Handbook of Ecological and Ecosystem Engineering. Группа авторов
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for adsorbent and enzyme binding sites, as well as the double pressure effect on plants and soil microorganisms [128]. Heavy metals inhibit microbial metabolism and enzyme activity, reducing the degradation efficiency of organic pollutants [129]. Thus, promoting remediation in a synchronized manner is not a simple task.
Revitalization by ecological engineering: Physical and chemical technologies for soil remediation, such as washing, electrokinetic remediation, chemical oxidation, nanomaterials remediation, adsorption, filtration, granular activated carbon, and photocatalysis, can remove heavy metals from soils but affect different ecological functions of the soil by changing either the pH, moisture content, or oxidation potential [130, 131]. For example, studies conducted by Ma et al. [127] showed that electroremediation technology combined with activated bamboo charcoal was effective in the simultaneous removal of 2,4‐dichlorophenol and Cd from a soil; however, the application of such technology seems to decrease the water content in soil significantly. Likewise, the studies of Villa et al. [130] showed that the Fenton process, despite being very efficient for removing DDT (dichlorodiphenyltrichloroethane) and diesel from contaminated soil, creates oxidizing conditions that contribute to the dissolution of organic matter initially present in the soil and increase the water solubility of hydrophobic compounds (like DDT), accelerating the possible contamination of ground and surface waters. Nevertheless, in some situations, the conventional Fenton process can be effective by treating the soil with lower organic content and pH and can be integrated with biological remediation, which is more economical and ecological [132]. Other technologies include soil replacement, consisting of diluting the contaminated soil with a large amount of clean soil, covering it, and mixing it with contaminated soil. This approach very effectively reduces the toxic effect of the heavy metals in soil but requires large volumes of work and earthworks that disturb the functioning of these ecosystems [133].
Revitalization by ecosystem engineering: Bioremediation technology minimizes the toxic effect of certain heavy metals in less toxic forms, revitalizing natural ecosystems through the use of microorganisms and enzymes that purify the contaminated environment [131]. Other variants include the remediation of heavy metal pollution in soil using a combination of microbial immobilization and carbon microspheres [134]. This approach leads to the formation of stable bacterial colonies and reduces heavy metal pollution. However, in the case of soils contaminated with heavy metals, this technology has some limitations: (i) heavy metals are not biodegradable; (ii) microorganisms produce toxic metabolites, which are eliminated during the process; and (iii) the microorganism's response to heavy metals depends on the concentration and availability of the heavy metals [131]. It is more efficient when associated with phytoremediation technology through symbiotic associations with plants [23].
Nanobioremediation is an emerging technology that uses biosynthesized nanoparticles for the decontamination (adsorption) of heavy metals from contaminated soils. The biological synthesis of nanoparticles using plant extracts or microorganisms can be an ecological and economical alternative to chemical and physical methods [135].
Biological remediation technologies like phytoremediation use plants' abilities and mechanisms (phytodegradation, phytostabilization, phytoextraction, and phytovolatilization), together with their microbial consortia, to effectively reduce the bioavailability of toxic heavy metals and degrade organic pollutants by metabolic decomposition [136–139]. These technologies have lower remediation costs and can be applied in situ and on a large scale, maintaining ecological sustainability through the use of plant organisms capable of restoring soil functions [140–142].
Recently, bioenergy crops have been considered for the simultaneous purpose of ecological restoration and biomass production [36–40, 143]. For example, switchgrass can be used to decompose (phytodegradation) and remove contaminants from the soil, including herbicides, trinitrotoluene, polychlorinated biphenyls, polynuclear aromatic hydrocarbons, chromium, and radionuclides [144]. Castor bean (Ricinus communis L.) is a primary colonizer suitable for dealing with toxic conditions in soils, probably because arbuscular fungi may be involved in root metal sequestration. Although it is not a metal accumulator, it can be useful in stabilizing mine tailings, promoting the reduction of the heavy metal's bioavailability, dispersion, and concomitant risks to human health [145]. In addition to extracting most toxic metals (Cd, Pb, Ni, As, Cu, etc.) as well as organic contaminants (pesticides), the plant can increase soil fertility and reduce soil erosion and is also important for generating biodiesel and employment/social development in local communities [146]. Miscanthus spp. can also tolerate soils that are poor in various elements, such as nitrogen, and contaminated with heavy metals and hydrocarbons, making it suitable for phytostabilization [147]. Aided phytostabilization technology combines soil amendments with the plant consortium and associated microorganisms and is well‐suited to stabilize mine tailings and acid mine drainage [148]. A. donax and Miscanthus spp. can also be used in the phytoextraction of Zn from soils and the phytostabilization of heavy metal contamination and preventing leaching/groundwater contamination [149]. The energy crops P. purpureum and Pennisetum thyphoideum can also be used successfully to aid the phytostabilization of mine tailings rich in the heavy metals Cd, Zn, and Pb [82].
This ability of certain energy crops to tolerate heavy metal contamination has a genetic cause: in giant reed, for example, the transport and detoxification of heavy metals seems to be related to the expression of glutathione reductase genes, and antioxidant activities and gene expression are specific to a particular toxic metal [150]. This knowledge can be important in genetic engineering. In addition, the inoculation of plant growth‐promoting bacteria may also increase efficiency in aided phytostabilization [83]. Also, mycological assisted phytoremediation enhancement of bioenergy crops can be a suitable approach to increase the efficiency of this technology. The inoculation of fungi in energy crops such as corn and sunflower and favoring their phytoremediation efficiency in soils contaminated with Cr, Cd, Pb, and Cu also increased their respective biomass productivity and oil production in sunflower [151].
2.3.3 Salinity and Sodicity
Description of the marginality factor: The salinization process is focused on arid and semi‐arid regions and depends on the soil texture and relative water and salt content [152, 153]. It consists of changes in the patterns of precipitation, evapotranspiration, and landscape hydrology caused by the presence of chlorides, sulfites, nitrates, and bicarbonates of sodium, calcium, magnesium, and potassium that affect the microbial activity of soils and vegetation zonation and cover [153]. Since natural biological, biochemical, hydrological, and erosional earth cycles can be affected by the salinization process, environmental and agricultural services can also be lost [154].
Revitalization by ecosystem engineering: The species A. donax L. can be classified as “moderately salt‐tolerant” [155]. At 42 dS/m, the giant cane's overall growth is affected, but with zero mortality [155]. Gas exchange of giant reed leaves and their overall tolerance to salt is greater than in many food crops (like corn and rice) and energy crops such as Miscanthus, Populus, and Salix that are indigenous in regions where giant reed is now an invasive species [155]. Although A. donax L. is naturally tolerant, some research has tried to improve this tolerance to salts and select which varieties are best adapted to these stress conditions. Both salinity and water stress affect stomatal closure and eventually decrease the photosynthetic rate of this crop, but that some giant reed ecotypes are more tolerant to salinity than to water stress [156]. Also, the inoculation of arbuscular mycorrhizal fungi (Rhizophagus intraradices and Funneliformis mosseae) in giant reed can improve phosphorous extraction by the roots and the plant's overall tolerance to salinity. The plants' nutritional status has been improved along with the efficiency of nutrient use and response to saline stress conditions [157].
Miscanthus tolerance to saline conditions seems to be lower than that of giant reed. Biomass yields of Miscanthus spp. were reduced by 50% at 10.65 dS/m NaCl [158]. In greenhouse experiments, upland switchgrass showed low tolerance to 5 dS/m (with a reduction of 77% in the aboveground biomass, dry weight), while the lowland ecotype showed higher tolerance (with a reduction of 20% in the aboveground biomass, dry weight) [61].
2.4 Economic Revitalization of Degraded