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Mercury (Hg) speciation was compared in French Guiana pristine soils and in Hg contaminated soils impacted by former (~1950's) gold-mining activities which used Hg for gold amalgamation. Four selective extractions were performed on soil samples to assess the fraction of Hg present as Hg(II) and bond to organic matter (extracted by NH 4 OH and KOH), to amorphous iron oxides (ascorbate) and to soil components other than refractory minerals (HCl/HNO 3 ). In addition, pyrolysis was used to quantify the content of elemental Hg in contaminated soils. X-ray diffraction (XRD) and X-ray fluorescence micro-mapping (µXRF) were used in combination to selective extractions to assess the nature of targeted components, the possible overlaps between the different extraction procedures and the spatial correlation between Si, K, Fe, Au and Hg.In soil profiles from pristine toposequences, Hg concentrations (0.01-0.49 µg.g -1 ) decreased with increasing depth in soil matrix. Hg concentrations also decreased from ferralsols to acrisols and further to gleysols. In pristine soil matrix, Hg was mainly associated to the clay size fraction (< 2 µm) which was mainly constituted of amorphous and crystalline Fe oxides (Al-substituted goethite and hematite), gibbsite and fine organic matter (OM), whose relative abundances vary along the soil association. Total Hg concentration was positively correlated with sulfurs and organic carbon suggesting the association of Hg with OM sulfur-bearing functional groups.Gleysols were depleted in Hg because of the prevailing reducing conditions that lead to the dissolution of iron oxides. In the same soil profiles, Hg concentrations in ferruginous nodules, which make up most of the soil coarse fraction (> 2 mm), were similar to those reported in the pristine soil matrix. These nodules mainly contained Al-substituted hematite and goethite and were especially abundant upslope in 2 ferralsols and acrisols. Gold-mined gleysols were strongly disorganized by former activities as neither the original structure nor the texture was preserved. Soil granulometry was dominated by gravels, sands and silts. Hg concentrations (0.09-9.22 µg.g -1 ) largely exceeded those in pristine soils. µXRF allowed the identification of Au-amalgamated Hg and of elemental Hg droplets. Pyrolysis confirmed Hg to be mainly present in its elemental form in contaminated soils. Selective extractions showed additional minor contributions of Hg(II) associated to OM, and to Al or Fe oxides. The combination of selective extractions with XRD and µXRF data showed that extraction efficiency is strongly dependent on the soil type, and that this efficiency needs to be determined on a soil-by-soil basis for Hg speciation studies.KOH extraction was especially delicate as crystalline and amorphous oxides were extracted together with organic matter.

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Landscape dynamics result from forestry and farming practices, both of which are expected to have diverse impacts on ecosystem services (ES). In this study, we investigated this general statement for regulating and supporting services via an assessment of ecosystem functions: climate regulation via carbon sequestration in soil and plant biomass, water cycle and soil erosion regulation via water infiltration in soil, and support for primary production via soil chemical quality and water storage. We tested the hypothesis that patterns of land-cover composition and structure significantly alter ES metrics at two different scales. We surveyed 54 farms in two Amazonian regions of Brazil and Colombia and assessed land-cover composition and structure from remote sensing data (farm scale) from 1990 to 2007. Simple and well-established methods were used to characterize soil and vegetation from five points in each farm (plot scale). Most ES metrics were significantly correlated with land-use (plot scale) and land-cover (farm scale) classifications; however, spatial variability in inherent soil properties, alone or in interaction with land-use or land-cover changes, contributed greatly to variability in ES metrics. Carbon stock in above-ground plant biomass and water infiltration rate decreased from forest to pasture land covers, whereas soil chemical quality and plant-available water storage capacity increased. Land-cover classifications based on structure metrics explained significantly less ES metric variation than those based on composition metrics. Land-cover composition dynamics explained 45 % (P < 0.001) of ES metric variance, 15 % by itself and 30 % in interaction with inherent soil properties. This study describes how ES evolve with landscape changes, specifying the contribution of spatial variability in the physical environment and highlighting trade-offs and synergies among ES. (Résumé d'auteur

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Summary Ferralsols have a macrostructure that is weak to moderate and a strong microstructure consisting of near spherical microaggregates. We have studied the evolution of the structure under changing land use in two Ferralsols under a native Cerrado and under pasture sowed after recent clearing by measuring bulk density. We studied the microaggregates' characteristics and resulting porosity using scanning electron microscopy and mercury porosimetry, respectively. Microaggregates 50–300 μm in diameter are attributed to termites. They were in a much closer arrangement from the surface to 1 m depth under pasture than under Cerrado. Larger microaggregates, 500–1000 μm in diameter, that are closely packed, resulted from earthworm activity. They were more numerous under pasture than under Cerrado. Thus within a few years, clearing and grazing have caused a dramatic decrease in microstructure that is one of the rare favourable characteristics of Ferralsols for agriculture. This evolution that affects the subsoil to 80–90 cm cannot be attributed to compaction by mechanical deforestation alone but seems to result from a change in the faunal activity in the soil. Finally, the packing of the clay particles within the microaggregates was similar whatever the type of microaggregate and land use. Our results accord with earlier studies showing that the porosity resulting from clay packing varies little in Ferralsols in which the fine material is mainly kaolinite. Evolution de la porosité et de la microagrégation dans des Ferralsols argileux du Cerrado Brésilien après déforestation et installation d'un pâturage Résumé Les Ferralsols possèdent une macrostructure qui est faiblement à modérément développée et une microstructure très nette sous la forme de microagrégats approximativement sphériques. Nous avons étudié l'évolution de la structure lors d'un changement de mode d'usage du sol dans deux Ferralsols argileux sous végétation naturelle de type Cerrado et sous pâturage semé après déforestation. Nous avons mesuré la densité apparente et décrit la microstructure et la porosité qui en résulte en microscopie électronique à balayage et en porosimétrie au mercure. Des microagrégats de 50 à 300 μm de diamètre ont été attribués à l'activité des termites; leur assemblage est plus compact jusqu'à 1 m de profondeur sous pâturage que sous Cerrado. Des microagrégats de plus grande taille, de 500 à 1000 μm de diamètre, et en assemblage compact ont aussi été observés. Ils résultent de l'activité de vers de terre et sont plus nombreux sous pâturage que sous Cerrado. Ainsi en quelques années, la mise en place d'un pâturage après déforestation a pour effet une décroissance importante de la microagrégation qui est l'une des rares caractéristiques favorables des Ferralsols pour l'agriculture. Cette transformation qui affecte le sol en profondeur ne peut pas être attribuée à un compactage lors la déforestation mécanisée mais apparaît être en revanche la conséquence d'une évolution de l'activité de la faune du sol. Enfin, la porosité résultant d...

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