89]. Bioengineering techniques for slope stabilization and landslide mitigation are already using many plant species that are being investigated for bioenergy purposes. The roots of giant reed and P. purpureum, for example, can act in sloping areas by a variety of mechanical mechanisms: (i) they reinforce the soil, increasing shear resistance, like jute nets, jute reinforcement grids, and geotextiles; (ii) they provide support to slope soils through buttress and arching, since they can anchor deeply in firm strata, similar to shear wrenches and screws or pretensioned rock anchors; (iii) they help compact the soil, increasing the densification and solidification of soil materials, similar to wooden, concrete, or lime piles; and (iv) they unite surface soil particles, increasing cohesive and adhesive forces and reducing susceptibility to erosion and landslides [90, 91].
2.2.5 Dryness
Description of the marginality factor: Dryness is a severe water stress that induces changes in the maturity time of plants and losses in their biomass yields, causing leaf senescence and increasing the stem/leaf ratio as well the translocation of nutrients to the roots [92, 93]. Dryness induces the closure of stomata, affecting the photosynthetic process and leading to less water loss through transpiration [94, 95]. Dryness can also be seen as the ratio between annual precipitation and potential evapotranspiration in a particular area, causing marginality [61]. When dry conditions develop in soil, its surface becomes dry and hard, making root penetration difficult. However, in some soils, the deepest layers can contain enough moisture for plant growth. Even if some roots manage to penetrate these deeper layers, deleterious effects should be expected to occur in the more superficial soil layers [96].
Revitalization by ecosystem engineering: In dry conditions, cytokinin concentrations on plants decrease, stoma closes, and growth is redirected from the aerial components of the plants to the underground ones. The inoculation of plants with cytokinin‐producing bacteria in water deficit conditions can be an efficient approach to overcome dryness [97]. Drought tolerance of the bioenergy crop giant reed (A. donax L.) is high, and it is well‐suited to warm, temperate, and semi‐arid environments with high temperatures and prolonged summer dryness [98]. The same authors pointed out Miscanthus spp., switchgrass, and other fast‐growing, fast‐yielding energy crops exhibited high tolerance to drought [65]. However, during the establishment phase of these perennials, it is essential to avoid drought. From the second year onward, drought tolerance increases since rhizomes are already established, reaching water sources from deeper soil horizons [61]. Despite this, the productivity of these crops can be affected by drought.
Techniques such as mutagenesis have been identified as valid strategies for increasing the genetic diversity of giant reed, increasing its performance under dry soil conditions [99]. Moreover, giant reed can maintain its photosynthetic capacity and leaf growth to minimize the stress caused by drought [100]. In fact, giant reed can change its water sources depending on water availability, maximize water uptake efficiency, and satisfy evapotranspirative demands [101, 102]. When water is scarce, the giant reed species can improve the efficiency of water use and maintain high productivity. This fact indicates that the species can grow under conditions of moderate irrigation during periods of drought.
Saccharum spontaneum spp. aegyptiacum is another potential bioenergy crop species that produces high biomass yields while tolerating environments characterized by drought stress, high temperatures, and high vapor pressure deficit [94]. It is a C4 plant with a rapid growth rate, few or no natural enemies, and very active assimilation rates during drought‐stress periods, and it can use water very efficiently [103]. Saccharum spontaneum spp. aegyptiacum is so well adapted to drought conditions in Southern Europe that it displays biomass yields even higher than Miscanthus spp. and A. donax grown in the same experimental conditions and under well‐watered, rainfed conditions [104].
Reed canary grass shows high plasticity and is one of the best candidates for most marginal land revitalization. It can be distributed in and dominate various habitats, including wetlands and riparian zones; and in a cool temperature climate, it can withstand flooding for long periods and exhibits excellent drought tolerance [98]. The energy crops perennial ryegrass (Lolium perenne L.), Panicum spp., and Sorghum spp. are also well adapted and resistant to drought conditions [105, 106].
2.2.6 Waterlogging
Description of the marginality factor: Waterlogging means excess water in the root zone. Excessive soil moisture adversely affects crop growth and development by causing restricting rooting conditions and low‐oxygen stress and reducing soil strength for workability and trafficability [10, 107]. Flooding induces changes in soil pH and redox potential [108]. Today this factor affects up to 10% of global land area [109].
Revitalization by ecosystem engineering: Flooding tolerance found in halophytes and glycophytes is often associated with the production of adventitious roots containing aerenchyma, resulting in an internal supply of O2 and preventing anoxia in tissue [110]. Flooding causes a reduction in the photosynthetic capacity of many plants, such as L. perenne [108]. Waterlogged marginal land can be revitalized through the adaptation and growth of plants for biomass production.
Flooded soils are characterized by the absence of oxygen and reduced chemical conditions. The adaptation of a plant species to these conditions depends on the growth capacity of its roots and its aeration. The effectiveness of aeration within the roots of plant species such as Vetiveria zizanoides, wetland grass species, sorghum, maize, and sugarcane is determined by: (i) anatomical characteristics such as the length of the aerenchyma, dimensions of the stele, cortical cell layout pattern, and existence of barriers to radial O2 losses; (ii) morphological characteristics such as root diameter and the number of lateral roots; and (iii) physiological processes, such as respiratory demand for oxygen [111]. Sunflowers and maize also display adaptations to flooded conditions/low levels of oxygen in the soil, such as elongation of the roots and inhibition/reduction of the growth aerial plant components [112]. Aquaporin genes of Sorghum bicolor L. seem to be related to the answer to waterlogging stress [113]. Liu and Jiang [114] reported that waterlogging and submergence stress decreased the root activity of SOD, catalase, peroxidase, and ascorbate peroxidase in perennial ryegrasses, indicating that maintaining antioxidant activity and carbohydrate and minimizing lipid peroxidation could contribute to better waterlogging and submergence tolerance in that bioenergy crop. Giant reed quickly adapts to flooding conditions with the absence of oxygen. After prolonged oxygen scarcity, the species can readjust and restore its physiological conditions. Depending on the severity of the stress, it can adapt its uptake of photosynthetic CO2, related to a combination of stomatal and mesophyll diffusional constraints [115]. Japanese sugarcane and S. bicolor are pointed out as bioenergy crops tolerant to multiple environmental stresses in marginal land, tolerating waterlogging conditions [116].
2.3 Revitalization of Chemical Factors
2.3.1 Acidity
Description of the marginality factor: Soil surface acidity can reduce nutrient availability in soil solutions, increasing the concentration of heavy metals (especially aluminum) and their toxic effects on decomposition and nitrogen fixation microbial communities [61, 117, 118]. The occurrence of toxicity caused by aluminum depends on the interaction of this element with other limiting factors such as nitrogen, phosphorus, pH, and iron [119].
Revitalization strategy: According to Zhao and collaborators [119], three proposals can be applied to acidic soils to improve plant growth: (i) combined application of lime and nutrients, depending on the type of soil and plants; (ii) identification and development of tolerant plant species of multiple stressors; and (iii) inoculation of plants with beneficial microorganisms.
Revitalization by ecosystem engineering: Plants can reduce the toxicity of aluminum ions by exuding
