**4. Discussion**

Summarizing the data on the quantitative content of the studied plant antioxidant system components of *Echinochloa frumentacea* showed that the plant is characterized by

a good adaptive potential on soils with polyelement anomalies caused by technogenic pollution of metallurgical industries: the content of carotenoids increases from 16 to 34% on the soils of the SPZ of metallurgical enterprises. The amount of AA and GSH in plants grown on experimental soils are 4–19% higher compared to the soils of the background zone (Table 9).

**Table 9.** Components of the *Echinochloa frumentacea* antioxidant system on soils with polyelement anomalies.


↑ higher or ↓ lower compared to background soil.

However, intense pollution of the soils of the Embankment caused inhibition of the synthesis of carotenoids and consumption of the synthesized low molecular weight antioxidants (ascorbic acid and glutathione) necessary for the urgen<sup>t</sup> adaptive response of the plant to prevent the effects of oxidative stress [74]. In bean and rice HM-stressed plants, other authors also observed a significant increase in the content of these antioxidants compared to the control [75,76].

*Echinochloa frumentacea* on soils with multielement contamination accumulated similar amounts of Mn compared to the literature data for *Echinochloa colona* (L.) Link (213 mg/kg) [15]. The accumulation of Zn by shoots of Japanese millet was 2 times lower than for *Echinochloa colona*; however, on soils with the highest Zn content, its accumulation increased by 1.7 times.

The calculation of the removal of elements from soils by *Echinochloa frumentacea* during the growing season of 1.5 months is presented in Table 10.

**Table 10.** Removal of elements from soils *Echinochloa frumentacea* in the case of polyelement pollution, g/ha.


\*—*p* < 0.05 for the difference between experimental and background samples.

The calculation of the removal of elements on the dry biomass of plants showed that on experimental soils with polyelement anomalies, the analyzed plant is not suitable for

Pb and As phytoremediation. The removal of these elements from the soil by shoots was 0.35–084 g/ha and 0.22–0.49 g/ha, respectively. Bioaccumulation of Pb was at its maximum in the root system and constituted 1.57–7.38 mg/kg dry weight. The content of Pb in the shoots was lower and varied within 0.14–0.88 mg/kg dry weight. At the same time, in plants grown on soils with a multiple excess of the MPC for Pb by the end of the growing season, up to 7.42 mg/kg of dry weight of Pb was accumulated in the aboveground mass. These figures are lower compared to those available in the literature for other cereals, e.g., sorghum 107–378 g/ha [77].

The content of V in the root system of *E. frumentacea* on contaminated soils reached 9–48 mg/kg dry weight, depending on the soil. At the same time, the root system performs a barrier function and the transfer of V into the shoots was very low, 0.01–0.05. The content of V in the shoots was 0.43–0.87 mg/kg dry weight. The removal of V from contaminated soils by shoots was 0.89–1.12 g/ha.

The amount of Ni and Cu removed from the polluted soils increased with an increase in pollution level and was 5.26–5.95 g/ha for Ni and 34–52 g/ha for Cu. The content of Cu in the shoots was 11.5–27.6 mg/kg of dry weight, and in the root system, 16.5–96.6 mg/kg of dry weight. The plant can be used for long-term phytoextraction of the indicated elements when the MPC is exceeded up to 2 times.

The removal of manganese from soils, in which its content exceeds the MPC more than 4 times, amounted to 290 g/ha. These values are not sufficient for remediation of the soil when MPC is multifold exceeded because the remediation period can be more than 20 years.

The removal of Zn from contaminated soils was 168–508 g/ha and it was maximal on the most polluted soils. The content of zinc in the shoots of *E. frumentacea* grown on soils, where the MPC was exceeded by less than twice, was 109 mg/kg; while at multiple excess of MPC (4.4 times) this increased to 454 mg/kg of dry weight. Zinc is a mobile element and is well accumulated by the plant. The removal of the element for *Sorghum bicolor* and *Zea mais* due to the formation of the maximum biomass, according to the literature data, was 1223–1998 g/ha [77]. An increase in the accumulation of zinc and the removal of the element from contaminated soils makes it possible to recommend this culture for phytoremediation in grass mixtures with other crops, for example, with a representative of cruciferous plants, *Brassica napus*.

Studies on the rhizosphere microflora of *Echinochloa* plants are rare [78–80], and we did not find any data on the impact of technogenic soil pollution on the structure of microbial communities in the root zone of *E. frumentacea*. At the same time, the study of the root microflora of remediating plants and the isolation of rhizospheric microbial strains that are resistant to the presence of pollutants (metals) in the environment and have a plant growth stimulating potential will make it possible to find a suitable microbial inoculant as a tool to increase the effectiveness of the remediation capabilities of *E. frumentacea* used for the restoration of technogenically disturbed soil ecosystems. Previously, a positive effect on the growth of *E. frumentacea* when inoculated with PGPR (plant growth-promoting rhizobacteria) strains was shown both in pure soil conditions [81,82] and in the presence of heavy metals (Cd, Ni, Pb, Cu) and As [83].

It is known that heavy metals have a grea<sup>t</sup> effect on bacterial communities of soil, contributing to an increase or decrease in bacterial abundance, species diversity, and alterations in dominant and subordinate species [84]. However, a negative effect of heavy metals on the composition and abundance of soil actinomycetes and fungal communities is also noted [85–87].

In this study, we demonstrated that the abundance of the main groups of microorganisms (bacteria, actinomycetes, and micromycetes) in the rhizosphere of *E. frumentacea* depended on the type of soil in which the plants were grown and, probably, was determined by its characteristics. The minimal number of all groups of microorganisms studied (on average, two times lower than in the uncontaminated control soil) was noted in the Embankment soil, which contains the highest concentrations of both inorganic (heavy

metals) and organic (oil products) pollutants. A two-fold increase in the number of bacteria was observed in the rhizosphere of plants grown in the soils of Tulachermet and Lenin Ave., a 1.7-fold increase in the number of actinomycetes, and 1.5-fold increase in the number of micromycetes were observed in the soil of KMP, which could be due to specific plant stimulation of microbial taxa resistant to heavy metals.

The rhizosphere populations of *E. frumentacea* grown in the background soil of Yasnaya Polyana were characterized by the lowest taxonomic diversity compared to the rhizomes of plants grown in contaminated urban soils. The species richness index was maximal for rhizosphere communities of plants grown on Tulachermet soils. The taxonomic structure of the rhizospheric microbiomes of *E. frumentacea* was represented by the dominant bacterial phyla Proteobacteria and Actinobacteria, as well as the phyla Planctomycetes, Chloroflexi, Acidobacteria, and Bacteroides. Compared to the control soil, the cultivation of *E. frumentacea* plants in urban soils led to a change in the ratio of the main phyla. The share of Proteobacteria (~52% in control, 27–36% in urban soils) and Firmicutes (>4% in control, <2–4% in urban soils) decreased, but the shares of Actinobacteria (~18% in control, 22–31% in urban soils), Planctomycetes (~4% in control, 6–10% in urban soils), Chloroflexi (~3% in control, 6–7% in urban soils), Acidobacteria (~4% in control, 5–9% in urban soils), and Bacteroides (~5% in control, 4–8% in urban soils) increased (Table A1).

The data of metagenomic analysis characterizing the rhizospheric microbiome of *E. frumentacea* plants grown on different soils make it possible to assume that the dominant families are Gaiellaceae and Nocardioidaceae (Actinobacteria), Sphingomonadaceae, Hyphomicrobiaceae, Comamonadaceae, Oxalobacteraceae, and Xanthomonadaceae (Proteobacteria), and the families Pirellulaceae (Planctomycetes), Cytophagaceae, and Chitinophagaceae (Bacteroides) can participate in the formation of the rhizobiome of *E. frumentacea* (Table A1). Sun et al. (2018) suggested a potential ecological role for Gaiellaceae in metal-contaminated soils, finding their increased numbers in sequencing libraries and a close significant correlation with various metals' or metalloids' content. In our study, in four out of five soil samples, the Gaiellaceae family was dominant, reaching a maximum in the Tulachermet soil. It can be assumed that representatives of the Gaiellaceae family can form the so-called "core" rhizobiome of *E. frumentacea* plants, being present in its rhizosphere under various conditions. Representatives of the Planctomycetes and Bacteroides phyla also seem to contribute to the formation of the "core" rhizobiome of *E. frumentacea*. Thus, Pirellulaceae (Planctomycetes), Cytophagaceae, and Chitinophagaceae (Bacteroides) families make up a significant share in the rhizospheric microbial populations of plants grown in various soils.

Thus, the qualitative and quantitative changes in the rhizospheric microbial populations of *E. frumentacea* under the influence of technogenic soil pollution have been found.
