Fougerite a Natural Layered Double Hydroxide in Gley Soil: Habitus, Structure, and Some Properties

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Geochemical Methods to Assess Agriculture Sustainability

Facing global changes and the challenge of food security, scientists are being questioned by decision-makers and stakeholders on the sustainability of agro-systems. The main difficulty in dealing with this question is to obtain enough data over long periods of time. Monitoring slow drifts and weak noises is needed to forecast tipping points that can jeopardize the present steady state. High-resolution datations by radiocarbon coupled with detailed palynological determinations in sediments, historical archives on yields and crop quality, and high-frequency field in situ measurements give information on climatic changes from multi-secular to seasonal and hourly time scales. In the long term, climatic forcing dominates agriculture performance, at that time only organic agriculture, with oscillations between prosperity and misery driven by climate and intermediated by civilization flourishing and collapsing; in the medium term, in modern agriculture, irrigation provides a provisional buffering effect on yield and crop quality despite present warming; in the short term, either under non-fertilized forested ecosystem or intensive rice cropping, the same patterns are evidenced and point to the importance of soil microflora shifting from aerobiosis to anaerobiosis. In all cases, geochemistry offers appropriate tools to decipher the climate-soil-agriculture complex interplay

Kinetic Citrate-Bicarbonate Extraction of Green Rust Fougerite and Mineralogical Controls by XRD and Mössbauer Spectroscopy

Fougerite (IMA 2003-057) is a layered double hydroxysalt, the natural mineral of green rusts (GRs) and has been observed as a major iron phase in hydromorphic soils and several reductic environments. Its general formula is [Fe1−x 2+ Mgy 2+ Fex 3+(OH)2+2 y ]x+[(x)OH−,mH2O]x− where x=Fe3+/Fetot.. Natural GRs in the environment are difficult to study due to their reactivity and their small concentration. Chemical extractions with citrate-bicarbonate (CB), and mineralogical controls by XRD and Mössbauer spectroscopy (MS) were performed at different times of contact (0, 1, 6, 48, 168 and 504 h). This protocol was applied on soil samples taken in a reductic horizon at 80 cm depth of the Gleysol where fougerite was originally characterized. Kinetic extraction of Si by CB reached slowly 1.8% of absolute total concentration in the soil after 504 h. Al extracted by CB increased quickly to 9.4% until 6 h and then gradually until 48 h to reach 12% and stayed constant until 504 h. Fe and Mg increased quickly respectively to 23% and 56% after 6 h and then gradually and linearly to 97% and 69%. Kinetic extractions by CB reveal two distinct mineral compartments: - a first mineral form of Si-Al with Fe and Mg as minors; - then fougerite with a very nice congruence with Mg, showing that fougerite contains about 10% of Mg. After 168 h, CB extraction on the initial sample did not affect significantly any of the main peaks of the XRD patterns. MS on initial soil sample before CB treatment showed characteristic hyperfine interaction parameters of fougerite- GR, with x=Fe3+/Fetot. mole ratio of 0.43. After 6 and 48 h of CB extraction, x mole ratios were respectively 0.406 and 0.346. After 168 and 504 h of CB treatments, spectra did not show any peak. This disappearance of fougerite-GR, attested by MS, and the dynamics of iron extraction by CB in kinetics proved the selectivity of CB procedure to dissolve fougerite. (Texte intégral

Evaluation of streamwater composition changes in the Vosges Mountains (NE France): 1955–2005

n 1995, in the southwestern Vosges Mountains (NE France), 158 of 395 streams (40%) had a pH lower than 5.5 at baseflow. As elsewhere in Europe, acid deposition has decreased since the seventies, as has base cation deposition. In order to assess the response of streamwater to decreasing deposition, we compared their present chemical composition to their former composition. All comparisons showed a decrease in sulphate concentration, which was greater on granite than on sandstone. In addition calcium, magnesium and aluminium concentrations generally decreased. Acidity in streams draining granite decreased in spring, especially during the eighties, decreases were not observed on sandstone. Continuous monitoring of 5 streams since 1998 confirmed that Al concentrations decreased while changes in pH were small. Chemical trends in streams from the Vosges massif fell between those measured in Northern Europe and Central Europe. This study provides the first broad-scale overview of surface water acidification and recovery in France and emphasizes the need for continuous monitoring to assess long-term changes in aquatic ecosystems

Le phosphore dans l'environnement : bilan des connaissances sur les impacts, les transferts et la gestion environnementale

La question du phosphore est devenue depuis quelques années en France une question environnementale majeure. Cela tient à plusieurs évolutions : - le principe de l’équilibre de fertilisation présent dans la directive européenne "Nitrate" a été jusqu’ici principalement invoqué dans la règlementation française pour les notrates et peu pour le phosphore, alors que d’autres pays européens l’ont précocement introduit dans leur règlementation ; - le développement des phénomènes d’eutrophisation des eaux douces, avec en particulier des blooms de cyanobactéries, a contribué à attirer l’attention sur les flux et les concentrations de phosphore ; - le rôle du phosphore dans certains impacts environnementaux côtiers et en particulier dans certaines dystrophies phytoplanctoniques, même s’il est loin d’être aussi manifeste que dans le cas des eaux douces. Tout cela conduit à repositionner la question du phosphore au centre des préoccupations environnementales. Et il en résulte d’ores et déjà des évolutions dans différents documents règlementaires. Il nous a donc semblé important d’organiser un séminaire sur cette question, la première jounrée à Paris à l’Institut océanographique, la seconde à Rennes dans le cadre du Carrefour des gestions locales de l’eau