Vers de terre et services ecosytémiques en riziculture pluviale a Madagascar.. Connaissances des processus et propositions d'innovations Agricoles.
Thèse soutenue le 20 juillet 2018 à l'Université d'Antananarivo à Madagascar
Soil organisms play a key role in the soil ecological functions at the basis of soil ecosystem services: (i) recycling of soil nutrients, (ii) decomposition of organic materials, (iii) maintenance of soil structure, and (iv) regulation of pests and pathogens. With the outcome to improve the productivity and sustainability of cropping systems in Madagascar, the present thesis aims to better understand the role of earthworms in the soil ecological functions and plant responses, and to test the feasibility of earthworm inoculation in field conditions in the rainfed rice systems in the Highlands of Madagascar. Numerous experiments at different scales were conducted in order to better understand the role of these organisms in very nutrient and carbon poor, phosphorus-fixing soils such as Ferralsols on hills in Malagasy Highlands.
In the laboratory, our results show that earthworms (Pontoscolex corethrurus) positively affect plant nutrition and growth. The magnitude of this positive effect depends essentially on the responsiveness of cultivars to the presence of earthworms. We found an increase ranged from 11% to 355% of the amounts of total N accumulated in the aerial parts of rice. For the total P amounts, such increase varied between 37% and 145%. In addition, aboveground biomass increases from 27% to 99% in the presence of earthworms. A part form plant growth and nutrition, earthworms also have an important role in the regulation of pathogens. The interaction of earthworm and silicon inputs reduces the overall severity of blast disease while increasing plant growth.
In the field, the earthworm inoculation increases by 44% and 14%, respectively the total N and total P amounts in the aboveground biomass of plant at rice harvest. In addition, earthworms positively affect aboveground biomass (+ 28%) and grain yields of rice (+ 45%) after 3 years. An improvement of C sequestration is also induced by earthworm presence. The annual C stock increases by 0.61 MgC ha-1 year-1 in soils where earthworms were introduced during three years. This is probably related to the improvement of soil structure by an increase (+ 11%) of the soil aggregation.
Thanks to the beneficial effects of earthworms in several agroecosystem services, the earthworm biofertilization or the optimization of their activity through various practices appear as promising ways to meet the context of ecological intensification of agriculture. This innovative practice could be a key solution for improving agricultural production in low input tropical agro-ecosystems.