## 4 Discussion  
### Contaminant Concentrations  
In this study, different remediation strategies were applied in an attempt to (i) decrease the concentration of the target contaminants (TPHs, PAHs) in the studied industrial soil, and (ii) improve soil health. Apart from biostimulating soil microbial biomass and activity, the application of organic amendments provides beneficial macro- and micronutrients for plant establishment and growth, and often improves soil physicochemical characteristics (and, hence, soil health).  
The industrial activity in the studied site, where the soil was collected from, began in the early 1970s and lasted until the mid-2000s, so it appears conceivable that a significant portion of the organic contaminants present in the collected soil were strongly bonded to the soil matrix and recalcitrant to biodegradation. Then, prior to the application of these remediation treatments, the contaminated industrial soil was supplemented with rhamnoid biosurfactant and sodium dodecyl benzenesulfonate (an ionic surfactant) to increase the bioavailability, and then degradability, of the sorbed recalcitrant organic contaminants. Regrettably, as mentioned above, the applied remediation treatments were not able to significantly reduce the concentration of the target contaminants. Actually, at the end of the experiment, there were no significant differences among treatments, including the untreated control (unplanted, non-bioaugmented control). Then, the observed reduction in contaminant concentrations, compared to the initial values (see above), was most likely due to soil manipulation, not to the applied remediation treatments. One of the possible reasons for this lack of contaminant degradation might be that the concentrations of the surfactants used here (rhammolipids: 10 mg kg\({}^{-1}\) soil; sodium dodecyl benzenesulfonate: 50 mg kg\({}^{-1}\) soil) were not high enough to achieve the desired objective. We chose these surfactant concentrations following [PERSON] et al. [58] who reported highest reductions in PAH concentrations in a long-term contaminated soil using these very same concentrations. Nonetheless, in another study [59], the most effective degradation of carbendazim (a stable benzimidazole fungicide) was achieved at 50 mg rhamnoid kg\({}^{-1}\) soil (in contrast, the application of 150 mg rhamnoid kg\({}^{-1}\) soil inhibited carbendazim degradation). Other authors [60] observed that the application of rhamnoids at 150 mg kg\({}^{-1}\) soil was most appropriate for the remediation of a diesel-oil contaminated soil through bioaugmentation with a bacterial consortium. The most appropriate (most effective) dose of surfactant depends on many different factors, including the nature, concentration and recalcitrance of the target contaminants, as well as the soil type and its physicochemical characteristics. On the other hand, the application of organic amendments to soil, in addition to promoting plant growth and stimulating soil microbial activity, can sometimes result in a chemostabilization effect on some contaminants, thus hindering their degradation. This chemostabilization effect has nevertheless been more reported for toxic heavy metals [61].