Caractérisation des risques de contamination des agrosystèmes périurbains de Dakar par les éléments traces métalliques

L’application de produits résiduaires organiques (PRO) améliore la fertilité des sols, mais induit des effets négatifs sur les agrosystèmes, en raison de la présence de contaminants tels que les éléments traces métalliques (ETM). Cette étude vise à évaluer l’influence de deux doses (20 et 60 T.ha-1) d’apport de boue de station d’épuration STEP et de fiente de volaille sur la disponibilité de quatre ETM (Ni, Cd, Cr et Pb) dans un arenosol et un fluvisol. Une incubation de 88 jours en conditions contrôlées a été conduite sur les deux sols mélangés aux différentes doses de boue et de fiente. La disponibilité des ETM dans les sols incubés a été évaluée par la technique DGT (Diffusive Gradient in Thin films). Les boues ont abaissé le pH des deux sols tandis que les fientes ont alcalinisé l’arenosol durant les 14 premiers jours d’incubation. Ces PRO ont augmenté les concentrations de Ni, Cd, Pb et Cr disponibles dans les sols. Les  disponibilités du Ni, Cd et Pb sont plus élevées dans l’arenosol que dans le fluvisol. La boue à 60 T.ha-1 a induit les plus fortes disponibilités en Cd et Pb tandis que la fiente à 60 T.ha-1 a induit les plus fortes disponibilités en Ni et Cr dans les deux sols.Mots clés : Eléments traces métalliques, arenosol, fluvisol, boue d’épuration, fiente.  CHARACTERIZATION OF CONTAMINATION RISKS OF THE  SUBURBANAGROECOSYSTEMS OF DAKAR BY TRACE ELEMENTSOrganic wastes (OW) spreading improve soil fertility, but can induce negative effects on agroecosystems, due to the presence of contaminants such as trace elements (TE). This study aims to evaluate the impacts of two doses (20 and 60 T.ha-1) of sewage sludge and poultry manure on the availability of four trace elements (Ni, Cd, Cr and Pb) in arenosol and  fluvisol. Soils mixed with different doses of sewage sludge and poultry manure were incubated for 88 days under controlled conditions. The  availability of trace elements in incubated soils was evaluated by the DGT (Diffusive Gradient in Thin films) technique. Sewage sludge has decreased the pH of both soils while poultry manure induced an alkalinization of arenosol during the first 14 days of incubation. OW increased Ni, Cd, Pb and Cr availability in the soils. Availability of Ni, Cd and Pb were higher in arenosol than in the fluvisol. Sewage sludge at 60 T.ha-1 induced the highest availability of Cd and Pb while the poultry manure at 60 T.ha-1 induced the highest availability of Ni and Cr in both soils.Keywords : Trace elements, arenosol, fluvisol, sewage sludge, poultry manure

Carbon storage and DNA absorption in allophanic soils and paleosols

Andisols and andic paleosols dominated by the nanocrystalline mineral allophane sequester large amounts of carbon (C), attributable mainly to its chemical bonding with charged hydroxyl groups on the surface of allophane together with its physical protection in nanopores within and between allophane nanoaggregates. C near-edge X-ray absorption fine structure (NEXAFS) spectra for a New Zealand Andisol (Tirau series) showed that the organic matter (OM) mainly comprises quinonic, aromatic, aliphatic, and carboxylic C. In different buried horizons from several other Andisols, C contents varied but the C species were similar, attributable to pedogenic processes operating during developmental upbuilding, downward leaching, or both. The presence of OM in natural allophanic soils weakened the adsorption of DNA on clay; an adsorption isotherm experiment involving humic acid (HA) showed that HA-free synthetic allophane adsorbed seven times more DNA than HA-rich synthetic allophane. Phosphorus X-ray absorption near-edge structure (XANES) spectra for salmonsperm DNA and DNA-clay complexes indicated that DNA was bound to the allophane clay through the phosphate group, but it is not clear if DNA was chemically bound to the surface of the allophane or to OM, or both. We plan more experiments to investigate interactions among DNA, allophane (natural and synthetic), and OM. Because DNA shows a high affinity to allophane, we are studying the potential to reconstruct late Quaternary palaeoenvironments by attempting to extract and characterise ancient DNA from allophanic paleosol

Orbital forcing of glacial/interglacial variations in chemical weathering and silicon cycling within the upper White Nile basin, East Africa: Stable-isotope and biomarker evidence from Lakes Victoria and Edward

On Quaternary time scales, the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae (notably diatoms, for which Si is an essential nutrient). Large tropical river systems dominate the export of Si from the continents to the oceans. Here, we investigate variations in Si cycling in the upper White Nile basin over the last 15 ka, using sediment cores from Lakes Victoria and Edward. Coupled measurements of stable O and Si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and Si cycling, while the abundances of lipid biomarkers characteristic of terrestrial/emergent higher plants, submerged/floating aquatic macrophytes and freshwater algae document past ecosystem changes. During the late-glacial to mid-Holocene, 15–5.5 ka BP, orbital forcing greatly enhanced monsoon rainfall, forest cover and chemical weathering. Riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower Si-isotope values. Since 5.5 ka BP, increasingly dry climates and more open vegetation, reinforced by the spread of agricultural cropland over the last 3–4 ka, have reduced dissolved silica inputs into the lakes. Centennial-to millennial-scale dry episodes are also evident in the isotopic records and merit further investigation

Using a combined stable isotope -speciation approach to understand the impact of long-term spreading of organic effluents on agricultural soils

International audienceRecycling of organic waste (OW) as fertilizers on farmlands is a common practice as it provides a cost effective and sustainable way of managing OW. However, it represents a major source of contaminants such as copper (Cu) and zinc (Zn) that may pose potentially negative environmental impacts (Lavado et al., 2005). To limit the environmental impact of OW spreading on farmlands, agronomical doses are calculated based on the nitrogen, phosphorus and potassium requirements of crops. Consequently, lower quantities of Cu and Zn are introduced in soil following OW application (López-Rayo et al., 2016). However, the long-term impacts of OW spreading at the set agronomical doses on the accumulation and fate of Cu and Zn in soils are scant. This study was designed to fill this gap by using a combination of Cu and Zn speciation characterization and isotopic compositions in OW and soils. We studied four soils with contrasting physicochemical properties (luvisols, nitisols, calcisols and arenosols) and the OW used in four long-term field experiments. We observed, in agreement with published literature, that in OW, Cu and Zn speciation is affected by the OW treatment (anaerobic digestion and composting) and by the physicochemical conditions during storage. In the studied OW samples, Cu and Zn occurred mainly or entirely as sulfides in raw and anaerobically digested pig slurries whereas only oxidized species were present in composted household wastes (Zn-phosphate, Cu bound to organic matter). In two field experiments selected for this study, little or no change in Cu and Zn speciation was observed between control and amended soil due to OW application at agronomical doses. Thus, the interest in analyzing the isotopic signatures to investigate whether Cu and Zn originating from OW application can be traced in such cases

Using a combined stable isotope -speciation approach to understand the impact of long-term spreading of organic effluents on agricultural soils

Recycling of organic waste (OW) as fertilizers on farmlands is a common practice as it provides a cost effective and sustainable way of managing OW. However, it represents a major source of contaminants such as copper (Cu) and zinc (Zn) that may pose potentially negative environmental impacts (Lavado et al., 2005). To limit the environmental impact of OW spreading on farmlands, agronomical doses are calculated based on the nitrogen, phosphorus and potassium requirements of crops. Consequently, lower quantities of Cu and Zn are introduced in soil following OW application (López-Rayo et al., 2016). However, the long-term impacts of OW spreading at the set agronomical doses on the accumulation and fate of Cu and Zn in soils are scant. This study was designed to fill this gap by using a combination of Cu and Zn speciation characterization and isotopic compositions in OW and soils. We studied four soils with contrasting physicochemical properties (luvisols, nitisols, calcisols and arenosols) and the OW used in four long-term field experiments. We observed, in agreement with published literature, that in OW, Cu and Zn speciation is affected by the OW treatment (anaerobic digestion and composting) and by the physicochemical conditions during storage. In the studied OW samples, Cu and Zn occurred mainly or entirely as sulfides in raw and anaerobically digested pig slurries whereas only oxidized species were present in composted household wastes (Zn-phosphate, Cu bound to organic matter). In two field experiments selected for this study, little or no change in Cu and Zn speciation was observed between control and amended soil due to OW application at agronomical doses. Thus, the interest in analyzing the isotopic signatures to investigate whether Cu and Zn originating from OW application can be traced in such cases

Using a combined stable isotope -speciation approach to understand the impact of long-term spreading of organic effluents on agricultural soils

Recycling of organic waste (OW) as fertilizers on farmlands is a common practice as it provides a cost effective and sustainable way of managing OW. However, it represents a major source of contaminants such as copper (Cu) and zinc (Zn) that may pose potentially negative environmental impacts (Lavado et al., 2005). To limit the environmental impact of OW spreading on farmlands, agronomical doses are calculated based on the nitrogen, phosphorus and potassium requirements of crops. Consequently, lower quantities of Cu and Zn are introduced in soil following OW application (López-Rayo et al., 2016). However, the long-term impacts of OW spreading at the set agronomical doses on the accumulation and fate of Cu and Zn in soils are scant. This study was designed to fill this gap by using a combination of Cu and Zn speciation characterization and isotopic compositions in OW and soils. We studied four soils with contrasting physicochemical properties (luvisols, nitisols, calcisols and arenosols) and the OW used in four long-term field experiments. We observed, in agreement with published literature, that in OW, Cu and Zn speciation is affected by the OW treatment (anaerobic digestion and composting) and by the physicochemical conditions during storage. In the studied OW samples, Cu and Zn occurred mainly or entirely as sulfides in raw and anaerobically digested pig slurries whereas only oxidized species were present in composted household wastes (Zn-phosphate, Cu bound to organic matter). In two field experiments selected for this study, little or no change in Cu and Zn speciation was observed between control and amended soil due to OW application at agronomical doses. Thus, the interest in analyzing the isotopic signatures to investigate whether Cu and Zn originating from OW application can be traced in such cases