The behavior of nickel isotopes at the biogeochemical interface between ultramafic soils and Ni accumulator species

International audienceUltramafic derived soils are characterized by low nutrient soils, a low Ca:Mg ratio, and high metal contents such as Ni, Co and Cr. The vegetation growing on these soils is highly adapted and includes both Ni hyperaccumulator and accumulator species. Today, approximately 530 Ni hyperaccumulator species are listed worldwide and the Ni concentration can be extremely high, e.g. up to 25% in latex from Pycnandra acuminata (Sapotaceae), a tree found in New Caledonia. The aim of this study is to identify the potential role of Ni hyperaccumulator plants in the Ni biogeochemical cycle at the soil surface by using Ni isotopes. A set of Ni hyperaccumulator and Ni accumulator plants as well as topsoils were sampled on the Barro Alto and Niquelândia ultramafic complexes (Goiás State, Brazil). Three Ni hyperaccumulator plants were collected: Justicia lanstyakii, Heliotropium aff. salicoides, Cnidoscolus aff. urens, as well as one Ni accumulator plant, Manihot sp. The isotopic compositions of the whole plants were determined and compared to those of the bulk topsoils and DTPA-extractable Ni. The topsoils exhibited δ60Ni values ranging from −0.30 ± 0.06‰ to 0.16 ± 0.05‰. The DTPA-extractable Ni in the topsoils ranged from 94 to 623 mg kg−1, i.e. 0.9–4.9% of the total soil Ni and was found to be isotopically heavier than the corresponding topsoil (from −0.30 ± 0.05‰ to 0.34 ± 0.08‰). The δ60Ni values for the Ni accumulator plants showed an enrichment in heavy Ni isotopes in the aerial parts of the plant compared to the roots, whereas similar δ60Ni values for the roots, stems and aerial parts suggested that no significant fractionation results from Ni uptake and translocation in Ni hyperaccumulator plants. Moreover, the aerial parts (i.e. leaves and flowers) from all of the plants analyzed showed the highest Ni concentrations and the heaviest δ60Ni values up to 1.21 ± 0.05‰. The enrichment in heavy Ni isotopes in the leaves (0.09 ± 0.06‰ < Δ60Nileaves-soil < 1.06 ± 0.03‰) may result in a heavy Ni input in the litter during organic matter restitution. There is a non-negligible amount of Ni uptake by Ni accumulator and Ni hyperaccumulator plants and this may modify both the Ni isotope composition at the soil-plant interface and the overall cycle of Ni in surface soil

Nickel mass balance and isotopic records in a serpentinic weathering profile: Implications on the continental Ni budget

International audienceDuring serpentinite rock weathering, Ni is concentrated in the regolith owing to residual and secondary enrichment, forming a Ni-rich deposit under tropical conditions. This study presents geochemical, isotopic, and mineralogical data for Ni from the Serra do Puma Complex (SPC) weathering profile in Carajas (Brazil). The Ni fluxes, redistribution, and Ni isotopic fractionation magnitudes were quantified along the entire weathering profile: rock, saprock, lower and upper saprolite, and limonite. The results show that chlorite and serpentine are the primary Ni-scavenging phases in saprolite, whereas Fe oxyhydroxides are the main Ni-hosting minerals in the limonite unit. The mass balance model confirmed a global Ni gain at the weathered profile scale, with larger Ni enrichment in the upper saprolite. The isotopic dataset obtained in this study contributes greatly to the knowledge of the current Ni cycle on regional and global scales. The detailed insights into Ni isotopes in the SPC, coupled with chemical and mineralogical composition, allow for the first time a Ni mass balance and Ni isotopic values for an entire weathering profile in an Amazonian context. The Ni isotopic profile agrees with the preferential retention of light Ni isotopes in the residual material relative to the parent rock during weathering processes, with the Δ60Ni limonite-saprock of −0.72‰. In the SPC, major isotopic fractionation was notably recorded during saprolitization, while limonitization was accompanied by an overall Ni loss without significant Ni isotopic fractionation. As already observed in previous studies, heavier Ni isotopes, preferentially leached during weathering processes, can be further mobilized downward and lost from the profile or incorporated in secondary minerals in the saprolite and limonite sections. The Ni isotopic mass balance model indicated that the Ni loss from the weathered serpentinite profile was isotopically heavy, in agreement with the isotopically heavier composition of the dissolved load of Amazonian rivers. The isotopically light Ni pool, associated with the significant Ni gain encountered in the upper saprolite, is notable in the SPC weathering profile and confirms the existence of a light Ni isotope reservoir in the continent

Detangling past and modern zinc anthropogenic source contributions in an urbanized coastal river by combining elemental, isotope and speciation approaches

International audienceThe accumulation of trace metals in the environmental compartments of coastal rivers is a global and complex environmental issue, requiring multiple tools to constrain the various anthropogenic sources and biogeochemical processes affecting the water quality of these environments. The Valao fluvio-estuarine system (Rio de Janeiro, Brazil) presents a challenging case of a coastal river contaminated by both modern and historical anthropogenic metal sources, located in the land and in the intra-estuary, continuously mixed by tidal cycles. This study employed a combination of spatial distribution analysis of trace metals including gadolinium (Gd), zinc (Zn) isotopic analyses, and X-ray absorption spectroscopy (XAS) to distinguish between these sources. The concentrations of metals in both dissolved (water samples) and surficial sediment compartments (Suspended Particulate Matter and sediment samples) display an overall enrichment trend from upstream to downstream. Multivariate statistical analysis allows to discriminate geogenic elements derived from watershed geology (Ti, K, and Mg) vs anthropogenic contaminants from urban runoff and domestic sewage discharges (Cu, Cr, Pb, Zn, and Gd); and legacy metal contaminants (Zn and Cd) remobilized from ancient metallurgical wastes and transported upstream in the estuary during tidal cycles. The anthropogenic Gd concentration in the dissolved compartment increases along the watercourse, highlighting continuous ongoing sewage discharge. Zinc solid speciation also indicates that Zn contribution from legacy metallurgy waste is primarily associated with sulfide-Zn and Zn-phyllosilicate in the outlet estuary, while in upstream sediments of fluvio-estuarine system, Zn is found bound to organic matter. Zinc isotope systematically reveals a progressive downstream shift to heavier isotope compositions. Upstream, the relatively pristine site and the urbanized section of the river exhibit a relatively uniform δ66/64Zn value (+0.20 ± 0.07 ‰) in suspended particulate matter (SPM) and surficial sediments. These results indicate that domestic sewage discharges contribute to Zn enrichment in sediments of the Valao fluvio-estuarine system but without modifying its isotope signature in sediments. The sediment of the downstream estuarine section shows a heavier δ66/64Zn value (+0.48 ± 0.08 ‰), indicating the strong influence of the intra-estuarine source identified as the historical metallurgic contamination. An integrated view of the geochemical tracers allows thus inferring that the untreated sewage and legacy metallurgical contamination are the primary sources of anthropogenic Zn contamination. It highlights the progressive mixing along the estuarine gradient under tidal dynamics. The influence of the former source continuously expands from the headland towards the estuary

L'exposition "Sols fertiles, vies secrètes" du Sénat, ses panneaux de sensibilisation, leurs déclinaisons, leur diffusion et leurs impacts

International audienceThis paper deals with an exhibition: “Fertile soils and secret lives” designed at the initiative of Senat to raise public awareness of the soil resources knowledge and preservation issues. Exhibited for the first time in the Orangeries of the Luxembourg gardens in 2014,its design and diffusion involved the collaboration of numerous organizations, among which the French Soil Science Society. This very diverse exhibition attracted a very large and diverse public, with an estimated 50,000 persons involved between September 2014 and December 2019. The pedagogic posters designed on this occasion were further enriched, thanks to a collaboration with the CESE (Economic, Social and Environmental Council), with complementary posters about the links between soils and climate change. Then, they have been made available by the senate to AFES and declined in various formats (roll-ups, posters…) and presented in very numerous events (scientific congress of soil science, World Soil Day in France, open doors of agricultural high schools, libraries, …) to a very diverse public including professionals, students, general public and policy-makers. In this paper, we describe their dissemination and impact. These posters are still, and will be again, used. They are very good means to discover soils and to raise awareness about the major issues and challenges to which they contribute.Cet article présente une exposition, "Sols fertiles, vies secrètes", conçue à l'initiative du Sénat pour sensibiliser le grand public aux enjeux de la connaissance et de la préservation de nos ressources en sol. Exposée pour la première fois dans l'Orangerie des jardins du Luxembourg en 2014, sa conception et sa diffusion a mis à contribution de nombreux organismes, dont l'Association Française pour l'Etude du Sol. Cette exposition d'une grande diversité a touché un public nombreux et très divers avec selon nos estimations plus de 50 000 personnes touchées entre septembre 2014 et décembre 2019. Des panneaux pédagogiques ont été réalisés à cette occasion puis ont ensuite été enrichis grâce à une collaboration avec le CESE par des panneaux complémentaires sur le lien entre le sol et le climat. Ils ont été mis à disposition par le Sénat à l'AFES pour être déclinés sous différentes formes (roll-ups, affiches...) et présentés en de très nombreuses occasions (colloques de sciences du sol, Journée Mondiales des Sols, portes ouvertes de lycées agricoles, bibliothèques, …), en touchant un public varié (professionnels, étudiants, grand public, élus…). Nous décrivons ici leur diffusion et leur impact. Ces panneaux continuent, et continueront, d'être utilisés et constituent un excellent moyen de découverte des sols et de sensibilisation aux grands enjeux auxquels ils contribuent

Communiquer et sensibiliser le grand public aux sols

Cet article présente une exposition, "Sols fertiles, vies secrètes", conçue à l'initiative du Sénat pour sensibiliser le grand public aux enjeux de la connaissance et de la préservation de nos ressources en sol. Exposée pour la première fois dans l'Orangerie des jardins du Luxembourg en 2014, sa conception et sa diffusion a mis à contribution de nombreux organismes, dont l'Association Française pour l'Etude du Sol. Cette exposition d'une grande diversité a touché un public nombreux et très divers avec selon nos estimations plus de 50 000 personnes touchées entre septembre 2014 et décembre 2019. Des panneaux pédagogiques ont été réalisés à cette occasion puis ont ensuite été enrichis grâce à une collaboration avec le CESE par des panneaux complémentaires sur le lien entre le sol et le climat. Ils ont été mis à disposition par le Sénat à l'AFES pour être déclinés sous différentes formes (roll-ups, affiches...) et présentés en de très nombreuses occasions (colloques de sciences du sol, Journée Mondiales des Sols, portes ouvertes de lycées agricoles, bibliothèques, &#8230;), en touchant un public varié (professionnels, étudiants, grand public, élus&#8230;). Nous décrivons ici leur diffusion et leur impact. Ces panneaux continuent, et continueront, d'être utilisés et constituent un excellent moyen de découverte des sols et de sensibilisation aux grands enjeux auxquels ils contribuent

Soil sampling and preparation for monitoring soil carbon

There is an urgent need for standardized monitoring of existing soil organic carbon stocks in order to accurately quantify potential negative or positive feedbacks with climate change on carbon fluxes. Given the uncertainty of flux measurements at the ecosystem scale, obtaining precise estimates of changes in soil organic carbon stocks is essential to provide an independent assessment of long-Term net ecosystem carbon exchange. Here we describe the standard procedure to monitor the soil organic carbon stocks within the footprint of an eddy covariance flux tower, as applied at ecosystem stations of the Integrated Carbon Observation System. The objectives are i) to ensure comparability between sites and to be able to draw general conclusions from the results obtained across many ecosystems and ii) to optimize the sampling design in order to be able to prove changes in time using a reduced number of samples. When sampling a given site at two periods, the objective is generally to assess if changes occurred in time. The changes that can be detected (i.e., demonstrated as statistically significant) depend on several parameters such as the number of samples, the spatial sampling design, and the inherent within-site soil variability. Depending on these parameters, one can define the 'minimum detectable change' which is the minimum value of changed that can be statistically proved. Using simulation studies, we address the trade-off between increasing the number of samples and getting lower minimum detectable changes of soil organic carbon stocks

Mapping black carbon content in topsoils of central France

International audienceBlack Carbon (BC) is an important carbon pool due to its relative stability in soil. Thus, it is essential to determine the amount of BC in soil to have a better understanding of the global carbon cycle. The spatial distribution of BC was determined in the central region of France in relation to the main controlling factors. BC was measured for topsoil at 158 sites in the French soil monitoring network on a regular 16 × 16-km grid. A linear mixed model (LMM) which included fixed effects (linear relationships between BC content and covariates) and spatially correlated random effects was used for mapping BC to aid explanation. Covariates were selected from a set of factors linked to the BC cycle using the Akaike Information Criterion (AIC). The results show high variability in BC content with a minimum of 0.9%, a maximum of 32% and an average of 5.3% for total organic carbon. The fine-earth fraction and clay content gave the best statistical explanation for the spatial distribution of BC. Data on these covariates were not available in total for the whole study area, and therefore we reselected covariates using the fine-earth amount and density of fires from burning crop residues