## 1 Introduction
Hyperaccumulators are plants that can accumulate PTEs in their live tissues at levels which can be a hundred or thousand times higher than is common for a majority of plant species [16]. In hyperaccumulators, the process takes place without any influence on their growth and development [16; 17]. Hyperaccumulators are able to translocate xenobothics into aerial parts of the plant, so that the accumulation in shoots is higher than that in roots [16; 17]. The concentration of PTEs in the shoots should also be higher than their concentration in the soil [17]. It has been shown that wastes from agricultural production can be used in biogas plants [18], which can ensure material reuse, energy recovery, and control of greenhouse gas emissions. The use of mixed cropping systems is nowadays very promising. These systems increase diversity, bring better protection against weeds, and reduce the loss of water from agricultural soils [19; 20]. Moreover, the biomass from mixed cropping systems is suitable for biogas production [19; 20]. The use of two or more crops in one cropping system may have a higher phytoremediation effect, which can subsequently influence the outcome of biogas production. Moreover, the use of various plant materials in anaerobic digestion can significantly affect the composition and utilization of digestate [21]. Supplementation with the resultant digestate resulted in increased yield and nutritional composition of various grown crops in further cultivation [21; 22; 23]. Therefore, harvesting technologies have recently been intensively studied [24]. However, reutilization of plants after the phytoremediation of PTEs can pose a risk of these pollutants being released back into the environment due to the decomposition of plant residues [14; 16]. When these plant residues are used as a forage, PTEs can accumulate in the food chain and become a risk to human health. Therefore, many technologies for the removal of plant residues are used, such as pyrolysis [25]. Biochar from pyrolysis can be used as a sorbent of dyes or in agriculture [26]. Recently, the production of biomethane from phytoremediation of plant residues has been researched. This method may provide a possible compensation for the costly phytoremediation process [27]. In this paper, the possible effectiveness of white sweet clover for accumulation of PTEs (potential inhibitors of biomethane production) is assessed. White sweet clover is a legume, which means that it is able to assimilate nitrogen, thanks to the symbiosis with Rhizobium sp. microbes. Using legumes on arable land results in increased content of N in the soil through biological fixation [18; 19; 20]. This effect leads to the reduced need for mineral fertilizers. The reduced application of mineral N fertilizers is in line with the European Green Deal, which expects that mineral N fertilizers will be reduced by 20% by 2030 [28]. White sweet clover (_Meliotus albus_ Med.) contains coumarin, a secondary metabolite that plays a role in defense against pathogens. There are also indications that coumarins are important in Fe deficiency responses and even in the induced systemic resistance of plants [29]. Coumarines have a negative impact on anaerobic digestion if the biomass containing them is used for biogas production [30]. This causes lower production of methane during anaerobic digestion. However, [PERSON] et al. [31] found that feeding the biogas reactor microbiome with coumarin-rich feedstocks while maintaining coumarin concentrations below 0.5 g/L allows microbes to adapt to coumarins through structural and functional community reorganization and coumarin degradation [21]. The main goal of this study was to evaluate the ability of phytoextraction of various PTEs, using the tested crop _Meliotus albus_ Med., Meba, on soils with naturally abundant PTEs. The partial goal was to find out whether, in the case of excessive accumulation of PTEs in the plant biomass, this biomass can be potentially toxic for further use, for example in biogas or silage production.