Jean-Pierre Boudot

Chromium availability in ultramafic soils from New Caledonia

The sources and potential availability of chromium (Cr) on soils formed on ultramafic rocks were investigated with mineralogical studies and selective chemical extractions. Soil solutions were collected in the field (i) along a soil toposequence under natural vegetation with ceramic cups; (ii) under grass in a mandarin trees plantation with tension-free tube lysimeters. On selected soil solutions, the Cr(VI) was determined colorimetrically with the s-diphenylcarbazide method and total Cr by ICP-AES and speciation of Cr(VI) was performed with the MINEQL+ V 4.5 software. The main mineralogical sources of Cr were Cr-substituted goethite and chromite. Up to 90 mg kg(-1) of Cr was extracted by KH(2)PO(4), whereas KCl extractable Cr was very low, indicating that exchangeable Cr was mainly in the highly toxic Cr(VI) form in these soils. Under natural vegetation, the Cr concentrations in the soil solutions remained relatively low (<20 microg l(-1)) due to the high retention of the Cr(VI) anions by Fe-oxides. The Cr concentrations were larger in well aerated colluvial soils, where high levels of Mn-oxides are able to oxidize Cr(III) to Cr(VI), than in piedmont soil where the Mn-oxide content is lower, or in alluvial soils from the lowlands, where waterlogging occurs. Cr concentrations reached 700 microg l(-1) in the field that was fertilized with high amount of phosphorus, due to the exchange of Cr(VI) with phosphate. In such conditions, toxicity phenomena for crops can be expected.

Macroinvertebrate community loss as a result of headwater stream acidification in the Vosges Mountains (N-E France).

The relationships between water chemistry and aquatic macroinvertebrate communities of 41 headwater streams were studied in the Vosges Mountains (N-E of France) in an attempt to assess the impact of acidification on macroinvertebrate diversity. The taxa richness of macroinvertebrates decreased drastically in headwater streams which were characterized by low pH, low calcium and high aluminum content. All taxonomic groups were affected, but Molluscans, Crustaceans and Ephemeroptera disappeared totally from strongly acidified streams. Simple indices based on taxa richness such as the coefficient of community loss may provide accurate tools to quickly assess the impact of acidification on macroinvertebrate communities. Despite the reduction of atmospheric SO2 emissions, acidification of freshwater in the Vosges Mountains continues to affect streams which were believed in the past to constitute refuge biotopes for numerous species. Consequently, acidification represents a real threat for numerous invertebrates. This study arises the question of the evolution in the future of headwater stream ecosystems. Urgent decisions and interventions are required to preserve non-acidified streams and to restore impacted ecosystems while awaiting spontaneous recovery.

European Red List of dragonflies

The European Red List is a review of the conservation status of c.6,000 European species (mammals, reptiles, amphibians, freshwater fishes, butterflies, dragonflies, and selected groups of beetles, ...

Influence of mature Douglas fir roots on the solid soil phase of the rhizosphere and its solution chemistry

Plants can induce significant changes in the rhizosphere through the uptake of water and ions, the exudation of organic compounds and the activities of micro-organisms. The aim of the present study was to assess the influence of tree roots on the chemistry (pH, exchangeable cations, total organic carbon) of both the solid phase of the soil and the soil solutions, extracted by centrifugation, under a mature Douglas fir stand over two distinct seasons (March and September, 1999). The chemical characteristics of either the solid soil phase or the soil solutions of the rhizosphere were found to be different from those of the bulk soil. The cation exchange capacity, base saturation and organic C were all greater in the rhizosphere than in the bulk soil, as a possible result of rhizodeposition, incorporation of decaying root material and micro-organism activity. The concentration of all elements increased in the rhizosphere solutions as compared to the bulk soil solutions, except for P. The pH was lower in the rhizosphere than in the bulk soil for both the solid soil phase and the soil solutions. Despite the greater overall Al concentration of the rhizosphere solutions, as compared to the bulk soil solutions, we suggest that in both, Al toxicity was efficiently restricted by both high Ca + Mg contents and Al complexation with various ligands.

Consideration on the occurrence of the Al13 polycation in natural soil solutions and surface waters

Abstract Equilibrium speciation calculations were performed (1) for soil solutions and streamwaters collected in central and eastern France and (2) for simulated waters at 0 and 25°C, to assess the highest concentration of Al13 that could be reached in waters in the absence of complexing ligands other than OH−. A comprehensive and updated set of aqueous Al species, including polymeric hydroxyaluminosilicates (HAS), and their corresponding thermodynamic formation constants, were used. Results suggest that the concentration of the Al13 polycation in natural waters has been largely overestimated in some past studies using equilibrium models to calculate Al speciation, owing to oversimplification (many Al ligands not considered) and the unrecognised temperature dependence of some formation constants. The Al13 concentration in mildly acidic natural waters may not exceed a few μmol l−1 at AlT on the order of 10−4 mol l−1 and should be less than 1 μmol l−1 at AlT=10−5 mol l−1. Monomeric Al–Si species may not significantly interfere with the formation of Al13, but the formation of both HAS polymers (proto-imogolite precursors) and organo-Al complexes have a marked detrimental effect on the Al13 concentration. The maximum concentration of Al13 decreased upon increasing temperature from 0 to 25°C. In contrast, the pH range wherein Al13 may occur increases slightly with temperature and the most acidic pH value above which Al13 may be formed has been underestimated. At T=25°C, the Al13 polycation may be a significant Al species (4 to 5% of AlT) at pH 10−4 mol l−1. The results of this study and the use of HAS polymers to calculate Al speciation in moderately natural acidic soil solutions were in better accordance with soil mineralogy. This research suggests strongly that Al13 should be negligible in natural soil and surface waters and may not control either Al3+ activity or Al-trihydroxide formation through polymerisation/depolymerisation steps. Also, from a biological point of view, the toxicity of Al13 to plants and aquatic organisms in natural conditions may be considered to be very low.

Validation of an operational procedure for aluminium speciation in soil solutions and surface waters

Abstract Based on various methods, this paper proposes an optimized procedure to assess Al speciation in soil and surface waters. A 5-s flash extraction of some Al inorganic species by 8-hydroxyquinoline is proposed to replace the widely used rapid extraction (15 s) procedure. All species of Al relevant to natural waters were for extractibility under the recommended conditions. The speciation of this pool may be computed by equilibrium modelling. The Al-F species were not extracted and must be computed separately by equilibrium modelling without too many uncertainties. They cannot be determined using the free F-selective electrode method, because of interferences with other ions that are too strong. The polymeric and colloidal Al species are derived from the unrecovered Al in ion chromatography using a Dionex CS-3 cationic analytical column, for which the dimeric AlSi species extracted during the 8-hydroxyquinoline extraction step must be added. Organic Al was considered as being the amount of Al not accounted for by these combined procedures. An example of speciation is given with respect to some spring waters from the Vosges mountains in France. Often less than one-third of Al was found as toxic species, which identifies the need of such determinations to assess Al toxicity in acidified ecosystems. Among the non-toxic species, the AlSi complexes were found to sometimes reach up to 50% of the total Al, which emphasizes the great influence of silica on Al toxicity in surface waters.

Occurrence of non-monomeric species of aluminium in undersaturated soil and surface waters: consequences for the determination of mineral saturation indices

The occurrence of a polymeric and/or colloidal Al pool was demonstrated on an experimental basis in various natural acid soil solutions and stream waters, even in the pH range 3.3–4. A number of these waters were found to be undersaturated with respect to various relevant mineral phases (amorphous Al(OH)3 and AIPO4, gibbsite, boehmite, variscite, imogolite, kaolinite, halloysite, jurbanite, basaluminite and alunite). Non-monomeric Al species should not occur in undersaturated aqueous systems in equilibrium; therefore the polymeric/colloidal Al pool observed in these undersaturated waters cannot be in equilibrium with monomeric Al. As a consequence, the traditional determination of saturation indices (S!) by model equilibrium calculation using total concentrations of every element is considered to be unreliable, and any valid procedure should involve the determination of monomeric species (or a part of them) by analytical speciation.

Soil solutions and surface water analysis in two contrasted watersheds impacted by acid deposition, Vosges mountains, N.E. France: interpretation in terms of Al impact and nutrient imbalance

Two acid watersheds in the Vosges mountains (N.E. France), one with a podzol, the other with an acid brown soil, were monitored by analysing soil solutions and two related springs for total chemistry and Al speciation. High concentrations of inorganic Al did not occur in the podzol upper horizons but were evident in the corresponding spring, together with low concentrations of Ca. In the acid brown soil area, high concentrations of inorganic Al occurred in the leaching water draining the upper soil horizons, but not in the undrained water nor in the spring, the latter exhibiting rather high Ca content. In both watersheds, needle yellowing in conifers could be observed and might be ascribed to Mg deficiency rather than to Al toxicity.

Bryological patterns and streamwater acidification in the Vosges Mountains (N.E. France): An analysis tool for the survey of acidification processes

The relationships between water chemistry and aquatic bryoflora in the Vosges Mountains (Eastern France) were studied in an attempt to survey the processes of acidification. 6 chemical variables (pH, Mg2+, Ca2+, K+, NO3−, Al) were used to explain the segregation of the bryophytic communities concerning 19 species in 31 study sites. Ca2+, Mg2+ and a lesser extend pH and Al are the four variables best explaining the relationships between the bryophytes and the chemical data. Acidophilous species as Marsupella emarginata seem to be sensitive to high cations concentrations. On the contrary neutrophilous species as Rhynchostegium riparioides may be more sensitive to the toxicity of protons or Al than to lacks of cations. The combinations of bryophytes are used to reconstitute the chemical variables and the precision of the modelling is assessed.

Evidence for the redissolution of soil spodic horizons under the influence of acid deposition in the Vosges mountains (north-eastern France)

Abstract The speciation of soluble Al present in leaching waters was studied in an acid watershed with podzolic soils from the upper soil horizons to two spring waters. The main pattern of soil water circulation during high rainfall events was found to be vertical leaching through the upper soil eluvial horizons, then lateral flowing within the upper part of the deep spodic horizons. Between the bottom of the eluvial horizons and the spring waters, organic Al disappeared, wheteas new inorganic monomeric Al species appeared. The latter were ascribed to the redissolution of the amorphous Al previously accumulated within the soil spodic horizons rather than to the dissociation of pre-existing F-bound Al or organic Al species. Atmospheric pollution-derived acidity was involved in this process, rather than soil-derived acidity. As such horizons contain high amounts of potentially toxic Al and trace metals in readily mobilizable forms, such a redissolution constitutes a true chemical time bomb for the future.