity in an adsorbed state. One of the enzymes which underwent rapid degradation in solution was protected by the presence of ferrallitic soils but not by the vertisols. These fungic phosphatase samples were purified and partially characterized. The fractions of S. collinitus phosphatases retained on hydrophobic chromatography column showed enhanced activity in contact with mineral clays with respect to solution. The hypothesis of dimeerisation on the clay surfaces was advanced to explain the unexpected enhancement of catalytic activity in an adsorbed state of the purified fractions.MONTPELLIER-SupAgro La Gaillarde (341722306) / SudocSudocFranceF

International Centre for Guidance Studies (iCeGS) Annual Review (2020)

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Interacting effects of land use type, soil microbes and plant traits on aggregate stability

Soil aggregates are critical to soil functionality, but there remain many uncertainties with respect to the role of biotic factors in forming aggregates. Understanding the interacting effects of soil, land use type, vegetation and microbial communities is a major challenge that needs assessment in both field and controlled laboratory conditions, as well as in bulk and rhizosphere soils. To address these effects and their feedbacks, we first examined the influence of soil, root and litter characteristics along a land use gradient (ancient woodland, secondary woodland, grassland, pasture and arable land) on microbial community structure (in both bulk and rhizosphere soil), as well as on aggregate stability. Then, we performed an inoculation experiment where we extracted soil columns from the arable and secondary woodland and used a third unstructured loamy soil as a control. We sterilized these three soils to remove microbial communities, and then either inoculated the tops of sterilized soil columns with soil from the secondary woodland or the arable field sites. Control columns of all soil types were not inoculated. In a fully-crossed design, we planted two species possessing distinct root system morphological traits: Brachypodium sylvaticum (fibrous system with many thin and fine roots) and Urtica dioica (taproot system with few fine roots). After four months, microbial communities (in bulk and rhizospheric soil) and aggregate stability were measured, along with root traits. In both the field and laboratory experiments, bacterial (16S) and fungal (ITS) biodiversity was determined using high throughput sequencing. In the field study we found that: i) there were strong relationships between aggregate stability and microbial community composition that were driven by land use, ii) the relationship between aggregate stability along the land use gradient and the trophic nature of bacterial communities was not significant, but that certain soil, root and litter parameters shaped bacterial phyla, with oligotrophic bacteria conditioned by the rhizosphere niche, and copiotrophic phyla more dependent on bulk soil conditions, iii) land use gradient (from woodland to arable), reduced the relative abundance of saprotrophic and ectomycorrhizal fungi with an increase in the relative abundance of Ascomycota and a reduction in the relative abundance of Basidiomycota. In the laboratory experiment we found that: i) the inoculation of sterilized soils with soils from the field significantly increased aggregate stability in control soil that was initially poorly structured, ii) the effects of inoculation on aggregate stability were similar when either secondary woodland or arable soils were used as inoculums and iii) these effects were affected significantly by root length density. Our results show that microbial communities influence soil structure and that bacterial communities are intimately associated to rhizospheric conditions and root traits (of which root length density was the most pertinent)

Effect of organic matter on the adsorption of trace amounts of caesium by soil

International audienc

Le sol : facteur déterminant dans le devenir du césium dans l'environnement

National audienc

Adsorption of radiocaesium on various soils : interpretation and consequences of the effects of soil : solution ratio and solution composition on the distribution coefficient

International audienc

On the production of the distribution coefficient of 137Cs in soils from clay mineralogy

International audienc

Sensitivity analysis of the distribution coefficient, Kd, of nickel with changing soil chemical properties

Correspondance: [email protected] audienceNo simple universal test exists to predict accurately the mobility and availability of trace metals in soil. The principals behind existing tests are briefly discussed. One of the reasons for their limited success is that soil tests do not take into account the temporal and spatial variations in soil composition and hence metal speciation. In this study a simple variation on a classical technique is proposed. The. parameter measured is the distribution coefficient, Kd, defined as the ratio of concentrations in the adsorbed and solution phases. It has been obtained using a radiotracer. Variations of soil composition have been chemically simulated by addition of simple chemicals, taking particular account of changing composition in the rhizosphere. The approach has been applied to the distribution of nickel. The effect of concentration in Ni, other similar divalent cations (Co2+, Cu2+, Zn2+, Pd2+, Sn2+, Pb2+) and an indifferent cation, Ca2+, have been investigated. The adsorption isotherm was strongly nonlinear and could be fitted well to a Freundlich isotherm. Competitive adsorption led to a decrease in Ni sorption when other cations were present. Their effect depended upon their chemical similarity to Ni, in particular their softness. The adjustment of pH within a range often induced by biological activity was found to produce various effects, depending on the soil. In general, Kd increased as pH was increased towards neutrality and in some cases a marked decrease was observed at alkaline pH. The effect of addition of organic compounds was also complex and soil-dependent and could not be simply attributed to the formation of solution phase complexes. The sensitivity of Kd to the chosen parameters is discussed with respect to the likely consequences on mobility. Some likely mechanisms are briefly presented. Finally, the limitations inherent to this albeit useful approach are discusse

Direct and indirect effects of organic matter on metal immobilisation in soil

International audienc

Cs adsorption on the clay-sized fraction of various soils: effect of organic matter destruction and charge compensating cation

International audienc