## 3 Results and Discussion
### Plant Response at Microcosm Scale  
Biomass data are shown in Table S2 and Figure 1. As can be seen, there are no significant variations in growth when the different contaminated soils are considered. For each species, the values of biomass produced also seem to depend little on the sampling depth (see samples description in Table 1). In general, plants sown in the contaminated soil did not demonstrate any visible signs of stress or phytotoxicity. In the case of microcosms not treated with PGPB, however, there is a tendency for lower biomass production than growth on uncontaminated soil (CT) taken as a reference. This is especially evident in the aerial part, with an average growth reduction of about 33% for _M. sativa_, 38% for _L. albus_, and 40% for _Z. mays_. On the other hand, the addition of PGPB shows a significant positive effect, which succeeds in overcoming plant growth difficulties, as confirmed by the production of fresh biomass at levels similar to or even higher than the control. This increase is up to 50% higher than in the respective case without PGPB, with results particularly evident in the aerial part of _L. albus_ in soil D and _M. sativa_ in soils A and B (Figure 1). This is because PGPB create an environment that allows the plant to withstand the pollution-driven abiotic stress by increasing the bioavailability of essential nutrients such as iron, nitrogen, and phosphorus, or by producing phytohormones like indoleacetic acid (IAA). This results in better plant health and, ultimately, higher biomass production, which is one of the core pieces of data needed to identify a successful phytoremediation intervention [14]. The hydrocarbon content in plants (Table 2) was evaluated for both the C > 12 fraction and the C \(\leq\) 12 fraction (of minor significance). To assess the origin of hydrocarbons in plants, plants were also grown (in duplicate) on control soil from organic farming, which was characterized by very low values of hydrocarbons, showing considerable variability in the concentration of these compounds. Since corn is the most widely grown plant in the area, special attention was paid in checking the behavior of this species, so as also to protect the food chain from the potential \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{_Zea mays_} & \multicolumn{2}{c}{_Medicago sativa_} & \multicolumn{2}{c}{_Lupinus albus_} \\ \hline  & **C \(\leq\) 12** & **C \(>\) 12** & **C \(\leq\) 12** & **C \(>\) 12** & **C \(\leq\) 12** & **C \(>\) 12** \\ \hline A & 0.91 \(\pm\) 0.16 & 215 \(\pm\) 100 & 4.10 \(\pm\) 0.9 & 931 \(\pm\) 217 & 0.83 \(\pm\) 0.28 & 227 \(\pm\) 51.6 \\ A+ & 1.07 \(\pm\) 0.49 & 207 \(\pm\) 35.3 & 4.30 \(\pm\) 1.59 & 988 \(\pm\) 144 & 1.16 \(\pm\) 0.53 & 240 \(\pm\) 42.1 \\ \hline B & 0.98 \(\pm\) 0.18 & 185 \(\pm\) 117 & 3.40 \(\pm\) 0.7 & 675 \(\pm\) 149 & 1.10 \(\pm\) 0.25 & 167 \(\pm\) 85.9 \\ B+ & 1.07 \(\pm\) 0.18 & 162 \(\pm\) 53.6 & 2.50 \(\pm\) 0.46 & 664 \(\pm\) 81.5 & 0.91 \(\pm\) 0.30 & 144 \(\pm\) 51.6 \\ \hline C & 0.90 \(\pm\) 0.22 & 228 \(\pm\) 133 & 4.60 \(\pm\) 1.0 & 1050 \(\pm\) 340 & 0.87 \(\pm\) 0.46 & 273 \(\pm\) 45 \\ C+ & 0.92 \(\pm\) 0.21 & 238 \(\pm\) 47.0 & 4.40 \(\pm\) 1.28 & 1073 \(\pm\) 239 & 0.95 \(\pm\) 0.49 & 260 \(\pm\) 97.0 \\ \hline D & 0.85 \(\pm\) 0.10 & 183 \(\pm\) 111 & 4.30 \(\pm\) 1.0 & 1271 \(\pm\) 381 & 0.84 \(\pm\) 0.33 & 264 \(\pm\) 68.6 \\ D+ & 1.01 \(\pm\) 0.14 & 180 \(\pm\) 39.7 & 5.10 \(\pm\) 0.35 & 1266 \(\pm\) 302 & 0.88 \(\pm\) 0.27 & 274 \(\pm\) 49.1 \\ CT & n.d. & 122 \(\pm\) 67.0 & 1.23 \(\pm\) 0.81 & 253 \(\pm\) 136 & 4.00 \(\pm\) 1.56 & 265 \(\pm\) 75 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Concentration (mg kg\({}^{-1}\)d.m.) of hydrocarbons (C \(\leq\) 12 and C \(>\) 12) contained in shoots of plants grown on different soils (A–D) compared to control (CT) during microcosm tests. Data reported as the mean of replicates with the related standard deviation. (+) indicates the addition of PGPB.