Hermine Huot

Pedogenetic Trends in Soils Formed in Technogenic Parent Materials

Abstract Soils formed in technogenic parent materials are proliferating, and their pedogenesis remains inadequately understood. On investigation of soils formed in various technogenic materials reported in the literature and of a Technosol developing on iron industry deposits, potential specificities of pedogenesis within technogenic materials are discussed. Analyses have shown that pedogenetic processes observed in these soils are similar to those occurring in natural soils. However, some particularities have been highlighted, such as the coexistence of processes rarely encountered simultaneously in natural environments or high rates of soil development during the first stages of weathering. Technogenic materials display highly diverse constituents, wide spatial variability, and temporal discontinuities as a result of human activities. These inherent features, in interaction with other soil-forming factors, may govern pedogenesis by generating the following conditions: (i) a high diversity of pedogenetic processes, including processes occurring usually in dissimilar climatic regions; (ii) localized processes and distinct rates and orientations of soil development over short distance; and (iii) a succession and/or a superimposition of processes, constrained by the deposition of materials and human operations. This creates the potential for the simultaneity of processes coexisting rarely in natural soils developing in the same climatic region. These soils are likely to have a polycyclic evolution and to evolve toward groups of soils other than Technosols, depending on the dominant processes. Further investigations of the processes and their potential interactions occurring in a larger range of soils formed in technogenic materials are required to better predict the evolution of these soils.

A Technosol as archives of organic matter related to past industrial activities

To better understand formation, functioning and evolution of a Technosol developing on a former settling pond of iron industry under forest cover, organic matter (OM) of layers along the soil profile was investigated. Spectroscopic and molecular analyses of extractable OM gave information on OM origin and state of preservation. In the surface layer, OM fingerprints indicated fresh input from vegetation while they revealed well preserved anthropogenic compounds related to industrial processes in deeper layers. OM variability and distribution according to the layers recorded deposition cycles of industrial effluents into the pond. Thus, the Technosol can be considered as archives of past industrial activities. The preservation of anthropogenic OM could be connected with mineralogy, high metal contents and particular physical properties of the Technosol.

Litter chemistry prevails over litter consumers in mediating effects of past steel industry activities on leaf litter decomposition

Soil pollution has adverse effects on the performance and life history traits of microorganisms, plants, and animals, yet evidence indicates that even the most polluted sites can support structurally-complex and dynamic ecosystems. The present study aims at determining whether and how litter decomposition, one of the most important soil ecological processes leaf, is affected in a highly trace-metal polluted site. We postulated that past steel mill activities resulting in soil pollution and associated changes in soil characteristics would influence the rate of litter decomposition through two non-exclusive pathways: altered litter chemistry and responses of decomposers to lethal and sub-lethal toxic stress. We carried out a litter-bag experiment using Populus tremula L. leaf litter collected at, and allowed to decompose in, a trace metal polluted site and in three unpolluted sites used as controls. We designed a fully-factorial transplant experimental design to assess effects of litter origin and exposure site on the rate of litter decomposition. We further determined initial litter chemistry, fungal biomass, mesofauna abundance in litter bags, and the soil macrofauna community. Irrespective of the site of litter exposure, litter originating from the polluted site had a two-fold faster decomposition than litter from the unpolluted sites. Litter chemistry, notably the lignin content, seemed most important in explaining the degradation rate of the leaf litter. Abundance of meso and macro-detritivores was higher at the polluted site than at the unpolluted sites. However, litter decomposition proceeded at similar rates in polluted and unpolluted sites. Our results show that trace metal pollution and associated soil and litter changes do not necessarily weaken consumer control on litter decomposition through lethal and sub-lethal toxic stress.

Lysimeter monitoring as assessment of the potential for revegetation to manage former iron industry settling ponds

To assess the impact of metal-rich brownfields on groundwater quality, the fluxes in a Technosol developed on a former iron industry settling pond were studied. Intact soil monoliths (1 m(2) × 2 m) were extracted and placed in lysimeters. Dynamics of fluxes of metals and solutes under varying vegetation cover were monitored over the course of four years. Soil hydraulic properties were also determined. Results showed that the Technosol has a high retention capacity for water and metals, in relation to its mineral components and resulting chemical and physical properties. As a consequence, metal fluxes were limited. However, soluble compounds, such as SO4(2-), were found at significant concentrations in the leachates. The presence of a dense and deeply-rooted vegetation cover limited water- and solute-fluxes by increasing evapotranspiration and water uptake, thereby reducing the risks of transfer of potentially toxic compounds to local groundwater sources. However, vegetation development may induce changes in soil chemical (e.g. pH, redox potential) and physical properties (e.g. structure), favoring metal mobilization and transport. Revegetation is a valuable management solution for former iron industry settling ponds, provided vegetation does not change soil physico-chemical conditions in the long term. Monitored natural attenuation is required.

Water, sediment and agricultural soil contamination from an ion-adsorption rare earth mining area

Due to their specific properties, ion-adsorption rare earth mine sites may be a threat for adjacent environments. This work was undertaken to assess whether former mining operations on ion-adsorption rare earth mine sites have a significant impact on water bodies and soils of the surrounding environments. Tailing soil materials, stream waters and sediments, and farmland soils were collected from one of the largest ion-adsorption rare earth mine sites worldwide (Southern China). Total concentrations of rare earth elements (REEs), Fe, Al, etc., and pH were measured. Results revealed high concentrations of REEs in tailing soils (392 mg kg-1), stream waters (4460 μg L-1), sediments (462 mg kg-1) and farmland soils (928 mg kg-1) in comparison with control sites. In the tailing profiles, light REEs (LREEs) were preferentially leached compared to middle REEs (MREEs) and heavy REEs (HREEs). Anomalies in tailings and stream water indicated strong soil weathering (Eu) and leaching activities (Ce) within the tailings. The MREE enriched pattern in stream water was more related to water parameters such as Al and Fe oxides, and ligands, than to the source of REEs. Anomalies also indicated that REEs contamination in the farmland soils was mainly originated from the stream water contaminated by the leaching from the tailings. In conclusion, a heavy REEs pollution was recorded in the surrounding environment of ion-adsorption rare earth mine. REEs fractionation, Ce and Eu anomalies provided an insight to the understanding of REEs leaching and soil weathering processes, and REEs environmental fate in rare earth mining area.