Assisted phytoremediation of a former mine soil using biochar and iron sulphate: Effects on As soil immobilization and accumulation in three Salicaceae species.

Metal(loid) accumulation in soils, is of increasing concern because of the potential human health risks. Therefore, metal(loid) contaminated sites need rehabilitation. It is becoming increasingly popular to use phytoremediation methods for the reclamation of sites containing metal(loid)s. However, plant establishment and growth on contaminated soils can be difficult due to high metal(loid) concentrations and poor fertility conditions. Consequently, amendments, like biochar and iron sulphate, must be applied. Biochar, obtained from plant biomass or animal wastes pyrolyzed under minimal oxygen supply, showed beneficial effects on soil properties and plant growth. Iron sulphate can effectively immobilize anions, thus mitigating metal(loid) toxicity and hence promoting plant development. This study aimed to assess the effect of two different modalities of biochar amendment application (top third of the tube and all tube height) combined with iron sulphate addition on the physico-chemical properties of a mining polluted soil and the growth and metal(loid) uptake of three Salicaceae species. A 1.5 year mesocosm experiment under field condition was conducted using a former tin mine contaminated by arsenic, amended with biochar and iron sulphate and vegetated with three Salicaceae species. Results showed that the combination of biochar and iron sulphate improved soil characteristics by increasing pH and electrical conductivity and reducing soil pore water metal(loid) concentrations. Between the two biochar application methods, the addition of biochar on the all tube height showed better results. But for such contaminated soil, biochar, in combination with iron sulphate, had no positive effect on plant growth, for all species tested and especially when incorporating on the top third of the tube. Finally, S. purpurea presented high root metal(loid) concentrations associated to the better growth compared to P. euramericana and S. viminalis, making it a better candidate for phytostabilization of the studied soil.

Fast-growing willows significantly reduce invasive knotweed spread.

Competitive interactions seem to play a major role in invasive plant success. However, they have mostly been addressed through the invader impacts on other species of the plant community and rarely through the way plant communities can contain alien species. Understanding such mechanisms would help in designing restoration projects using plant community competitive properties to control invasive populations. In this study, we looked at the role of competitive interactions in the success of Fallopia japonica (Houtt.) Ronse Decraene using a native willow frequently used in bioengineering techniques: Salix viminalis L. S. viminalis has a high growth rate and is, as such, a potential candidate to compete with F. japonica in restoration projects of invaded areas. Both species were grown in semi-controlled conditions in mesocosms (truck dumpsters), alone or in competition. Morphological traits (plant height, specific leaf area) as well as biomass (aboveground and underground) were measured on each species during two growing seasons. We also quantified spatial expansion of F. japonica. Even under a dense canopy of S. viminalis, F. japonica was able to survive and grow. However, its performance was significantly reduced compared to monocultures and its spatial colonization was less extended. Although S. viminalis biomass was affected by F. japonica, F. japonica expressed competitive stress through a modification of ramet density and height. There was no significant effect of F. japonica on S. viminalis height, enabling this species to dominate. Synthesis and applications: We conclude that S. viminalis succeeded in reducing F. japonica growth by developing a competitive canopy. Bioengineering techniques aiming at restoring a competitive neighborhood can control F. japonica. However, F. japonica's broad underground extension should be taken into account in any management strategy in order to successfully limit its development and spatial spread.