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International audience; Many forest soils are acidic and have very low plant-available pools of magnesium. Past and present sylvicultural, nutritional and/or climatic pressures endured by forest ecosystems can result in net losses of nutrients and ecosystem function losses. Liming with a carbonate product is an alternative to counteract these degradations but the effects of liming on the biogeochemical cycling of nutrients over time and the dynamics of Mg released from liming products are still unclear. We studied the Mg isotopes composition in four paired-treatment experimental beech forest ecosystems in northern France. At the sites where dolomitic lime was applied, the variation in exchangeable and foliar δ26Mg demonstrated the direct contribution of dolomite-derived Mg to the replenishment of topsoil exchangeable pools and to tree nutrition improvement: dolomite-derived Mg was incorporated into the biological cycling which allows its retention on the mid to long term in the soil–plant system. At the sites limed with calcium carbonate, the changes in exchangeable and foliar Mg contents and δ26Mg observed on the long term suggest that the applied product contained a small amount of Mg and/or that Mg cycling changed after liming, to cope in particular with the low Mg availability. Lastly, our results highlight the high δ26Mg of the organic layer (humus): fractionation processes occurring within this layer (mineralization/ageing of organic matter, preferential retention of 26 Mg) could explain these singular signatures that could greatly influence the topsoil Mg exchangeable pools.

The quality of the plant production in periurban agriculture is subjected to question given the potential contamination of soils that can affect the crops. The levels of contamination of soils and vegetables by Metallic trace elements (MTE) as copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb) in the district of Abidjan, have been evaluated. Multi-sites survey of cropping areas of sweet potato and Hibiscus was conducted in three municipalities in Abidjan (Port-Bouët, Yopougon and Bingerville) in relation to the intensity of industrial and commercial activities. The site of Bingerville has been used as the reference site referring to the low activities. Soil samples (in 0-20 cm and 20 - 40 cm), combined with that of plants (leaf, stem, and root), and water were collected, transported in laboratory for analysis. The total amounts of MTE in soil, as well as the different fractions extracted were determined in addition to the respective concentration in plants (Hibiscus and Sweet potato). Toxic level of Pb (< 8 mgkg-1) was observed in the plant organs collected at Port-Bouët site indifferently to crops while lowest content of Pb (35.5 mgkg-1) was accounting for the soil of Yopougon (39.8 mgkg-1). A neutral pH of the soil has been considered more favorable to the contamination of plants in Pb on the polluted sites somewhat differing for extractable fractions. The acidity and small width of leaf as observed for Hibiscus, were identified as the control factors of crop contamination in periurban agroecosystems prone to Pb pollution. To strengthen the consistency of the knowledge, studies of the interaction between Pb and Zn as well as the translocation of Pb in the plants to tubers are suggested in the tropical ecosystems.

International audience; Inadequate urban waste management in Côte d’Ivoire is the heavy metal source in soils used to produce vegetables. This represents a potential risk of food contamination. In order to find an efficient technique for soils depollution before their use in agriculture, an experimental study had demonstrated the ability of species of Acacia (mangium, auriculiformis and crassicarpa) to extract heavy metals from landfill soils. This work was undertaken to determine rhizosphere processes controlling Acacia growth in polluted soil. An experiment was carried out in hydroponic culture and on thin layer of landfill polluted soil during 3 months. During this experiment, growth parameters, soil chemical properties, heavy metal contents in soil (total and various fractions) and in plants were measured at the end of experiment. Results show that plant growth is related to the development of strategies in root zone. Plants released protons in their rhizosphere and lowering the redox potential, especially under Acacia crassicarpa, which stored the most level of heavy metal in in biomass. The other species produce organic acids allowing plants to grow with high biomass in soil with high content of heavy metal. Acacia crassicarpa is therefore the species better suited for depollution of landfill soil.

This study addresses the different biogeochemical parameters that control the dynamics of Hg, which is a less-studied metal in the Ebrie Lagoon. During two hydrological seasons, the dry season and the rainy season, we regularly sampled and analysed various compartments (e.g. sediments and fishes (Tilapia sp.)) of the lagoon. Thus, the physicochemical parameters were measured in situ (e.g. temperature, pH, salinity, redox potential and dissolved oxygen, total dissolved organic carbon, nitrates and sulphates), and the microbiological parameters (e.g. cultivable cells, total enzymatic activity and catabolic activity) were measured to establish the seasonal variations in the links between Hg and biogeochemical parameters through multivariate statistical analyses. The bioavailability of Hg from an unpolluted site was studied by comparing the ratios of fish and sediment. The results indicated that the seasons influenced the different biogeochemical factors, although for some factors, the variations were not significant. This influence was more pronounced in the dry season than in the rainy season. The impact of microbial activities and organic matter on Hg dynamics was observed in all seasons. However, other factors, such as pH, temperature, salinity, Eh and sulphates, influenced the dynamics of Hg only in the dry season.

Symbiotic ectomycorrhizal fungi mobilise nutrients from both organic and inorganic substrates and supply them to their host plants. Their role in mobilising base cations and phosphorus from mineral substrates through weathering has received increasing attention in recent years but the processes involved remain to be elucidated. We grew selected ectomycorrhizal and nonmycorrhizal fungi in axenic systems containing mineral and organic substrates and examined their capacity to fractionate and assimilate stable isotopes of magnesium. The mycorrhizal fungi were significantly depleted in heavy isotopes with the lowest Δ26Mg values (the difference between δ26Mg in fungal tissue and δ26Mg in the substrate) compared with nonmycorrhizal fungi, when grown on mineral substrates containing granite particles. The ectomycorrhizal fungi accumulated significantly higher concentrations of Mg, K and P than the nonmycorrhizal fungi. There was a highly significant statistical relationship between δ26Mg tissue signature and mycelial concentration of Mg, with a clear separation between most ectomycorrhizal fungi and the nonmycorrhizal fungi. These results are consistent with the idea that ectomycorrhizal fungi have evolved efficient mechanisms to mobilise, transport and store Mg within their mycelia. This article is protected by copyright. All rights reserved.

Dissolved organic carbon (DOC), especially low molecular mass organic acids (LMMOAs) derives principally from biota degradation process in which soil microorganisms are the main actors and from roots exudates. The presence of LMMOAs led to an increase of availability and mobility of metallic elements through the formation of organo-metallic complex. In tropical soils, very few information about LMMOAs quantification and their role in the biogeochemical process related to trace metals cycling was available. Quantification of LMMOAs is limited due to their low concentration and rapid degradation. Until now, the role of microbial activity as well as LMMOAs in the biogeochemical cycle of metallic elements in tropical soils has not been investigated. The present study was conducted to evaluate the effect of microbial activity and biomass on the availability and mobility of metallic elements (Fe, Al and Hg) in two tropical soils, Ferralsol and Acrisol. We also quantified LMMOAs contents in soil solutions and addressed to their role in the mobilization of metals. Utilization of Diffuse Gradient in Thin film (DGT) method permits to analyze bioavailable metal in both fractions: organically complexed and free metals. The results show that the quantity of Fe, Al and Hg labile were higher in Ferralsol than Acrisol soils. This was more accentuated for the 50 cm-depth of soils where the microbial activities and the organic carbon content were important. Concentration of LMMOAs of Ferralsol and Acrisol were lower in compare to coniferous and deciduous forest soils. Proportions of LMMOAs in DOC were very small at 10.5% and 6.85% in the Ferralsol and Acrisol soils, respectively. The mobilization of Fe, Al and Hg in Ferralsol and Acrisol soils appeared to vary depending on the soil physico-chemical characteristics (sorption capacities and metals content) and also on the microbial biomass and activity. Soil pH influences the acidity of the functional groups in organic molecules and consequently their speciation. In addition, low pH increase proton competition within acidic functional groups involved in coordinate bond. The content of CEC in Ferralsol is higher than Acrisol that is related to the high contents of clay and organic carbon. Low CEC content can result in a decrease of retain of the cationic trace metals. Low CEC content led to a decrease of the capacity of retaining of metallic elements in tropical soils in compare to temperate soils.

Phlogopite weathering experiments were carried out under aerobic conditions in a closed system to study Mg isotopic fractionation during the dissolution and processes involved in the presence of bacterial strains. Four different bacterial strains with different metabolisms were chosen. Biotic experiments were performed in batch reactors at 24 °C for 12 days. In parallel, abiotic phlogopite weathering experiments were also performed using organic and nitric acids. Citric and gluconic acids were used to model the effects of chelating agents produced by bacteria, and nitric acid was used to model the effects of acidifying agents. The results indicate that both decreases in pH and the production of metabolites during heterotrophic bacterial activities significantly accelerate the release of Mg and Si in solution. Also, at a given pH, the fraction of elements released in solution is greater in the presence of bacteria or citric acid compared to that released in the presence of nitric and gluconic acids. Magnesium isotopic analyses indicate that the solutions obtained using nitric acid and acidifying bacterial strains display, on average, δ26Mg values that are close to or slightly heavier (by 0.4‰) than those of fresh phlogopite (−1.2‰ ± 0.08). In contrast, in experiments performed with citric acid, the Mg leached into solution is slightly enriched in light isotopes by −0.3‰ relative to the initial phlogopite, while its binding to organics is expected to be related to its preferential enrichment in heavy isotopes. Despite its small range of variations, the δ26Mg values of solutions vary inversely with pH, thus suggesting that secondary phases preferentially enriched in 26Mg at the highest pH values may play a key role.

International audience; This study investigates the potential of Mg isotopes as tracers of biogeochemical processes in a small-forested catchment located on sandstones extremely poor in Mg-bearing minerals. The average delta Mg-26 is -0.63 +/- 0.12 parts per thousand and 0 +/- 0.14 parts per thousand for local rainwater and bedrock, respectively. From the C horizon to the upper eluvial (E) horizon, soil delta Mg-26 (from 0.0 +/- 0.14 parts per thousand to 0.25 +/- 0.14 parts per thousand) is close to the bedrock value, while more than 70% of Mg is lost, suggesting a small isotopic shift during illite dissolution. The surface soil horizon (A(h)) delta Mg-26 is close to plant delta Mg-26, and especially to the grass delta Mg-26 value (-0.49 parts per thousand). The bulk delta Mg-26 of trees and grass (-0.32 parts per thousand and -0.41 parts per thousand, respectively) are higher than the average delta Mg-26 values of the soil exchangeable fraction (-0.92 parts per thousand to -0.42 parts per thousand), and of rainwater (-0.65 parts per thousand). Within plants, roots are enriched in heavy isotopes, whereas light isotopes are preferentially translocated and stored in the above ground parts. In Norway spruce, the older needles, forming the annual litterfall, are isotopically lighter and strongly depleted in Mg compared to more recent needles. Soil solution delta Mg-26 shifts seasonally, from low values, lower than rainwater and close to litterfall during a high rainfall period in spring, to higher values, close to soil delta Mg-26 in dryer periods of winter or summer. At the watershed scale, streamwater delta Mg-26 varies between -0.85 perpendicular to 0.14 parts per thousand and -0.08 perpendicular to 0.14 parts per thousand and delta Mg-26 values decrease linearly with discharge. The high streamwater delta Mg-26 at low flow, close to bedrock delta Mg-26, most likely reflects dissolution processes in the deep saprolite in relation to the very long water residence time. Conversely, we suggest that low stream level delta Mg-26 values are at least partly related to the contribution of surface flows from wet areas. Using a simple mass and isotopic balance approach, we compute that mineral dissolution rates in the soil (0.35 kg Mg ha(-1) year(-1)) presently compensate for Mg losses from the soil. (C) 2012 Elsevier Ltd. All rights reserved.

This study presents a chemical protocol for the separation of Mg that is particularly adapted to alkali-rich samples (granite, soil, plants). This protocol was based on a combination of two pre-existing methods: transition metals were first removed from the sample using an AG-MP1 anion-exchange resin, followed by the separation of alkalis (Na, K) and bivalent cations (Ca2+, Mn2+ and Sr2+) using a AG50W-X12 cation-exchange resin. This procedure allowed Mg recovery of ∼ 10 0 ± 8%. The [Σcations]/[Mg] molar ratios in all of the final Mg fractions were lower than 0.05. The Mg isotope ratios of eleven reference materials were analysed using two different MC-ICP-MS instruments (Isoprobe and Nu Plasma). The long-term reproducibility, assessed by repeated measurements of Mg standard solutions and natural reference materials, was 0.14‰. The basalt (BE-N), limestone (Cal-S) and seawater (BCR-403) reference materials analysed in this study yielded δ26Mg mean values of −0.28 ± 0.08‰, −4.37 ± 0.11‰ and −0.89 ± 0.10‰ respectively, in agreement with published data. The two continental rocks analysed, diorite (DR-N) and granite (GA), yielded δ26Mg mean values of −0.50 ± 0.08‰ and −0.75 ± 0.14‰, respectively. The weathering products, soil (TILL-1) and river water (NIST SRM 1640), gave δ26Mg values of −0.40 ± 0.07‰ and −1.27 ± 0.14‰, respectively. We also present, for the first time, the Mg isotope composition of bulk plant and organic matter. Rye flour (BCR-381), sea lettuce (Ulva lactuva) (BCR-279), natural hairgrass (Deschampsia flexuosa) and lichen (BCR-482) reference materials gave δ26Mg values of −1.10 ± 0.14‰, −0.90 ± 0.19‰, −0.50 ± 0.22‰ and −1.15 ± 0.27‰ respectively. Plant δ26Mg values fell within the range defined by published data for chlorophylls.

N-nitrogen is the most limiting element in rice-growing on lowland soils in the central part of Côte d'Ivoire. In addition to its direct effect, poor management of P and K fertilizers in basic manure can induce low yields and losses of currencies. However, inappropriate use of these nutrients (P and K) in these shallows can induce other adverse effects on crops and even on the environment. So, for the eficient management of lowland ricegrowing fertiliszation, three agronomic trials have been set up in the Central part of Côte d'Ivoire to determine the optimum P and K doses to improve fertilizer efficiency in the relevant agroecology. Three rates of PCa(H2PO4)2H2O [30, 60 and 90 kgPha-1 ] as well as three of K-KCl [25, 50 and 75kgKha-1 ] and their recommended rates (13kgPha-1 and 25kgKha-1 ) in the humid forest zone were the treatments. A total of 80kgNha-1 (urea) was applied in three splits to each of the micro-plots except in the control including no fertilizer. The rice variety named NERICA L19 was transplanted.An agronomic trial including eleven (11) treatments in three replications was laid out in a complete randomized blocks design The highest K recovery rate is obtained with the 25kgKha -1 rates, while the lowest rate is with 50kgKha-1 in the three trials. There is a gradual decrease in P-level recovery as rates increase, regardless of the test. Increased rates of P and K induce a decrease in agronomic efficiency. The partial productivity factor (PFP) of nutrients P and K decreases with increasing rates for all three trials. The highest grains and straw yields are obtained for 75kgKha-1 doses regardless of the dose of P. However, it should be noted that the different efficiencies obtained are better for the lower rates of P and K

Two higher plant species (rye grass and clover) were cultivated under laboratory conditions on two substrates (solution, phlogopite) in order to constrain the corresponding Mg isotope fractionations during plant growth and Mg uptake. We show that bulk plants are systematically enriched in heavy isotopes relative to their nutrient source. The Δ26Mgplant-source range from 0.72‰ to 0.26‰ for rye grass and from 1.05‰ to 0.41‰ for clover. Plants grown on phlogopite display Mg isotope signatures (relative to the Mg source) ∼0.3‰ lower than hydroponic plants. For a given substrate, rye grass display lower δ26Mg (by ∼0.3‰) relative to clover. Magnesium desorbed from rye grass roots display a δ26Mg greater than the nutrient solution. Adsorption experiments on dead and living rye grass roots also indicate a significant enrichment in heavy isotopes of the Mg adsorbed on the root surface. Our results indicate that the key processes responsible for heavy isotope enrichment in plants are located at the root level. Both species also exhibit an enrichment in light isotopes from roots to shoots (Δ26Mgleaf-root = −0.65‰ and −0.34‰ for rye grass and clover grown on phlogopite respectively, and Δ26Mgleaf-root of −0.06‰ and −0.22‰ for the same species grown hydroponically). This heavy isotope depletion in leaves can be explained by biological processes that affect leaves and roots differently: (1) organo–Mg complex (including chlorophyll) formation, and (2) Mg transport within plant. For both species, a positive correlation between δ26Mg and K/Mg was observed among the various organs. This correlation is consistent with the link between K and Mg internal cycles, as well as with formation of organo–magnesium compounds associated with enrichment in heavy isotopes. Considering our results together with the published range for δ26Mg of natural plants and rivers, we estimate that a significant change in continental vegetation would induce a change of the mean river δ26Mg that is comparable to analytical uncertainties.