The main objective of my research work is to quantify and limit the risk of soil, vegetables and water pollution from the re-use of organic matter (liquid manure, compost, sewage sludge, etc.) in agriculture.
The approach taken is on a variety of scales and consists of:
(i) in the laboratory: in soil columns, ongoing monitoring, under perfectly controlled conditions, of water and solute fluxes after organic waste inputs;
(ii) on the experimental plot: consideration of the specific behaviour of pollutants in soils and their interactions within the larger biogeochemical cycles;
(iii) at the regional level: characterization, mapping and ranking of soil properties that influence the transfer of potentially polluting elements from soil to watertables.
By 2025 in sub-Saharan Africa, nearly half of the population will live in large cities (Cohen, 2004; United Nations, 2006). In these urban and peri-urban contexts, agricultural development responds primarily to needs for food security and diversification of food resources while creating jobs and wealth (Ellis and Sumberg, 1998; De Bon et al., 2010). To intensify these productions in a context of increasing mineral fertilizer prices, the use of organic materials of agricultural (compost, manure, etc.), agro-industrial (slaughterhouse waste, etc.) or urban (sewage sludge, household waste, etc.) origin is frequent but their use is difficult to reason (Cofie et al., 2006 N'Dienor, 2006). Indeed, the optimal valorization in agriculture of these organic inputs will always be very dependent on the knowledge of their compositions, their properties and the soil-plant system within which they are valorized. If organic matter use practices increase without being properly controlled, these intensified peri-urban systems can lead to :
- health risks mainly on market gardening, in the very short term when products are consumed (theme to be developed in collaboration);
- the risks of soil and water resource pollution, in the short and long term, which are highly dependent on the environment (soil, subsoil) and the organic products brought in.
The increase in quantity and frequency of organic matter inputs requires improved knowledge on their transformations and their medium and long term impacts in this specific context of peri-urban tropical soils. Indeed, the mineralization of organic matter induces major changes in the physical, chemical and biological conditions of the soil-plant system. These inputs of organic matter are potentially a source of risk (major pollutants and traces, microbiological); they can also modify the speciation and behaviour of pollutants already present. To assess long-term impacts, the dynamics of organic matter must be studied together with those of potentially polluting elements (N, P, K, trace metals, etc.).
The first objective was to identify agro-pedological risk situations and associated pollution. For this, we have relied on the bibliography, the work carried out within the framework of the Isard project on Dakar and local expertise. We have supplemented this information with additional analyses on an ad hoc basis. The second objective is to reproduce these systems under controlled laboratory conditions and to couple different study scales. In batch mode, the properties of organic matter brought to the soil are assessed using biochemical fractionation and spectroscopic characterization techniques. The concentrations of pollutants, such as trace metals, and their speciation are measured in the various compartments. To complete these "structural" approaches, a more "functional" approach is realized with polarographic techniques or DGT (Diffusive Gradient in Thin films). In parallel to these approaches under static conditions, we follow, under dynamic water flow conditions, the mineralization of organic matter and the description of trace metals speciation by experiments on the scale of soil columns. The third objective is to have a sufficient database on system reactivity to feed biogeochemical simulation models in order to evaluate long-term impacts.
The main context is the island of Réunion, a young volcanic island in the tropics, with marked relief and, within a small area, a very wide variety of soils (andosols, podzols, nitisols, phaeozems, umbrisols, vertisols, etc.), climates (average annual precipitation ranging from 500 to 10,000 mm) and crops (sugarcane, market gardening, etc.). The re-use in agriculture of organic waste derived from agriculture (liquid manure, compost…), agro-industrial processes (vinasse from rum distilleries) or cities (sewage sludge) is a major issue—essentially an environmental issue, but also a socioeconomic one. Rapid population growth over the next twenty years will tend to promote urbanization, to the detriment of agricultural land.
Master's courses: M1 and M2 GEDAH ("Gestion durable des agro-écosystèmes horticoles", coordinator Karamoko Diarra), AFECA ("Agroforesterie, Ecologie et Adaptation", coordinator Sékouna Diatta) and BIOVEM ("Biotechnologies Végétales et Microbiennes", coordinator Ibrahima NDOYE) at UCAD (université Cheick Anta Diop of Dakar). Undergraduate soil science courses and master's courses in plant chemistry and biology in the tropical environment at the Université de la Réunion.
Reception and supervision of trainees and VCATs: agronomy schools, research and professional Master's in "soil/aquifer transfer" (Avignon), in soil science, in environmental impact studies, in "environment, technology and society", etc.
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