The assessment of the potential of two rangeland plants for absorption and accumulationof lead (Pb) in a contaminated calcareous soil

Introduction Recently, due to the enhancement of industrialization, urbanization, and disposal of wastes, fertilizers, and pesticides the concentration of heavy metals in agricultural soil has increased. Heavy metals are a serious threat to the environment due to their hazardous effects. Heavy metal contamination of the soil is of particular attention due to food security issues and several reported health risks to both human and living organisms. In addition, large areas worldwide are polluted by lead (Pb). One of the major problems in the process of Phytoremediation is the low solubility of heavy metals, such as lead in the contaminated soil. Phytoextraction is a solar-driven remediation technology which greatly reduces the costs and has minimum adverse side effects. Lead (Pb) is among the highly toxic and most common heavy metals at contaminated sites. It originates from various anthropogenic sources and causes a variety of health, environmental, and ecological problems. The weed plant species are usually of the quickly growing nature and have higher biomass under unfavorable environments. Their phytoremediation potential could be more effective in reducing food chain contamination and consequently the risk to human health. Therefore, the objective of this study was to assess the Pb remediation potential of Artemisia (Artemisia absinthium L.) and Xanthium (Xanthium strumarium L.) in contaminated calcareous soil. Materials and Methods This study was carried out under a greenhouse condition as a factorial experiment based on a randomized complete block design with two factors, including Pb concentration in four levels (0, 250, 500, and 1000 mg Pb kg-1 soil) and plant type in two levels of Artemisia (Artemisia absanthium L.) and Xanthium (Xanthium strumarium L.) and in three replications. In this study, the soil was selected and was spiked with 0, 250, 500, and 1000 mg Pb kg−1 soil.  Then plants were grown in pots containing the contaminated soil. At the end of the growth period, the dry weight of root and shoot, Pb concentration in the root and shoot of plants, and soil bioavailable Pb were measured. Also, the tolerance index (TI) of root and shoot was calculated by dividing the dry biomass of plant in each treatment by dry biomass in the control treatment at Pb0 mg kg-1 of the soil. Moreover, the stabilized Pb in roots (MS) and extracted Pb by shoots (ME) were calculated. For evaluating the ability of plants on uptake and shoot and root accumulation of Pb, mBCF (Modified bioaccumulation factor) and mBAF (bioconcentration factors) of shoot and root were calculated by dividing the Pb concentration in plant dry matter to bioavailable Pb concentration in soil and dividing the Pb accumulation in the plant fraction bioavailable metal content in the soil. In addition, the modified translocation factor (mTF) was calculated by dividing the Pb concentration in shoot dry matter by Pb concentration in root dry matter.   Results and Discussion Results of this study indicated that with increasing soil Pb contamination, the root and shoot dry weight and tolerance index of plants decreased, while shoot and root Pb concentration, stabilized Pb in roots and the extracted Pb from shoots increased. The highest and lowest relative shoot and root dry weight were observed in Pb0 and Pb1000 treatments, respectively. There was no significant difference in the tolerance index (TI) of plants. In this study, roots and shoots mBCF, obtained for both plants and different levels of Pb in soil, were above unity, indicating that the plant is able to take up and accumulate Pb. A. absanthium PGPR had higher mTF than X. strumarium plant at every concentration of soil Pb. The assessment of the phytoremediation performance clearly revealed that the amounts of all phytoextraction indices in A. absanthium were higher than X. strumarium, while all phytostabilization indices in X. strumarium were higher than X. strumarium.  In general, maximum Pb accumulation for root was recorded for X. strumarium (average of root mBAF, mBCF, and mTF 1.65 %, 5.48 and 0.97, respectively) and maximum accumulation of Pb in shoot was observed for A. absantium (average of shoot mBAF, mBCF, and mTF 2.79 %, 2.86, and 1.84, respectively). Conclusion It could be concluded that X. strumarium and A. absanthium, with high biomass in native condition, might be effective in phytostabilization and phytoextraction of Pb, respectively, especially in low levels of soil Pb contamination (250 and 500 mg kg-1).