Interactions between microorganisms - mercury - soil component : tools for assessing the quality of ecological restoration of gold mining sites in French Guiana

Depuis plus de 150 l’exploitation aurifère en Guyane française bouleverse le paysage en entrainant une déforestation massive des concessions minières. Les conséquences sont multiples, aussi bien pour l’environnement que pour la santé humaine. Pendant des décennies l’utilisation du mercure dans le processus d’extraction a entrainé une contamination du réseau hydrique et de la chaine trophique. Afin d’inscrire l’exploitation aurifère dans une optique de développement durable le code minier impose depuis 1998 une obligation de réhabilitation des sites exploités. Des méthodes de revégétalisation utilisant différentes essences végétales ont donc été adoptées afin d’accélérer les successions écologiques. Le contrôle de la qualité de restauration écologique est néanmoins difficile à mettre en œuvre et il est nécessaire d’avoir des outils de bio-géo-indications fiables et peu couteux pour estimer le recouvrement des fonctionnalités des écosystèmes et apprécier le risque de mobilités des éléments traces toxiques. Dans ce contexte, les objectifs de ce travail de thèse ont été d’évaluer la qualité de restauration écologique de sites miniers en Guyane française à travers une caractérisation des activités microbiennes, du fonctionnement des cycles biogéochimiques d’éléments majeurs (C,N,P) et de la dynamique du mercure. L’originalité de ce travail réside dans l’évaluation dans le temps et dans l’espace des interactions entre couvert végétal – propriétés physico-chimiques du sol – activités microbiennes – spéciation du mercure après revégétalisation et en faire un outils diagnostic de la réussite de cette restauration. Dans ce but, plusieurs campagnes d’échantillonnages ont été réalisées en Guyane sur un panel de sites miniers réhabilités, avec différents types de couverts végétaux. Des échantillons de sols ont été prélevés sur des sites revégétalisés avec des espèces de fabacées, et sur des sites non revégétalisés. Nous avons évalué les fonctionnalités des communautés microbiennes de ces sites avec plusieurs bio-marqueurs de la qualité du sol. Afin d’estimer le devenir du mercure, des mesures du mercure total ainsi que des spéciations opérationnelles et environnementales ont été réalisées. Ce travail de thèse a permis de mettre en évidence un effet positif de la revégétalisation sur la densité et les activités des communautés microbiennes telluriques. Les sites restaurés ont montré une biomasse microbienne, des taux de minéralisation des macroéléments (C,N,P), une diversité catabolique plus élevées que des sites non restaurés. Nos résultats confirment également que la restauration influence les activités des communautés microbiennes anaérobies. En effet, les travaux portant sur les communautés bactériennes ferri-réductrices (BFR) et sulfato-réductrices (BSR) montrent des activités variantes entre les modalités de revégétalisation. Ces différences sont à mettre en relation avec les propriétés physiques et chimiques du sol ainsi qu’avec la nature et la densité du couvert végétal. Les communautés BFR sont actives sur les sites réhabilités alors que les communautés BSR seraient davantage actives sur les sites faiblement restaurés. Ces différences ont des conséquences sur la mobilité du mercure. Nos travaux montrent également que si dans les sites revégétalisés le mercure est associé à des phases stables du sol alors que dans les sites non restaurés il est sous des formes solubles mobilisable dans le réseau hydrique, biodisponible pour les organismes de la chaine trophique. Pour conclure, l’approche interdisciplinaire proposée dans ce travail a permis d’identifier certains processus d’écologie microbienne fondamentaux impactant le recouvrement des fonctionnalités des écosystèmes miniers dégradés. Ce travail offre un outil d’évaluation de la qualité de la restauration écologique original, appliqué et prometteur, qui pourra intéresser les décisionnaires responsables de la réhabilitation des sites miniers en Guyane françaiseFor more than 150 years, gold mining in French Guiana has disrupted the landscape by causing massive deforestation of mining concessions. The consequences are multiple, both for the environment and human health. Indeed, for decades the use of mercury in the extraction process has led to pollution of aquatic systems, contaminating the food chain. In order to ensure that gold mining is carried out in a sustainable development perspective, the Mining Code has imposed an obligation since 1998 to rehabilitate the sites operated. Several revegetation methods using different plant species have been adopted to accelerate ecological succession. However, quality control of ecological restoration is difficult to implement and it is necessary to have reliable and inexpensive bio-geo-indication tools to estimate the recovery of the functionalities of anthropized ecosystems and assess the risk of mobility of toxic metallic elements.In this context, the objectives of this thesis work were to evaluate the quality of ecological restoration of mining sites in French Guiana through a characterization of microbial activities, the functioning of biogeochemical cycles of major elements (C,N,P) and the dynamics of mercury. The originality of this work lies in the evaluation over time and space of the interactions between vegetation cover - physico-chemical properties of the soil - microbial activities - mercury speciation after revegetation and making it a diagnostic tool for the success of this restoration. To this end, several sampling campaigns were carried out in French Guiana on a panel of rehabilitated mining sites, with different types of vegetation cover. Soil samples were taken from replanted sites with fabaceous species (A. mangium and C. racemosa), and from non-restored sites. We then evaluated the main functionalities of the microbial communities of these sites through the use of several soil quality bio-markers. In order to estimate the fate of mercury, measurements of total mercury as well as operational and environmental specimens were carried out. This thesis work revealed a positive effect of revegetation on the density and activities of soil microbial communities. Sites restored with fabaceous species have thus shown microbial biomass, macro-element mineralization rates (C,N,P), and catabolic diversity significantly higher than sites that have not been restored. The associations of fabaceous plants show more conclusive results than the use of monoculture in terms of microbial functionalities. While the activities of aerobic microorganisms have been impacted by the return of vegetation, our results confirm that restoration influences the activities of anaerobic microbial communities. Indeed, work on iron-reducing (IRB) and sulfate-reducing (SRB) bacterial communities shows varying activities between revegetation modalities. These differences are related to the physical and chemical properties of the soil and the nature and density of the vegetation cover. BFR communities are more active on rehabilitated sites while SRB communities are more active on poorly restored sites. These differences in activities have consequences on mercury mobility and methylation. Our work also shows that while mercury in re-vegetated sites is associated with stable phases of the soil, including iron oxides, in non-restored sites it is in soluble forms that can potentially be mobilized in the water network, bioavailable and assimilated by organisms in the food chain.In conclusion, the interdisciplinary approach proposed in this work made it possible to identify certain fundamental microbial ecological processes that impact the recovery of the functionalities of degraded mining ecosystems. This work provides an original, applied and promising tool for assessing the quality of ecological restoration that may be of interest to decision-makers responsible for the rehabilitation of mining sites in French Guian

Interactions microorganismes - mercure - composante du sol : des outils pour l'évaluation de la qualité de la restauration écologique des sites miniers aurifères en Guyane française

For more than 150 years, gold mining in French Guiana has disrupted the landscape by causing massive deforestation of mining concessions. The consequences are multiple, both for the environment and human health. Indeed, for decades the use of mercury in the extraction process has led to pollution of aquatic systems, contaminating the food chain. In order to ensure that gold mining is carried out in a sustainable development perspective, the Mining Code has imposed an obligation since 1998 to rehabilitate the sites operated. Several revegetation methods using different plant species have been adopted to accelerate ecological succession. However, quality control of ecological restoration is difficult to implement and it is necessary to have reliable and inexpensive bio-geo-indication tools to estimate the recovery of the functionalities of anthropized ecosystems and assess the risk of mobility of toxic metallic elements.In this context, the objectives of this thesis work were to evaluate the quality of ecological restoration of mining sites in French Guiana through a characterization of microbial activities, the functioning of biogeochemical cycles of major elements (C,N,P) and the dynamics of mercury. The originality of this work lies in the evaluation over time and space of the interactions between vegetation cover - physico-chemical properties of the soil - microbial activities - mercury speciation after revegetation and making it a diagnostic tool for the success of this restoration. To this end, several sampling campaigns were carried out in French Guiana on a panel of rehabilitated mining sites, with different types of vegetation cover. Soil samples were taken from replanted sites with fabaceous species (A. mangium and C. racemosa), and from non-restored sites. We then evaluated the main functionalities of the microbial communities of these sites through the use of several soil quality bio-markers. In order to estimate the fate of mercury, measurements of total mercury as well as operational and environmental specimens were carried out. This thesis work revealed a positive effect of revegetation on the density and activities of soil microbial communities. Sites restored with fabaceous species have thus shown microbial biomass, macro-element mineralization rates (C,N,P), and catabolic diversity significantly higher than sites that have not been restored. The associations of fabaceous plants show more conclusive results than the use of monoculture in terms of microbial functionalities. While the activities of aerobic microorganisms have been impacted by the return of vegetation, our results confirm that restoration influences the activities of anaerobic microbial communities. Indeed, work on iron-reducing (IRB) and sulfate-reducing (SRB) bacterial communities shows varying activities between revegetation modalities. These differences are related to the physical and chemical properties of the soil and the nature and density of the vegetation cover. BFR communities are more active on rehabilitated sites while SRB communities are more active on poorly restored sites. These differences in activities have consequences on mercury mobility and methylation. Our work also shows that while mercury in re-vegetated sites is associated with stable phases of the soil, including iron oxides, in non-restored sites it is in soluble forms that can potentially be mobilized in the water network, bioavailable and assimilated by organisms in the food chain.In conclusion, the interdisciplinary approach proposed in this work made it possible to identify certain fundamental microbial ecological processes that impact the recovery of the functionalities of degraded mining ecosystems. This work provides an original, applied and promising tool for assessing the quality of ecological restoration that may be of interest to decision-makers responsible for the rehabilitation of mining sites in French GuianaDepuis plus de 150 l’exploitation aurifère en Guyane française bouleverse le paysage en entrainant une déforestation massive des concessions minières. Les conséquences sont multiples, aussi bien pour l’environnement que pour la santé humaine. Pendant des décennies l’utilisation du mercure dans le processus d’extraction a entrainé une contamination du réseau hydrique et de la chaine trophique. Afin d’inscrire l’exploitation aurifère dans une optique de développement durable le code minier impose depuis 1998 une obligation de réhabilitation des sites exploités. Des méthodes de revégétalisation utilisant différentes essences végétales ont donc été adoptées afin d’accélérer les successions écologiques. Le contrôle de la qualité de restauration écologique est néanmoins difficile à mettre en œuvre et il est nécessaire d’avoir des outils de bio-géo-indications fiables et peu couteux pour estimer le recouvrement des fonctionnalités des écosystèmes et apprécier le risque de mobilités des éléments traces toxiques. Dans ce contexte, les objectifs de ce travail de thèse ont été d’évaluer la qualité de restauration écologique de sites miniers en Guyane française à travers une caractérisation des activités microbiennes, du fonctionnement des cycles biogéochimiques d’éléments majeurs (C,N,P) et de la dynamique du mercure. L’originalité de ce travail réside dans l’évaluation dans le temps et dans l’espace des interactions entre couvert végétal – propriétés physico-chimiques du sol – activités microbiennes – spéciation du mercure après revégétalisation et en faire un outils diagnostic de la réussite de cette restauration. Dans ce but, plusieurs campagnes d’échantillonnages ont été réalisées en Guyane sur un panel de sites miniers réhabilités, avec différents types de couverts végétaux. Des échantillons de sols ont été prélevés sur des sites revégétalisés avec des espèces de fabacées, et sur des sites non revégétalisés. Nous avons évalué les fonctionnalités des communautés microbiennes de ces sites avec plusieurs bio-marqueurs de la qualité du sol. Afin d’estimer le devenir du mercure, des mesures du mercure total ainsi que des spéciations opérationnelles et environnementales ont été réalisées. Ce travail de thèse a permis de mettre en évidence un effet positif de la revégétalisation sur la densité et les activités des communautés microbiennes telluriques. Les sites restaurés ont montré une biomasse microbienne, des taux de minéralisation des macroéléments (C,N,P), une diversité catabolique plus élevées que des sites non restaurés. Nos résultats confirment également que la restauration influence les activités des communautés microbiennes anaérobies. En effet, les travaux portant sur les communautés bactériennes ferri-réductrices (BFR) et sulfato-réductrices (BSR) montrent des activités variantes entre les modalités de revégétalisation. Ces différences sont à mettre en relation avec les propriétés physiques et chimiques du sol ainsi qu’avec la nature et la densité du couvert végétal. Les communautés BFR sont actives sur les sites réhabilités alors que les communautés BSR seraient davantage actives sur les sites faiblement restaurés. Ces différences ont des conséquences sur la mobilité du mercure. Nos travaux montrent également que si dans les sites revégétalisés le mercure est associé à des phases stables du sol alors que dans les sites non restaurés il est sous des formes solubles mobilisable dans le réseau hydrique, biodisponible pour les organismes de la chaine trophique. Pour conclure, l’approche interdisciplinaire proposée dans ce travail a permis d’identifier certains processus d’écologie microbienne fondamentaux impactant le recouvrement des fonctionnalités des écosystèmes miniers dégradés. Ce travail offre un outil d’évaluation de la qualité de la restauration écologique original, appliqué et prometteur, qui pourra intéresser les décisionnaires responsables de la réhabilitation des sites miniers en Guyane français

Conventional soil test phosphorus failed to accurately predict dissolved phosphorus release in agricultural hydromorphic soils in Brittany, Western France

Researches have proved that agricultural phosphorus (P) loss contributes significantly to surface water eutrophication. Various soil test P (STP) methods have been developed to assess the P loss risk from agricultural soils. In the intensively-cultivated Brittany region of Western France, hydromorphic soils in wetland domains exhibit high risks of leaching and transferring dissolved P - the most bio-available form of P - to surface waters. It remains unclear whether STP conventionally developed for well-drained soils can accurately predict the risk of dissolved P release from these hydromorphic soils. In this study, we measured the dissolved reactive P (DRP) concentrations in soil solutions sampled in situ from 26 hydromorphic soils in the Brittany region and examined their relationship with several STPs available on the corresponding soils, such as the degree of soil P saturation, the equilibrium soil P concentration, or the soil Olsen P, Dyer P, and water extractable P contents. DRP concentrations ranged from 0.01 to 0.310 mg P l−1 (mean = 0.075 mg P l−1), highlighting the potential of hydromorphic soils as hotspots for DRP release in agricultural landscapes. Correlations between DRP concentrations and STPs were relatively weak (0.09 < r2 < 0.64), indicating that conventional STPs are generally unable to accurately predict the DRP release risks in hydromorphic soils. Tentatively, Olsen P showed promises as a useful risk indicator, with a relatively high r2 value of 0.6 and wide inclusion in the current STP database, especially in the Brittany region. Nevertheless, this hypothesis requires further evaluation with additional data. This study confirms the high risk of dissolved P release from hydromorphic soils in agricultural wetland domains and emphasizes the need for developing specific risk assessment tools to these hydromorphic soils

Variability of phosphorus sorption properties in hydromorphic soils: consequences on P losses in agricultural landscapes

ncreasing concerns over water eutrophication due to agricultural phosphorus (P) loss have led to the development of indicators to assess the risk of P release from agricultural soils. Recently, a logarithmic equation linking the degree of phosphorus saturation (DPS) to the simple water-soluble P (WSP) content of soils has been proposed as a universal method to assess this risk based, however, mainly on the analysis of well-drained soils. Here, we studied the P sorption properties and DPS values of 69 hydromorphic soils from cultivated and uncultivated wetland zones located in Brittany, Western France, to test whether the method could also apply to poorly-drained soils. The bulk soil analysis showed that P contents of the studied hydromorphic soils were 30 to 80% higher than P contents normally found in Brittany soils, evidencing a possible P enrichment process. Adsorption isotherms revealed a surprisingly high variability in the P sorption properties as a function of the location of the soil (maximum P adsorption capacity ranging from 500 to 1850 mg kg-1), which is caused by variations in the phases controlling P sorption in soil (from clay to organic matter and/or iron and aluminum oxides, depending on the soil location). Distinct relationships between DPS and WSP values were also obtained depending on the location of the soils. The obtained DPS vs. WSP relationships showed that the P saturation threshold above which the risk of dissolved P release increases markedly is 30% lower on average for hydromorphic soils than for well-drained soils. Hydromorphic soils appear to be more at risk of releasing dissolved P at the same DPS values than well-drained soils. The present study indicates an underestimation of the P release risk from hydromorphic soils by the existing method developed for well-drained soils and calls for the development of specific risk assessment tools for hydromorphic soils, especially given on the strong spatial heterogeneity of their P sorption propertie

Positive Effect of Ecological Restoration with Fabaceous Species on Microbial Activities of Former Guyanese Mining Sites

Understanding ecological trajectories after mine site rehabilitation is essential to develop relevant protocols adapted for gold mining sites. This study describes the influence of a range of mine site rehabilitation and revegetation protocols on soil physicochemical parameters and microbial activities related to carbon, nitrogen and phosphorus cycles. We sampled soil from six rehabilitated mining sites in French Guiana with different plant cover (herbaceous, Cyperaceous, monoculture of Clitoria racemosa and Acacia mangium and association of C. racemosa and A. mangium). We measured the mineralization potential of organic matter by estimating the mineralization of carbon, nitrogen and phosphorus and the microbial catabolic diversity balance. The results showed an improvement in the quality of organic matter on revegetated sites with tree cover. On restored sites with fabaceous species, the microbial biomass is three times higher than non-restored sites, improving the rates of organic matter mineralization and restoring the catabolic diversity to the level of natural Guyanese soils. These results confirm that the establishment of fabaceous species under controlled conditions significantly improves the restoration of microbial communities in mining soils

Influence of Ecological Restoration on Mercury Mobility and Microbial Activities on Former Guyanese Mining Sites

As rehabilitation efforts in Guyana are recent, there is little information on the effect of different ecological rehabilitation protocols for Guyana's mining sites on biogeochemical cycles and mercury mobility. This study was conducted to assess the impact of different ecological restoration protocols on soil quality with the use of soil microbial indicators and by estimating the mercury mobility. We sampled soil from six rehabilitated mining sites in French Guyana with different ecological restoration procedures. We carried out measurements of enzymatic activities and an analysis of mercury environmental speciation to assess its potential toxicity according to a mobility gradient. The results obtained in this study show that the rehabilitation of mining sites has been carried out in a heterogeneous manner and soil quality is very variable, even in nearby sites. Sites that have been rehabilitated with fabaceous species have positive soil quality indicators. In addition, the results highlight a change in mercury mobility that is 82.1% correlated after co-inertia analysis with soil texture properties, which also confirms a direct effect of rehabilitation on mercury mobility. The non-restored sites had a much higher potential of mercury mobility and toxicity than the sites where ecological restoration was successful. These results highlight the positive effect of controlled rehabilitation and ecological restoration on microbiological activities and the potential toxicity of mercury

Influence of Ecological Restoration on Mercury Mobility and Microbial Activities on Former Guyanese Mining Sites

As rehabilitation efforts in Guyana are recent, there is little information on the effect of different ecological rehabilitation protocols for Guyana's mining sites on biogeochemical cycles and mercury mobility. This study was conducted to assess the impact of different ecological restoration protocols on soil quality with the use of soil microbial indicators and by estimating the mercury mobility. We sampled soil from six rehabilitated mining sites in French Guyana with different ecological restoration procedures. We carried out measurements of enzymatic activities and an analysis of mercury environmental speciation to assess its potential toxicity according to a mobility gradient. The results obtained in this study show that the rehabilitation of mining sites has been carried out in a heterogeneous manner and soil quality is very variable, even in nearby sites. Sites that have been rehabilitated with fabaceous species have positive soil quality indicators. In addition, the results highlight a change in mercury mobility that is 82.1% correlated after co-inertia analysis with soil texture properties, which also confirms a direct effect of rehabilitation on mercury mobility. The non-restored sites had a much higher potential of mercury mobility and toxicity than the sites where ecological restoration was successful. These results highlight the positive effect of controlled rehabilitation and ecological restoration on microbiological activities and the potential toxicity of mercury

Influence of Ecological Restoration on Mercury Mobility and Microbial Activities on Former Guyanese Mining Sites

As rehabilitation efforts in Guyana are recent, there is little information on the effect of different ecological rehabilitation protocols for Guyana’s mining sites on biogeochemical cycles and mercury mobility. This study was conducted to assess the impact of different ecological restoration protocols on soil quality with the use of soil microbial indicators and by estimating the mercury mobility. We sampled soil from six rehabilitated mining sites in French Guyana with different ecological restoration procedures. We carried out measurements of enzymatic activities and an analysis of mercury environmental speciation to assess its potential toxicity according to a mobility gradient. The results obtained in this study show that the rehabilitation of mining sites has been carried out in a heterogeneous manner and soil quality is very variable, even in nearby sites. Sites that have been rehabilitated with fabaceous species have positive soil quality indicators. In addition, the results highlight a change in mercury mobility that is 82.1% correlated after co-inertia analysis with soil texture properties, which also confirms a direct effect of rehabilitation on mercury mobility. The non-restored sites had a much higher potential of mercury mobility and toxicity than the sites where ecological restoration was successful. These results highlight the positive effect of controlled rehabilitation and ecological restoration on microbiological activities and the potential toxicity of mercury

The Impact of Ecological Restoration on Biogeochemical Cycling and Mercury Mobilization in Anoxic Conditions on Former Mining Sites in French Guiana

Successive years of gold mining in French Guiana has resulted in soil degradation and deforestation leading to the pollution and erosion of mining plots. Due to erosion and topography, gold panning sites are submitted to hydromorphy during rainfall and groundwater increases. This original study focused on characterizing the impact of hydromorphic anaerobic periods on bio-geochemical cycles. We sampled soil from five rehabilitated sites in French Guiana, including sites with herbaceous vegetation and sites restored with fabaceous plants, Clitoria racemosa (Cli) mon-oculture, Acacia mangium (Aca) monoculture, Clitoria racemosa and Acacia mangium (Mix) bi-culture. We conducted mesocosm experiments where soil samples were incubated in anaerobic conditions for 35 days. To evaluate the effect of anaerobic conditions on biogeochemical cycles, we measured the following parameters related to iron-reducing bacteria and sulfate-reducing bacteria metabolism throughout the experiment: CO2 release, carbon dissolution, sulphide production and sulphate mobilization. We also monitored the solubilization of iron oxyhydroxides, manganese oxides, aluminum oxides and mercury in the culture medium. Iron-reducing bacteria (IRB) and sulfate-reducing bacteria (SRB) are described as the major players in the dynamics of iron, sulfur and metal elements including mercury in tropical environments. The results revealed two trends in these rehabilitated sites. In the Aca and Mix sites, bacterial iron-reducing activity coupled with manganese solubilization was detected with no mercury solubilization. In herbaceous sites, a low anaerobic activity coupled with sulphide production and mercury solubilization were detected. These results are the first that report the presence and activity of iron- and sulfate-reductive communities at rehabilitated mining sites and their interactions with the dynamics of metallic elements and mercury. These results report, however, the positive impact of ecological restoration of mining sites in French Guiana by reducing IRB and SRB activities, the potential mobility of mercury and its risk of transfer and methylation

The influence of landscape spatial configuration on nitrogen and phosphorus exports in agricultural catchments

International audienceContext Nitrogen (N) and phosphorus (P) exports from rural landscapes can cause eutrophication of inland and coastal waters. Few studies have investigated the influence of the spatial configuration of nutrient sources-i.e. the spatial arrangement of agricultural fields in headwater catchments-on N and P exports. Objectives This study aimed to (1) assess the influence of the spatial configuration of nutrient sources on nitrate (NO3-) and total phosphorus (TP) exports at the catchment scale, and (2) investigate how relationships between landscape composition (% agricultural land-use) and landscape configuration vary depending on catchment size. Methods We analysed NO3- and TP in 19 headwaters (1-14 km(2), Western France) every two weeks for 17 months. The headwater catchments had similar soil types, climate, and farming systems but differed in landscape composition and spatial configuration. We developed a landscape configuration index (LCI) describing the spatial organisation of nutrient sources as a function of their hydrological distance to streams and flow accumulation zones. We calibrated the LCI's two parameters to maximise the rank correlation with median concentrations of TP and NO3-. Results We found that landscape composition controlled NO3- exports, whereas landscape configuration controlled TP exports. For a given landscape composition, landscape spatial configuration was highly heterogeneous at small scales ( 50 km(2)). Conclusions The spatial configuration of nutrient sources influences TP but not NO3- exports. An ideal placement of mitigation measures to limit diffuse TP export should consider both the hydrological distance to streams and flow accumulation zones