## 1 Introduction  
Bioremediation, or the use of microorganisms to detoxify or remove contaminants, has great potential for the remediation of contaminated soils [1; 2; 3]. Bioremediation can be performed through: (i) natural attenuation, or the natural process of contaminant degradation; (ii) biostimulation, or the modification of the environmental conditions to stimulate the biodegradation ability of indigenous microorganisms; and (iii) bioaugmentation, or the introduction of exogenous microorganisms with the capacity to degrade the target contaminants [4; 5; 6]. Industrial soils are most frequently affected by the presence of more than one contaminant, thus hindering the application of biological remediation techniques. For instance, in petroleum contaminated soils, aliphatic and aromatic hydrocarbons [including polycyclic aromatic hydrocarbons(PAHs)] are often mixed [7]. Among petroleum-derived contaminants, PAHs are of particular concern since they can seriously affect human health [8]. Rhizoremediation (i.e., the use of plants and their associated microorganisms to remediate contaminated soils, usually as a result of the stimulation of the catalytic activities of soil microorganisms by plant roots) has great potential for the remediation of organically-contaminated soils [9; 10], owing to the fact that plant roots emit exudates that provide nutrients and energy for rhizobacteria, which makes rhizosphere microbial communities more abundant and active than those in bulk soil [11; 12]. In particular, rhizoremediation has been suggested to be the primary mechanism responsible for hydrocarbon degradation in soil [13]. Actually, some root exudates, such as oxalic acid and citric acid, have the ability to desorb PAHs, thus facilitating their degradation by soil bacterial populations [7]. On the other hand, bacterial consortia with the metabolic ability to degrade organic contaminants are often added to contaminated soils in a process called bioaugmentation [5; 14]. In addition to contaminant-degrading bacterial strains, plant growth-promoting rhizobacteria and bacterial endophytes (i.e., strains isolated from the interior of plant tissues) have demonstrated their potential for phytoremediation, owing to their ability to stimulate plant growth and/or protect plants against contaminant toxicity through several mechanisms [15; 16]. Thus, it has been reported [17] that endophyte inoculation can improve the physiological status of _Festuca rubra_ plants by increasing the content of carotenoids, chlorophylls and the Fv/Fm ratio (an estimate of the photosynthetic efficiency of photosystem II) by 69, 65 and 37%, respectively, while also enhancing the values of a variety of microbial indicators of soil health. Furthermore, organic amendments, such as animal manure and compost, are recurrently used to improve soil physicochemical (e.g., porosity, aeration, water holding capacity, structural stability, nutrient availability) and biological (e.g., microbial biomass and activity) properties [18; 19], as well as to promote plant colonization and growth, during biological remediation processes. Composing organic amendments can minimize chemical and especially biological risks, such as, for instance, the presence of potential human pathogens [20]. An often-mentioned paradigm within the soil remediation field is that \"the ultimate goal of any soil remediation process must be not only to reduce the concentration of the target contaminants but, most importantly, to also improve soil health\" [4; 21; 22; 23; 24; 25]. In this respect, many physicochemical methods of soil remediation are known to cause a negative impact on soil health. As a matter of fact, some of them have been tagged as more damaging to the soil ecosystem than the contaminants themselves [4]. Indeed, many soil remediation methods actively reduce the concentration of the contaminants at the expense of negatively affecting the integrity of the soil ecosystem, i.e., soil health.