Willows Used for Phytoremediation Increased Organic Contaminant Concentrations in Soil Surface

Abstract

The Salix genus includes shrub species that are widely used in phytoremediation and various other phytotechnologies due to their advantageous characteristics, such as a high evapotranspiration (ET) rate, in particular when cultivated in short rotation intensive culture (SRIC). Observations made in past field studies suggest that ET and its impact on soil hydrology can also lead to increases in soil pollutant concentrations near shrubs. To investigate this, sections of a mature willow plantation (seven years old) were cut to eliminate transpiration (Cut treatment). Soil concentrations of polychlorinated biphenyls (PCBs), aliphatic compounds C10–C50, polycyclic aromatic hydrocarbons (PAHs) and five trace elements (Cd, Cr, Cu, Ni and Zn) were compared between the Cut and the uncut plots (Salix miyabeana ‘SX61’). Over 24 months, the results clearly show that removal of the willow shrubs limited the contaminants’ increase in the soil surface, as observed for C10–C50 and of 10 PAHs under the Salix treatment. This finding strongly reinforces a hypothesis that SRIC of willows may facilitate the migration of contaminants towards their roots, thus increasing their concentration in the surrounding soil. Such a “pumping effect” in a high-density willow crop is a prominent characteristic specific to field studies that can lead to counterintuitive results. Although apparent increases of contaminant concentrations contradict the purification benefits usually pursued in phytoremediation, the possibility of active phytoextraction and rhizodegradation is not excluded. Moreover, increases of pollutant concentrations under shrubs following migration suggest that decreases would consequently occur at the source points. Some reflections on interpreting field work results are provided.

Willow Aboveground and Belowground Traits Can Predict Phytoremediation Services

“…Finally, traits related to evapotranspiration are an important aspect of plant ecophysiology that will be important to investigate in future studies on short-rotation coppice. These traits are likely to be a crucial driver of TE movement in the soil, with sometimes counterintuitive accumulation in the surface soil surrounding the plants, due to mass movements of TE with abundant transpiration in humid climates [ 48 ].…”

“…In addition, the observed increases in final soil TE (ex. Cu and Zn) can be explained by the strong evapotranspiration of willow, acting as a soil solution pump [ 49 ], and converging contaminants toward their root systems and the surrounding soil horizons [ 48 , 50 ]. However, the present study does not provide mechanisms for the phytoremediation patterns but demonstrates the interest in conducting additional comparative ecological studies to correlate plant traits with their ability to provide phytoremediation services.…”

Ectomycorrhizal Fungi Dominated the Root and Rhizosphere Microbial Communities of Two Willow Cultivars Grown for Six-Years in a Mixed-Contaminated Environment

“…Fast-growing willow shrubs (Salix spp.) present attractive economic values as woody crops for biomass production [22,23], and they shown interesting versatility to be used in various environmental projects, to minimize the leaching of pesticides from agricultural fields [24,25], to treat contaminated leachate [26,27], or to remediate contaminated soil [28][29][30]. Consequently, the microbial community of Salix spp.…”

Cowpea induced physicochemical and biological rhizosphere changes in hydrocarbon contaminated soil

“…On the other hand, physiochemical properties of soils have a direct in uence on speci c microbes and plant root exudates (Dumbrell et al 2010). Unless plants and microbes are able to adapt to the soil extremes or optimum soil conditions are attained, the remediation process may be halted, poorly implemented or take longer time than necessary (Ho et al 2017;Fortin-Faubert et al 2021).…”