19 research outputs found
Role of the interactions bacteria - bacterivorous nematodes on the availability of N and P within rhizosphere of rice on Ferralsols in Madagascar : mechanisms and control drivers
Les objectifs de cette thĂšse Ă©taient (i) d'Ă©tudier les mĂ©canismes par lesquels les nĂ©matodes bactĂ©rivores rĂ©gulent les flux de nutriments (N et P) dans la rhizosphĂšre du riz pluvial dans un sol ferrallitique de Madagascar, (ii) d'Ă©tudier les effets des interactions bactĂ©ries â bactĂ©rivores sur les fonctions de la plante (croissance et nutrition), et (iii) d'Ă©valuer les principaux facteurs contrĂŽlant la boucle microbienne des sols dans une optique d'intensification. Notre modĂšle biologique comprenait Acrobeloides sp. (Cephalobidae) et Oryza sativa (Poaceae).Deux voies ont Ă©tĂ© proposĂ©es pour expliquer les effets positifs des bactĂ©rivores sur la disponibilitĂ© des nutriments et les fonctions de la plante: voie de minĂ©ralisation et voie d'exploration. Afin d'identifier l'implication de chaque voie dans les flux de P inorganique Ă l'interface sol-plante nous avons utilisĂ© le radio-isotope 32P comme traceur. Lorsque le pH du sol est corrigĂ© par l'ajout de dolomie, la prĂ©sence des nĂ©matodes bactĂ©rivores amĂ©liore la minĂ©ralisation nette de P, la production de biomasse et la nutrition de la plante sans modifier la ramification du systĂšme racinaire, contrairement Ă ce qui a Ă©tĂ© observĂ© par certains auteurs en prĂ©sence de protistes. En effet, en prĂ©sence de nĂ©matodes et de dolomie, la L-value dans les parties aĂ©riennes de la plante augmente de 49%, la biomasse totale de la plante augmente de 22%, et la quantitĂ© de P total dans les tissus de la plante augmente de 9%. Ainsi, Acrobeloides sp. stimule le prĂ©lĂšvement de P par la plante via la voie nutritionnelle ou voie de minĂ©ralisation.Les effets de Acrobeloides sp. sur les fonctions de la plante sont variables (positives, neutres, nĂ©gatives), en fonction des pratiques agricoles et des caractĂ©ristiques abiotiques du sol. Tout d'abord, ces effets peuvent dĂ©pendre de la variĂ©tĂ© de riz puisque l'attractivitĂ© des nĂ©matodes Acrobeloides sp. par la rhizosphĂšre du riz varie en fonction des variĂ©tĂ©s de riz pluvial, plus prĂ©cisĂ©ment en fonction de la composition et de la quantitĂ© d'exsudats racinaires libĂ©rĂ©s par la plante. Ensuite, l'activitĂ© mutualiste des nĂ©matodes est influencĂ©e (maximisĂ©e) par la prĂ©sence d'arbres (agroforesterie) dans la parcelle de riz pluvial. En plus des plantes, les paramĂštres Ă©daphiques Ă©taient des facteurs clĂ©s. L'activitĂ© mutualiste des nĂ©matodes est fortement limitĂ©e par le pouvoir fixateur des Ferralsols. Ainsi, l'ajout de dolomie limite la capacitĂ© de sorption de P du sol et amĂ©liore l'activitĂ© mutualiste des nĂ©matodes bactĂ©rivores. Enfin, l'activitĂ© mutualiste des nĂ©matodes sur la croissance et la nutrition du riz pluvial est favorisĂ©e par les valeurs Ă©levĂ©es de pH et de teneur en Mg dans le sol. Pour conclure nous avons trouvĂ© qu'il est possible d'intensifier l'activitĂ© mutualiste des nĂ©matodes bactĂ©rivores dans un sol tropical pauvre Ă travers des pratiques agricoles spĂ©cifiques (gestion des sols, agroforesterie).Using Acrobeloides sp. (Cephalobidae) and Oryza sativa (Poaceae), we aimed at (i) studying the effects of bacterivorous nematodes on plant functions growing in a nutrient-poor soil in Madagascar, (ii) testing the mechanisms involved in the mutualistic activity of the bacterivorous nematodes in a strong P-depleted Ferralsol from the highlands of Madagascar, and (iii) identifying the agronomic and edaphic factors that can drive the soil microbial loop.Two pathways have been proposed to explain the positive effects of bacterial-feeders on nutrients availability and plant functions: mineralization pathway and exploration pathway. In order to test the involvement of each pathway in the mutualistic activity of bacterivorous nematodes, we assessed the effect of nematodes inoculation on inorganic P flows from soil to plant using the 32P labelling technique. When the soil pH was corrected with dolomite lime, we showed that the presence of Acrobeloides sp. improved net P mineralization and plant (Oryza sativa) functions (growth and nutrition) but did not alter lateral root growth as protists do. Indeed, in the presence of nematodes and dolomite, the shoot L-value increased by 49%, the plant total biomass increased by 22% and the plant total P amount increased by 9%. Thus, Acrobeloides sp. increased plant P uptake through the "nutritional" or "mineralization" pathway.The effects of Acrobeloides sp. on Oryza sativa functions were variable (positive, neutral or negative) according to agricultural practices and soil abiotic variables. First, these effects may depend on the rice cultivar used in the experiment. Indeed, the nematodes attractancy to root-adhering soils varied according to upland rice cultivar, probably according to the amount and composition of root exudates. The mutualistic activity of nematodes is also influenced (maximized) by the presence of trees (agroforestry) in the rice field. Besides plants, the abiotic soil parameters were key drivers. The mutualistic activity of nematodes was strongly limited by the ability of Ferralsols to precipitate and chemisorb phosphates. The addition of dolomite lime limited phosphates sorption on soil and enhanced the mutualistic activity of the nematodes. Also, the high values of soil pH and Mg content tended to increase the mutualistic activity of the bacterivorous nematodes on rice growth and nutrition. To conclude, we found that it is possible to drive the mutualistic activity of bacterivorous nematodes in poor tropical soils through specific soil and agricultural practices
DĂFORESTATION ET GOUVERNANCE ENVIRONNEMENTALE : ANALYSE DE LA MISE EN ĆUVRE DES POLITIQUES DE CONSERVATION DES FORĂTS A MADAGASCAR - CAS DE LâAIRE PROTĂGĂE MAROMIZAHA
Les premiĂšres lois forestiĂšres malgaches datent de lâĂ©poque monarchique, et ont Ă©tĂ© stimulĂ©es par une vision conversationniste. Tenant compte du droit coutumier des populations riveraines qui ont longtemps considĂ©rĂ© la forĂȘt comme un moyen de subsistance, ces rĂšgles formelles du gouvernement sont souvent rĂ©adaptĂ©es en fonction du contexte au niveau local. Cette recherche a eu pour objectif principal de comprendre comment lâEtat malgache sâest occupĂ© de la dĂ©forestation (1) au niveau national par lâintermĂ©diaire des lĂ©gislations, puis (2) dans un cas concret qui est lâaire protĂ©gĂ©e Maromizaha. Ainsi, lâanalyse sâest basĂ©e essentiellement sur les processus de gouvernance et de rĂ©appropriation des lois au niveau local. Est-ce que les politiques puis les structures mises en place et les mĂ©canismes locaux contribuent Ă lâamĂ©lioration du processus de conservation des forĂȘts et donc Ă la rĂ©duction de la dĂ©forestation ? Tel est lâobjet principal de la recherche. Nous avons alors procĂ©dĂ© Ă une revue de la littĂ©rature scientifique sur les concepts fondamentaux autour desquels sâarticule notre sujet, puis effectuĂ© une pĂ©riodisation de lâĂ©volution de la politique de conservation des forĂȘts, pour enfin mener des entretiens auprĂšs des principaux acteurs locaux Ă Maromizaha et confronter le thĂ©orique aux rĂ©alitĂ©s locales. Les difficultĂ©s de mise en Ćuvre des politiques forestiĂšres Ă Maromizaha sâinscrivent dans des dynamiques conflictuelles qui opposent les besoins de survies des locaux matĂ©rialisĂ©s par des activitĂ©s Ă©conomiques Ă courts termes et une vision conversationniste sur le long terme de lâEtat.Master de spĂ©cialisation en dĂ©veloppement, environnement et sociĂ©tĂ©s, UniversitĂ© catholique de Louvain, 201
RÎle des interactions bactéries-nématodes bactérivores sur la disponibilité du N et P au sein de la rhizosphÚre du riz sur sol ferrallitique à Madagascar : mécanismes et facteurs de contrÎle
Using Acrobeloides sp. (Cephalobidae) and Oryza sativa (Poaceae), we aimed at (i) studying the effects of bacterivorous nematodes on plant functions growing in a nutrient-poor soil in Madagascar, (ii) testing the mechanisms involved in the mutualistic activity of the bacterivorous nematodes in a strong P-depleted Ferralsol from the highlands of Madagascar, and (iii) identifying the agronomic and edaphic factors that can drive the soil microbial loop.Two pathways have been proposed to explain the positive effects of bacterial-feeders on nutrients availability and plant functions: mineralization pathway and exploration pathway. In order to test the involvement of each pathway in the mutualistic activity of bacterivorous nematodes, we assessed the effect of nematodes inoculation on inorganic P flows from soil to plant using the 32P labelling technique. When the soil pH was corrected with dolomite lime, we showed that the presence of Acrobeloides sp. improved net P mineralization and plant (Oryza sativa) functions (growth and nutrition) but did not alter lateral root growth as protists do. Indeed, in the presence of nematodes and dolomite, the shoot L-value increased by 49%, the plant total biomass increased by 22% and the plant total P amount increased by 9%. Thus, Acrobeloides sp. increased plant P uptake through the "nutritional" or "mineralization" pathway.The effects of Acrobeloides sp. on Oryza sativa functions were variable (positive, neutral or negative) according to agricultural practices and soil abiotic variables. First, these effects may depend on the rice cultivar used in the experiment. Indeed, the nematodes attractancy to root-adhering soils varied according to upland rice cultivar, probably according to the amount and composition of root exudates. The mutualistic activity of nematodes is also influenced (maximized) by the presence of trees (agroforestry) in the rice field. Besides plants, the abiotic soil parameters were key drivers. The mutualistic activity of nematodes was strongly limited by the ability of Ferralsols to precipitate and chemisorb phosphates. The addition of dolomite lime limited phosphates sorption on soil and enhanced the mutualistic activity of the nematodes. Also, the high values of soil pH and Mg content tended to increase the mutualistic activity of the bacterivorous nematodes on rice growth and nutrition. To conclude, we found that it is possible to drive the mutualistic activity of bacterivorous nematodes in poor tropical soils through specific soil and agricultural practices.Les objectifs de cette thĂšse Ă©taient (i) d'Ă©tudier les mĂ©canismes par lesquels les nĂ©matodes bactĂ©rivores rĂ©gulent les flux de nutriments (N et P) dans la rhizosphĂšre du riz pluvial dans un sol ferrallitique de Madagascar, (ii) d'Ă©tudier les effets des interactions bactĂ©ries â bactĂ©rivores sur les fonctions de la plante (croissance et nutrition), et (iii) d'Ă©valuer les principaux facteurs contrĂŽlant la boucle microbienne des sols dans une optique d'intensification. Notre modĂšle biologique comprenait Acrobeloides sp. (Cephalobidae) et Oryza sativa (Poaceae).Deux voies ont Ă©tĂ© proposĂ©es pour expliquer les effets positifs des bactĂ©rivores sur la disponibilitĂ© des nutriments et les fonctions de la plante: voie de minĂ©ralisation et voie d'exploration. Afin d'identifier l'implication de chaque voie dans les flux de P inorganique Ă l'interface sol-plante nous avons utilisĂ© le radio-isotope 32P comme traceur. Lorsque le pH du sol est corrigĂ© par l'ajout de dolomie, la prĂ©sence des nĂ©matodes bactĂ©rivores amĂ©liore la minĂ©ralisation nette de P, la production de biomasse et la nutrition de la plante sans modifier la ramification du systĂšme racinaire, contrairement Ă ce qui a Ă©tĂ© observĂ© par certains auteurs en prĂ©sence de protistes. En effet, en prĂ©sence de nĂ©matodes et de dolomie, la L-value dans les parties aĂ©riennes de la plante augmente de 49%, la biomasse totale de la plante augmente de 22%, et la quantitĂ© de P total dans les tissus de la plante augmente de 9%. Ainsi, Acrobeloides sp. stimule le prĂ©lĂšvement de P par la plante via la voie nutritionnelle ou voie de minĂ©ralisation.Les effets de Acrobeloides sp. sur les fonctions de la plante sont variables (positives, neutres, nĂ©gatives), en fonction des pratiques agricoles et des caractĂ©ristiques abiotiques du sol. Tout d'abord, ces effets peuvent dĂ©pendre de la variĂ©tĂ© de riz puisque l'attractivitĂ© des nĂ©matodes Acrobeloides sp. par la rhizosphĂšre du riz varie en fonction des variĂ©tĂ©s de riz pluvial, plus prĂ©cisĂ©ment en fonction de la composition et de la quantitĂ© d'exsudats racinaires libĂ©rĂ©s par la plante. Ensuite, l'activitĂ© mutualiste des nĂ©matodes est influencĂ©e (maximisĂ©e) par la prĂ©sence d'arbres (agroforesterie) dans la parcelle de riz pluvial. En plus des plantes, les paramĂštres Ă©daphiques Ă©taient des facteurs clĂ©s. L'activitĂ© mutualiste des nĂ©matodes est fortement limitĂ©e par le pouvoir fixateur des Ferralsols. Ainsi, l'ajout de dolomie limite la capacitĂ© de sorption de P du sol et amĂ©liore l'activitĂ© mutualiste des nĂ©matodes bactĂ©rivores. Enfin, l'activitĂ© mutualiste des nĂ©matodes sur la croissance et la nutrition du riz pluvial est favorisĂ©e par les valeurs Ă©levĂ©es de pH et de teneur en Mg dans le sol. Pour conclure nous avons trouvĂ© qu'il est possible d'intensifier l'activitĂ© mutualiste des nĂ©matodes bactĂ©rivores dans un sol tropical pauvre Ă travers des pratiques agricoles spĂ©cifiques (gestion des sols, agroforesterie)
Micro-food web interactions involving bacteria, nematodes, and mycorrhiza enhance tree P nutrition in a high P-sorbing soil amended with phytate
International audiencePhytate is considered a poorly available plant P source but proved to be useful for particular soil bacteria strains. In soil-free conditions, it has been shown that bacteria locked up the mineralized phosphorus from phytate whereas bacterial grazers like nematodes were able to deliver P to plants. Here, we aimed to determine if the interactions between phytate-mineralizing bacteria, bacterial grazer nematodes, and mycorrhizal fungi could increase plant P acquisition from phytate in high P-adsorbing soils. Pinus pinaster was grown in a Cambisol supplemented with phytate. Plants, whether associated or not associated with the ectomycorrhizal fungus Hebeloma cylindrosporum, were either inoculated or not inoculated with the phytase-releasing bacteria Bacillus subtilis and the bacterial-feeding nematode Rhabditis sp. After 100 days, the dual inoculation of bacteria and nematodes significantly increased net plant P accumulation. We observed that, on average, mycorrhizal plants accumulated more P in their shoots than non-mycorrhizal plants. However, the highest plant P acquisition efficiency was found when the three soil organisms were present in the P. pinaster rhizosphere. We conclude that, in a highly inorganic P-fixing soil, plant P acquisition from phytate strongly depends on the grazing of phytate-mineralizing bacteria. Our results confirm the importance of the soil microbial loop to improve plant P nutrition from phytate, which should be considered a route to improve the utilization of this source of poorly available P by plants
Impacts of agronomic practices on in-situ bacterivorous nematode-induced benefits on rice growth and nitrogen nutrition in Madagascar
International audienc
Do trophic relationships in soil enhance plant P nutrition? Phytate mineralization as a case study
Phosphorus (P) strongly limits plant productivity as plants can only absorb free inorganic orthophosphate (Pi) at very low concentrations in soil solution. However, soil contains high levels of poorly available P, especially phytate, considered as the most abundant plant-unavailable organic P source. Here, we investigated a new strategy based on rhizosphere trophic relationships to mobilize P from phytate. We hypothesized that the interactions between plant (Pinus pinaster), phytase-producing bacteria (Bacillus subtilis), mycorrhizal fungi (Hebeloma cylindrosporum), representing the more widespread strategy to improve plant P acquisition, and bacterial grazer nematodes (Rhabditis sp.) may improve plant P acquisition from phytate and thus P sustainability from soil organic P. We grew seedlings jn microcosms containing soil with Pi or phytate, with or without the above-mentioned organisms for 2.5 months. With Pi, no significant differences were observed among inoculation or mycorrhizal treatments. In contrast, with phytate, nematode grazing was required for non-mycorrhizal plants to acquire P into their shoots. In mycorrhizal plants, bacteria alone improved net P accumulation and nematode grazing enhanced this positive effect. Soil microbial P contents and in situ probing of bacterial phytase gene expression are currently under study to understand better the mechanisms underlying the strong positive effects of nematode grazing on plant P nutrition. Our first results indicate that the use of trophic relationships should be considered as a sustainable strategy for plant P nutrition to enhance organic P cycling and P availability in the rhizosphere