Diversité végétale en phytoremédiation : la complémentarité fonctionnelle pour gérer efficacement la contamination multiple des sols

Phytoremediation is a plant-based technology using plant’s natural abilities to remove contaminants from their substrate. This technique comes with numerous advantages compared to conventional contaminated soil management. Since it is usually implemented in situ, it rarely involves soil excavation and transport, which contributes to its negligible environmental impact and affordable cost. These economic and environmental advantages add to the valuable esthetic and ecological properties of the plant systems involved in phytoremediation. By its nature, soil contamination can represent a challenge for phytoremediation. Chemical compounds inside the soil of degraded areas are often miscellaneous as well as heterogeneously distributed. This level of complexity in soil contamination must be taken into account when designing phytoremediation initiatives. The scientific literature reports several plant species presenting characteristics of interest for phytoremediation such as inorganic element absorption or organic compounds degradation by the internal metabolism or the rhizospheric community that is supported by the plants. However, although these plant species are numerous, only a small number are known to possess remediation abilities towards multiple compounds. In a context of complex soil contamination, it therefore appears wise to assemble plant species in order to obtain a set of remediation abilities that will be adapted for the target site. In addition to this technical argument, such an approach promoting diversity goes along with studies highlighting benefits of biodiversity for productivity and general functioning of agro-ecosystems. In order to explore the relevance of adopting of a diversified approach in phytoremediation, three studies were conducted. A first investigation was led on the area of a former decantation basin. The objective was to investigate the influence that the spatial distribution of the contaminants could have on the species repartition of a ruderal plant community. A multivariate analysis was used to demonstrate that most of the species spatial pattern could be explained by the contaminant distribution on this site. This evidence suggests the existence of ecological niches of resistance to pollutants and therefore supports the use of different plant species for phytoremediation of complex contaminated soil. A second study conducted as a pot trial aimed at exploring the potential of four commercially available species i.e. willow (Salix miyabeana `SX67`), alfalfa (Medicago sativa), indian mustard (Brassica juncea) and tall fescue (Festuca arundinacea), for phytoremediation of a soil contaminated by three trace elements, namely silver (Ag), copper (Cu, and zinc (Zn). Each of the species demonstrated phytoextraction abilities but only for one of the trace elements. The results gathered from this experiment point towards the existence of species-specific remediation abilities and suggest that plant combination could be appropriate for remediation of soil contaminated by multiple elements. A third experiment in mesocosm directly investigated the effect of plant diversity in phytoremediation by comparing one, two and three species systems for remediation of a soil contaminated by eight trace elements. In monoculture, S. miyabeana, M. sativa and F. arundinacea demonstrated complementary biomass allocation patterns as well as distinct remediation abilities. While for the remediation of a specific trace element assembling species did not seem to enhance phytoremediation efficiency, when considering a set of trace elements simultaneously, an approach combining species seems to address most efficiently a complex soil contamination issue. Facilitative interactions towards nitrogen accessibility were also identified in certain assemblages, adding to the interest of combining species in phytoremediation. This thesis gathered important scientific evidences suggesting that assemblages of species should be employed for phytoremediation of soil contaminated by multiple compounds.

Keyphrases: Phytoremediation, Multiple contamination, Soil, Ecology, Complementarity, Trace elements, Willow, Alfalfa, Tall fescue