# The Suitability of Short Rotation Coppice Crops for Phytoremediation of Urban Soils
## 1 Introduction  
Environmental contamination from potentially toxic elements (PTE) is one of the most important issues at global level, especially in urban areas, as they are ubiquitous contaminants and could represent a threat to the health of the citizens [1; 2; 3]. In urban areas, different contamination sources coexist, with both site-related (point) and non-site (diffuse) sources, such as dust deposition, as well as traffic and parent material [4]. Current remediation methods, including excavation and transport to landfill sites, chemical and thermal treatments, incineration, and vitrification are the most frequently applied [5; 6], although they have high operational costs and, potentially, a higher social and environmental impact in urban areas, for example negatively affecting the biological fertility of the soil for a new use [7]. In addition, these methods are suitable for point pollution but not for diffuse contamination, especially over large areas. In the past decade, alternative techniques have been of increasing interest in an environmental sustainability perspective. In addition, there is public awareness on sustainable remediation together with the notion that soil is a non-renewable resource. Bioremediation techniques, mediated by biological agents such as microorganisms or plants, encompass a number of green and sustainable remediation options. Among these, phytoremediation has been studied as both an environmentally friendly and a cost-effective practice, in particular in areas where pollutants occur at relatively low concentrations [8; 9]. These areas are the most frequently found in cities due mainly to historic contamination from diffuse sources [10]. Moreover, in urban and suburban sites, soils can undergo a lower pressure for use: in fact, areas allocated to public green would have no alternative uses and could be considered stable in the short and medium term. These two aspects, which are relatively unique to urban and suburban areas, make phytoremediation a particularly attractive technique [11]. In urban areas, the soils provide also ecosystem functions, e.g., as barrier protecting water bodies from contamination, and provide an aesthetic and recreational essential function; thus an urban phytoremediation field trial would be a benefit to the landscape. Among phytoremediation techniques, phytostabilization and phytoextraction are the most used for PTE. Phytostabilization uses plants and microorganisms to diminish the PTE mobility in soil and transfer them to other compartments, through mechanical and biochemical processes [9]. Within an urban framework, phytostabilization techniques could be used as an initial step towards remediation because, although PTEs could be stabilized, the concentrations do not decrease and, if soil particles are moved (e.g., after wind or water erosion) or change oxidation state (e.g., due to flooding episodes), contaminated particles could exert their toxicity in their new environment. Phytoextraction has, conversely, the aim to remove the contaminants from the soil by either using hyper-accumulator plants [12] or plants with a lower specific uptake of pollutants but high growth rate and biomass production [9; 13]. The possible drawback of phytoextraction methods is that usually it requires long reclamation times to decrease total metal concentrations [14]. Based on field biomass production on moderately contaminated soils, phytoextraction would take decades to diminish concentrations to legislative values [9], thus the decrease of the bioavailable fraction would be a more realistic objective [15], as the readily soluble forms of PTE are the most dangerous for the environment [16]. This technique is currently the subject of many laboratory-scale and greenhouse experiments but is still rarely used in field trials, in particular in urban contexts [15; 17]. In open field conditions, the success of the phytoextraction depends on time, climate, and on the characteristics of the contaminated matrix.