Industrial hemp (Cannabis sativa L.)—a valuable alternative crop for growing in agricultural soils contaminated with heavy metals

Hemp (Cannabis sativa L.) is a multiuse plant, which has been abundantly studied for phytoremediation purposes in recent years. The majority of experiments were performed in greenhouses with potted plants where hemp showed promising results. Only few studies tested hemp on site in heavy metal–polluted agricultural soil in real environmental conditions and practical assessments of hemp phytoremediation feasibility are lacking. We conducted a comprehensive study using 2 legal industrial hemp varieties (Futura 75 and Tisza) at three differently polluted locations (heavily polluted location, HP; moderately polluted location, MP; and slightly polluted location, SP) in the heavy metal contaminated Celje valley in Slovenia and determined the content of Pb, Zn, and Cd in 5 plant organs/tissues. The yield of each organ/tissue was determined as well to enable us to calculate the phytoremediation potential (PP). On average, plants grown in the HP location accumulated the highest values of all examined elements, followed by plants from the MP location and plants from the SP location, showing that the content of heavy metals in soil influences the accumulation in plants. Accumulation of Pb/Zn/Cd by plant organs/tissues was distributed in the following order: inflorescences (Pb-4.10/Zn-92.8/Cd-0.50 mg/kg) > seeds (Pb-1.79/Zn-92.6/Cd-0.27 mg/kg) > roots (Pb-1.15/Zn-15.0/Cd-0.44 mg/kg) > stem bark (Pb-0.42/Zn-12.4/Cd-0.23 mg/kg) > stem woody core (Pb-0.34/Zn-4.6/Cd-0.15 mg/kg). The only exception was for Cd, where roots accumulated a higher value than seed, yet lower than inflorescences. PP was calculated by multiplying hemp tissue/organ yield by the relative concentrations of heavy metal. The highest PP for Pb and Cd were achieved at the HP location (3.80 and 0.23 g/ha/vegetation period). On the other hand, tissue/organ yield was more important for high PP of Zn, where the SP location reached the highest PP for Zn (148.5 g/ha/vegetation period) due to the highest yields. Only seeds from HP and MP locations accumulated a too high content of Pb; otherwise, all other fibers and seeds can be safely used in the textile and food industry. Results of this study showed that hemp cannot be considered an efficient plant for the phytomanagement of contaminated areas. Nevertheless, hemp cultivation in heavy metal–polluted agricultural soils seems feasible since the majority of tissues/organs were not contaminated and different products can be obtained from various parts of the hemp plant.