Biostimulants efficacy in growing Sonchus oleraceus plants in contaminated mining soil with potentially toxic elements (PTEs)

Evaluating the effects of innovative amendments on potentially toxic element (PTE) availability is essential for managing contaminated soils and enhancing agricultural productivity. In the present work, we investigated the mitigation capacity of various biostimulant formulations (Trichoderma sp., Phosbactin, Bacillus sp., Azospir, and humic substances) in Sonchus oleraceus grown in a severely contaminated soil with potentially toxic elements (PTEs) from the Lavrio mining area in Greece. We aimed to determine the impact of biostimulants on PTE mobility in soil and their accumulation in plant tissues. For this purpose, we conducted a pot experiment with seven treatments, i.e., non-contaminated soil (negative control), contaminated soil from Lavrio (positive control; PC), and five biostimulant treatments added in the Lavrio soil, resulting in 105 pots (7 treatments × 15 replicates). All treatments were fertigated with nutrient solution (200 mg L^-1 of N-P-K) at regular intervals. Significant variations in S. oleraceus growth and PTE concentrations were observed. The Lavrio soil (positive control) showed reduced plant growth parameters compared to the negative control. Specifically, the average plant height notably decreased from 21.63 cm to 15.92 cm, while the fresh biomass dropped by 60.4% (from 15.50 g to 6.14 g), indicating a negative impact on plant growth processes. The application of biostimulants showed no notable differences on growth parameters, except for the reduced plant height compared to the non-contaminated soil (negative control). Certain biostimulants, like Phosbactin, Azospir, Bacillus spp. (BAC), and humic and fulvic acids, tended to cause an increase in the DTPA extractability of Pb, Cu, and Zn, although only BAC caused a significant increase in Zn (589.92 mg kg^-1) compared to the PC (381.20 mg kg^-1). Moreover, S. oleraceus accumulated increased PTE concentrations, particularly Cd, Zn and Pb. The highest concentration of Cd was found in the roots, reaching 39.42 mg kg^-1 at AZO, while Zn concentration peaked at 2911.99 mg kg^-1 at PH. The bioaccumulation factors (BAF) indicated a high uptake of Cd and Zn, with values of 5.831 and 3.221, respectively. Pb exhibited the highest mobility, with a transfer factor (TF) exceeding 1, especially at AZO (TF = 2.817). In contrast, Cd and Zn demonstrated limited translocation potential (TF < 0.5). S. oleraceus displayed a hyperaccumulator behavior, despite limited translocation to aerial parts, making it potentially suitable for phytoremediation of contaminated soils. These findings demonstrate the complex interactions among biostimulants, soil, plant responses and PTE concentrations, highlighting the importance of choosing suitable biostimulants for the sustainable management of PTE-contaminated soils. Further investigation is needed to identify biostimulant mechanisms mitigating PTE negative effects.