Biogeochemical and microbiological processes involved in the rhizospheric area of Salicaceae grown on an amended technosol polluted by inorganic toxic elements: a phytostabilization study

The pollution of soils by metal(loid)s, resulting from anthropogenic activities, is an important issue and subject of research nowadays. Therefore, to remediate such sites, an environment friendly technique has developed over the last decades, called phytoremediation. The phytoremediation is the use of plants and their associated microbiota to remediate in situ contaminated area. Among plants that can be used in phytoremediation, Salicaceae showed good potential, due to their rapid and high biomass production associated to their tolerance to metal(loid)s. However, contaminated soils are often poor (extreme pH, low nutrient contents), which associated to their elevated metal(loid) concentrations hinder plant growth. That is why amendments must be applied. Three amendment types gathered attention over the last decades, compost, biochar and iron-based amendments. Compost can increase soil nutrient availability and immobilize metal(loid)s. Biochar can ameliorate soil conditions and sorb metal(loid)s. Both amendments showed good potential to improve soil properties, immobilize cations and thus ameliorate plant growth. However, they showed no or negative effects on anions like As. On the contrary, iron-based amendments, such as iron grit and redmud, have an affinity for As and can thus immobilize it. Finally, another parameter that can affect phytoremediation efficiency is the microbial community. Microorganisms are very sensible to the growing conditions and the measure of their diversity and activity can inform on the recovery of the soil during the phytoremediation process. Microorganisms can also affect phytoremediation efficiency: (i) through their effect on metal(loid)s, some bacterial strains showed the capacity to accumulated metal(loid)s or detoxify them and (ii) by increasing plant growth, through the secretion of plant growth promoting factors (i.e. hormone and siderophore production, phosphate solubilization). In this context, the aims of this PhD work were first to evaluate the effect of applying to a former mine technosol highly contaminated by As and Pb different amendments, alone or combined, on (i) soil physico-chemical properties, (ii) Salicaceae plant growth and metal(loid) accumulation, (iii) Salicaceae physiology and biochemistry and (iv) soil bacterial community. The second objective was to assess the effect of inoculating an endogenous bacterium, using two inoculation methods, on (i) soil physico-chemical properties, (ii) soil bacterial community and (iii) Salix viminalis growth and metal(loid) accumulation. To answer these objectives, a total of seven mesocosm experiments were performed. In general, these different experiments demonstrated that the addition of amendment improved the soil conditions and thus plant growth in most of the cases. Salicaceae plants accumulated As and Pb mainly in their roots. Moreover, the application of amendments had an effect on the root physiology of Salicaceae plants. Especially, the root proteome profiles of Salix viminalis were affected differently by the addition of biochar, compost and/or iron grit; whereas Salix triandra root showed high levels of stress markers on the unamended technosol, which were reduced following the application of redmud and biochar. In addition, the inoculation of an endogenous Bacillus strain also improved soil conditions and Salix viminalis growth, especially when the bacterium was previously sorbed on biochar surface. Finally, the study of the bacterial community showed an increase of the bacterial activity in the presence of amendments and following bacterial inoculation, in addition to a modification of the bacterial community composition. To conclude, this PhD work demonstrated that amendment application can improve soil conditions and plant growth; however these effects depend on the amendment(s) applied. This work was also one of the first showing that amendments can affect the plants at a biochemical level and comparing two bacterial inoculation methods, demonstrating the better effect of a biochar carrier inoculation.