**1. Introduction**

Global climate changes, pollution levels, and high-impact farming have induced a strong decline in soil quality, making the sustainable use of land more challenging than in the past [1]. For soils contaminated with trace element (TE) there are various phytomanagement options for reducing the environmental risks. Phytoextraction in conjunction with the application of conditioners can be considered as the way to minimize the dispersion and biological action of TE on soil and to increase vegetation cover on polluted soils [2–4].

**Citation:** Gorelova, S.V.; Muratova, A.Y.; Zinicovscaia, I.; Okina, O.I.; Kolbas, A. Prospects for the Use of *Echinochloa frumentacea* for Phytoremediation of Soils with Multielement Anomalies. *Soil Syst.* **2022**, *6*, 27. https://doi.org/10.3390/ soilsystems6010027

Academic Editors: Matteo Spagnuolo, Paola Adamo and Giovanni Garau

Received: 31 January 2022 Accepted: 14 March 2022 Published: 16 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Recently, the cultivation of non-food crops for the production of plant-based feedstock, the bioremediation of metal-contaminated soils, and risk managemen<sup>t</sup> have been developing at the field scale [5–9].

Chemically-induced hyperaccumulation is impaired by various environmental risks, e.g., metal leaching from the root zone and toxic effects on the microbiome [7,10]. Regarding secondary TE accumulators, the desirable characteristics for these plant species are (1) relatively fast growth and high biomass; (2) extended root system for exploring large soil volumes; (3) good tolerance to high concentrations of TE in plant tissues; (4) high translocation factor; (5) adaptability to specific environments/sites; (6) easy agricultural managemen<sup>t</sup> [11], (7) good interaction with associated bacteria [12,13].

Global climate change makes it possible to introduce relatively thermophilic new biomass crops for phytoremediation, especially species of the *Poaceae* family [14]. These plants, due to C4 assimilation pathways [15], are of particular interest for phytoremediation due to the production of significant biomass and, as a consequence, the accumulation of significant amounts of carbon and heavy metals.

The selective accumulation of Pb, Cu, Zn, and Cd in the roots and the possibility to remove the root remains makes technical sorghum, sugar sorghum, and *Sudan grass* extremely suitable for phytoremediation purposes [16]. Some sorghum cultivars show a high antioxidant status during the bioaccumulation of Pb and Cu on anthropogenically disturbed substrates with polyelement contamination [17].

The capacity of *Miscanthus* species to accumulate inorganic contaminants into the root system and to reduce the dissipation of persistent organic contaminants makes them good candidates for soil phytostabilization and phytodegradation. The noninvasive hybrid *Miscanthus giganteus*, with high lignocellulosic content, has a high potential in the biorefinery and bioenergy industries [18]. The applicability of *Arundo donax* for cadmium removal from contaminated soil and water was shown by Sabeen et al. [19].

*Echinochloa frumentacea* is a cost-effective crop through seeds and biomass production. It can be applied for sustainable crop rotation for the enhancement of soil development processes, nutrient cycles, and microbial community.

*Echinochloa frumentacea* was previously described as an effective extractor (As) ([20], responsive to fertilizers, hymexazole, and rhizobacteria inoculation [21]. There is the experience of using this species in the phytoremediation of polyelement-contaminated substrates such as the stabilization of municipal wastewater sludge [22] and phytoremediation of soil contaminated with cadmium, copper, and polychlorinated biphenyls [23]. *Echinochloa* species can also be used efficiently for chromium and cadmium extraction [24].

Other advantages are the sufficient diversity of varieties, their high seed germination (up to 10 years), that they develop well on soils poor in mineral nutrition, give from two to eight mows, and provide grain and green mass with average yields of 1.5–3 t/ha and 30–50 t/ha, respectively. The latter fact is very important for effective phytoextraction since TE removal by plants arises from two factors: (1) TE concentration in dry plant tissue [25] and (2) the amount of harvested biomass [9].

The acceptable concentration of TEs in plant raw material significantly expands the area of its valorization. Potential plant-based products are (1) non-food products such as biofuels [26], fibers, wood, essential oils, etc., and (2) animal feed, depending on the contamination level. Plant parts generally accumulate metals to different degrees [27], and some parts are usually less contaminated and may often be used for consumption or at least for non-food purposes even if the plant has been grown on contaminated or marginal soil [28]. Seeds generally accumulate the lowest TEs concentrations in a plant [29].

Other harvestable parts can be used in various processes, i.e., composting to fertilize TE-deficient soil, incineration, ashing, vacuum and oxidative pyrolysis, liquid extraction, the synthesis of hydrogen fuel, biofuels such as bioethanol, biogas, and activated carbons, hydrothermal oxidation, gasification, etc. In addition, metals can be recovered and reused for economic purposes by phytomining [30].

The objective of this study is to determine the phytoremediation potential and prospects for the use of *Echinochloa frumentacea* on urban soils with polyelement anomalies (especially for large urbanized ecosystems with high levels of industrial and vehicular pollution). During the study, the tasks were:


## **2. Materials and Methods**
