Rôle des interactions bactéries-nématodes bactérivors sur la disponibilité du N et P au sein de la rhizosphère du riz sur sol ferrallitique à Madagascar: mécanismes et facteurs de contrôle
Thèse soutenue le 29 janvier 2018 à Montpellier Supagro
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