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Limited Soil Drainage and Shallow Rooting Depth

Description of the marginality factor: Part of the growth and survival strategy of plants is related to the depth and distribution of their roots, especially in ecosystems where water is a limiting factor [65]. Here, roots play a predominant role for hydrological and biogeochemical cycling [66]. The depth of rooting is governed by the depth of water infiltration and is strongly influenced by the soil's properties, the moment and magnitude of water intake, and evaporative demand [66]. Plant species exposed to water stress, such as slight soil moisture deficiencies, have deeper root profiles [66]. In general, roots tend to be deeper in ecosystems limited by water, colder regions, humid climates, or places where the rainy season occurs during the winter. The deepest roots are found in arid environments where a long dry season occurs and water is limited, and where the penetration of the roots seems to be related to the depth of infiltration and the intensity of evaporative demand [66]. Also, soil compaction can lead to limited drainage, shallow rooting depths, negative effects on soil porosity and gas transport properties, reduction of critical levels of aerobic microbial activity, and promotion of N₂O emission [10,15–17, 67].

      Revitalization by ecological engineering: Microbially induced calcite precipitation has been referred to as a method that uses bacteria and cementation solutions to effectively reduce oil cracking [68]. Other physical methods and approaches to reduce the cracking of clayey soil include compaction control, geogrid, and plant root reinforcement [69]. Compaction control and similar technologies have high associated costs and low long‐term maintenance [68].

      Revitalization by ecosystem engineering: Introducing perennial and annual plants capable of enduring these soil conditions may be a solution. Mechanical stresses and chemical stimuli exerted by soil conditions may affect the ability of plant roots to penetrate the soil. The physical force that the soil exerts on the root of a plant that tries to penetrate along the soil profile is influenced by the soil's density, porosity, and, above all, moisture [70]. Several alternatives have been used, such as the plows and chisels, cover crops, and species with strong root systems and high biomass production, among other management techniques [70]. Other plant species have developed adaptive responses to these conditions by increasing or decreasing the rate of elongation of the apical region, swelling or shrinking its diameter [71]. In Europe, it is easier to find shallow soils in mountainous slope areas. A threshold of 30–35 cm relative to the rooting depth constitutes a severe restriction on the cultivation of perennial crops (or other crops) such as giant reed and switchgrass [61]. Shallow soil implies limited nutrients and water resources, as well as very limited soil workability [61].

      Biomass crops other than switchgrass and giant reed can be adapted to such soil conditions or modify their marginal aspects. Cardoon fields, for example, do not suffer from soil compaction [64, 72]. The roots of perennial plants can also accelerate the decomposition of organic matter in the soil in the long term through the action of biological and physical mechanisms [73]. Miscanthus spp. can be used to rehabilitate ecosystem functions and physical properties of marginal soils such as mine soils [74]. However, the mechanization associated with its cultivation can also cause the reverse effect. Switchgrass can also be well‐adapted to shallow soil depth and light soils with reduced fertility [75]. Introducing those industrial crops in these soil types can ameliorate the soils through the increment of organic matter and structure [76]. When explored for biomaterials, bioproducts, biochemicals, and bioenergy can help revitalize these areas economically.

      2.2.3 Unfavorable Texture and Stoniness

      Description of the marginality factor: Soil aggregates play an essential role in accumulating and stabilizing soil organic matter since they provide physical protection to soils [77]. Also, the soil's physical structure is essential for organic carbon storage [78]. So, topsoil's heavy clay texture affects plant growth and crop parameters and is directly related to plant nutrient supply, soil moisture conditions, and rooting conditions [10]. Soil texture affects the balance between water and gases, which is very stable over time independent of soil management [79]. Soil texture plays an important role in plant tolerance to drought at the germination stage [61]. Stoniness and dryness may influence water and oxygen availability in the root zone and soil stability [10].

Revitalization by ecosystem engineering: Soil ecosystems dominated by fungal communities show higher carbon retention than soil communities dominated by bacteria. Generally, afforestation is a suitable technique to improve carbon sequestration on marginal lands [80]. Switchgrass and giant reed are well adapted to a broad range of soils with different relative abundances of clay, silt, sand, organic matter, and coarse material [61]. Most mine soils are marginal soils with no topsoil, consisting of mine spoils whose properties can range from loose, coarse‐textured material with many rock fragments to highly compacted clayey material [81]. Industrial crops like Miscanthus spp., giant reed, switchgrass, P. purpureum, and others can tolerate mine tailings and produce quality biomass for energy and other purposes such that soil properties are restored [82–85]. Also, soybeans can be easily adapted to a variety of soil textures [86].

      2.2.4 Sloping Areas

      Description of the marginality factor: Sloping areas change in elevation with respect to planimetric distance [61]. Steep rocky slopes are marginal lands: they are composed of unstable and unbalanced ecosystems that are not favorable to germination, growth, and plant development and are characterized by high soil loss, high evapotranspiration, and low mechanical stability for root development [87]. Such steeper slopes tend to occur in higher areas of the landscape and usually induce more pronounced apparent dryness, often associated with shallow soils, leading to heavy runoff, mechanization difficulties, minimal cultivation opportunities and land management, a greater risk of soil degradation (erosion), and landslides [10].

      Revitalization by ecological engineering: Ecological engineering and the use of super‐absorbent polymers can improve the environment through the hydrologic effect of aboveground biomass, improving the adsorption and complexing capacity of hydrophilic functional groups, promoting the formation of a water‐blocking layer between soil particles and thus inhibiting moisture from moving from the soil surface to the atmosphere or to the slope's rock layer, enhancing water uptake and the utilization of water for plant growth, decreasing evapotranspiration, increasing germination rates in grass and woody plant species, and increasing plant survival [87]. This technology can also be used to improve and optimize the use of irrigation water in regions affected by drought, since water and nutrients can be stored and released when needed, enhancing the yields of crops [88].

      Revitalization by ecosystem engineering: Shallow soils are often combined with slopes, and plants like Miscanthus spp., giant reed, and switchgrass can easily adapt to such conditions [61, 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

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