Which Of The Following Would Decrease Soil Erosion? Choose All That Apply.

Pollution and Environmental Perturbations in the Global System

J. Maximillian , ... A.D. Matthias , in Environmental and Pollution Science (Third Edition), 2019

25.2.5 Soil Degradation

Soil degradation is the loss of land's production capacity in terms of loss of soil fertility, soil biodiversity, and degradation. Soil degradation causes include agricultural, industrial, and commercial pollution; loss of arable land due to urban expansion, overgrazing, and unsustainable agricultural practices; and long-term climatic changes. According to a recent report to the United Nations, almost one-third of the world's farmable land has disappeared in the last four decades. It was also reported that all of the World's topsoil could become unproductive within 60 years if current rates of loss continue. The issues of soil health and impacts on human well-being are discussed in detail in Chapter 27.

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Soil as a complex ecological system for meeting food and nutritional security

Fábio Carvalho Nunes , ... Majeti Narasimha Vara Prasad , in Climate Change and Soil Interactions, 2020

9.3 Soil Degradation: Impacts on Climate and Society

Soil degradation is the loss of the intrinsic physical, chemical, and/or biological qualities of soil either by natural or anthropic processes, which result in the diminution or annihilation of important ecosystem functions. The main causes of soil degradation and, consequently, the main threats to its ecological functions are erosion, organic matter decline, loss of biodiversity, compaction, sealing, point-source and diffused contamination, pollution, and salinization (Fig. 9.14) (Montanarella, 2007).

The severity of soil and landscape degradation depends on the initial status of the land, the magnitude of drivers that place pressure on land, the responses of the land system, and the impact of feedback from these responses on land resources (Juntti and Wilson, 2005).

Indicators of soil degradation can be visual, physical, chemical, biological, and integrative (Ribeiro et al., 2009). Visual indicators can be obtained from field observation or analysis of satellite images, radar, or aerial photographs. Observations include, for example, changes in soil color and forested area, evidence of ravines and gullies (Fig. 9.15), presence of weed species, monitoring of plant development, and sediment deposition.

Physical indicators can be measured by analyzing the arrangement of the solid fractions (coarse and fine) of the soil (Ribeiro et al., 2009). These manifest themselves, for example, as plant growth limitation, horizon thickness, textural gradient, permeability, porosity, density, penetration resistance, aggregate stability, infiltration, surface runoff, compaction, entrapment, temperature, and plasma-skeletal disjunction of the soil (Fig. 9.16).

Chemical indicators can be measured by monitoring soil pH, salinity (Fig. 9.17), organic matter content, cation and anion exchange capacity, nutrient cycling, and the presence of toxic or radioactive elements, while biological indicators may include measures of the presence of macro- and microorganisms, as well as their activities and byproducts.

Integrative indicators, also called key indicators, are used to assess soil quality and degradation. An integrative or key indicator should gather basic information about the composition, structure, and functions of the soil system. The objective is to reflect the interactions between different biotic and abiotic processes that express spatiotemporal transformations in the soil system, such as enzymatic activities and aggregation (Ribeiro et al., 2009).

It is important to note that the measurement of each indicator should express the direction (positive, negative, increase or decrease, etc.) and magnitude (as a percentage relative to a reference value) of variation, as well as the intensity, duration, and extension of variation (Ribeiro et al., 2009), so that it can measure the stage of soil degradation.

According to several authors, among them (Snakin et al., 1996), there are different stages of soil degradation, namely not degraded, weakly degraded, moderately degraded, highly degraded, and extremely degraded. These categories were obtained from experimental indicators of productivity and expert estimates (Table 9.5).

Table 9.5. Indicators of Soil Degradation

Indicator	Increasing Degree of Degradation
Indicator	ND	WD	MD	HD	ED
Fraction removed from horizon A (%)	<10	10–20	30–50	60–100	>100
Thickness of deposition layer (cm)	<1	1–3	4–10	11–20	>20
Reduction of nutrient content NPK (x)	<1.2	1.2–1.5	1.6–2.0	2.1–5.0	>5
Increase in soluble salt content (%)	<10	10–20	21–40	41–80	>80
Increase in exchangeable Na content (% da CTC)	<5	5–10	11–25	26–50	>50
Reduction in active microbial biomass (%)	<5	5–10	11–50	51–100	>100
Increase in soil density (x)	<1.10	1.10–1.20	1.21–1.30	1.31–1.40	>1.40

Degree of degradation: ND, not degraded; WD, weakly degraded; MD, moderately degraded; HD, highly degraded; ED, extremely degraded. X=number of times.

Adapted from Snakin, V.V., Krechetov, P.P., Kuzovnikova, T.A., Alyabina, I.O., Gurov, A.F., Stepichev, A.V., 1996. The system of assessment of soil degradation. Soil Technol. 8, 331–343. https://doi.org/10.1016/0933-3630(95)00028-3.

Other indicators of soil degradation can also be highlighted, especially those related to the presence of organic and inorganic contaminants, such as the presence of pathogenic substances or heavy and radioactive metals that exceed acceptable limits, according to representative entities like the World Health Organization and the United States Environmental Protection Agency, or by federal laws.

Desertification is a striking example of the impacts of soil degradation on climate and society (Fig. 9.18). According to the United Nations Convention to Combat Desertification, desertification is the degradation of land in arid, semiarid and dry subhumid regions resulting from various factors ranging from natural causes, such as climatic variation, to human activities, such as overgrazing, deforestation, and unsustainable agricultural activities (EU, 2018).

Desertification has been described as a phenomenon of natural and socioeconomic deterioration through the progressive reduction of biomass, reduction of rainfall, elevation of average temperature, soil infertility, intensification of erosion processes, reduction in the natural resilience of the land, reduction of water quality, decrease in the supply of food, increase in malnutrition and hunger, economic stagnation, and rural exodus (EU, 2018; Geist and Lambin, 2004).

Impacts of desertification on society stem from the degraded capacity of soil to perform its ecosystem functions. Soil deterioration caused by compaction, sealing, erosion, contamination, salinization, loss of biodiversity, or reduction of organic matter has direct and indirect negative impacts on the functions of regulation and/or support; provision; and information, culture, and religion, as shown in Tables 9.6 and 9.7.

Table 9.6. Impacts of Degradation on Soil Functions of Regulation and/or Support

Functions of Regulation and/or Support	Examples of Negative Impacts
Recharge of aquifers, control, and storage of water	Compaction, sealing, increased surface runoff, and soil erosion decrease water infiltration for the recomposition of aquifers.
Water purification, assimilation, and recycling of pollutants	Reduced water infiltration and loss of colloidal fractions of the soil diminish its adsorption capacity, compromising potential assimilation and recycling of nutrients and contaminants.
Regulation of floods	Soil compaction and sealing reduce infiltration and increase surface runoff, which favors the formation of flooded areas. In addition, soil erosion and sedimentation of river courses favors the formation of floods during pluvial episodes
Provide refuge, nurseries, and habitats for organisms	Erosion and contamination compromise the use of soil as a refuge, nursery and natural habitat by living organisms such as ants, earthworms, armadillos, and owls. In addition, soil compaction and sealing modify surface and subsurface hydrodynamics, water availability and temperature, which hampers soil use by certain living organisms
Support for engineering construction	Loss of soil by erosion, notably ravines and gullies, compromises the stability of engineering works such as houses, buildings, and roads. In specific circumstances, the soil may undermine abruptly, compromising not only engineering works, but also human lives
Support for bacterial culture for antibiotics production	Loss or diminution of soil biodiversity, as well as salinization and contamination, may make it unfeasible for use as a culture medium for the development of bacteria for the production of antibiotics
Support for raising livestock	Accentuated loss of soil due to erosion can compromise the use of an area for raising livestock by the appearance of ravines and gullies or by the commitment of buildings to house animals.
Nutrient cycling	Loss of micro- and macroorganisms, as well as colloidal fractions, diminishes the ability of soil to cycle nutrients.
Climate regulation	Loss of soil and its micro- and macroflora locally reduces the potential for evapotranspiration; aerosols, which act as cloud formers and condensation nuclei, alter the albedo and can contribute to increasing temperature. In wetlands and coastal plains, for example, organic soils in the process of degradation can release gases such as methane and carbon dioxide into the atmosphere, altering the equilibrium of the climate.

Table 9.7. Impacts of Degradation on the Soil Functions of Provision, and Information, Culture, Leisure, and Religion

Functions of Provision and Information, Culture, Leisure, and Religion	Examples of Negative Impacts
Natural production of food fiber and medicines; production of food fiber, medicines, and energy resources in cultivated areas	Compaction, sealing, leaching, salinization, erosion, and the loss of colloidal fractions decrease soil fertility, which leads to the reduction or stagnation of natural production and crop productivity.
Materials for ornaments, handicrafts and household utensils, and miscellaneous construction	Exacerbated exploitation of soil materials may deplete the source of materials, causing severe erosive processes such as gullies, silting of watercourses, and contamination.
Genetic resources and materials for pharmaceuticals and cosmetics	Loss or diminution of soil biodiversity, as well as its salinization or contamination, may make its use for obtaining genetic, pharmaceutic, and cosmetic resources, such as fungi, bacteria, clay compounds, salt, and other functional groups of organic matter, unfeasible.
Paleoenvironmental and cultural heritage information	Surface erosion and geochemistry can destroy environmental records important for understanding paleoscenarios, landscape dynamics, and the history of mankind.
Recreation/leisure, educational activities, and religious rituals	Loss of soil due to erosion, flooding, and/or contamination reduces areas that would be useful for educational, recreational, and religious activities. In addition, specific spaces considered sacred may cease to exist, which would completely remove anthropological relationships.

It is estimated that each year about 20,000 km² of productive land will be destroyed by desertification (estimates by the United Nations Environment Programme—UNEP) and that the Sahara desert has advanced, in some stretches, by up to 100 km. However, it is also important to emphasize the reduction of desertified surfaces in different areas, thus making global assessment complex (Conti, 2002).

Soil degradation and soil sensitivity to desertification appear to have intensified in recent decades in various parts of the world. There are estimates suggesting a potential global increase in soil erosion driven by the expansion of agricultural land (Borrelli et al., 2017), which will produce heterogeneous spatial patterns determined by the interaction of factors such as climate, changes in soil use, and anthropic pressures. Increasing levels of soil degradation and soil sensitivity to desertification are reflected in increasingly complex (and nonlinear) relationships between environmental and socioeconomic variables (Salvati et al., 2015).

Owing to the complexity and amplitude of the processes of soil degradation and desertification, it is important to adopt adequate evaluation methodologies. Different authors have applied analytical strategies and statistical methodologies capable of approaching and quantifying the spatiotemporal evolution of areas with degraded and/or susceptible soils (Nicholson, 2005; Prince, 2012; Prince et al., 1998; Salvati et al., 2015).

Nicholson (2005) used the vegetation index to study desertification in the Sahel between 1981 and 2005 and observed pulsating vegetation cover throughout the seasons. The author emphasized that placing plant growth data side by side with precipitation data is a good method for gauging whether a productive area is becoming desertic or not because, under normal conditions, plant growth increases or decreases in synchrony with precipitation.

It is important, however, to be cautious because the use of the vegetation index can also represent shallow soils, rock outcrops, and fields that are no longer being cultivated. Fieldwork and satellite imagery with the higher spatial resolution are required for further clarification, but the vegetation index certainly helps to select places where more detailed studies should be conducted (Anyamba and Tucker, 2005; Nicholson, 2005; Prince, 2012, p. 201).

Salvati et al. (2015) used a new and promising approach in the study of soil degradation that considers, in an integrated way, biophysical and socioeconomic aspects—the complex adaptive systems (CASs) approach. CASs are special cases of complex systems that are made up of multifaceted components that adapt to a changing environment (Holland, 2006).

CASs are complex and simulate nonlinear relationships among their components, characterized by positive and negative feedback mechanisms. They are adaptive because the actors of the system self-organize according to the external and internal inputs that are simultaneously determinants and products of the function of the system (Salvati and Zitti, 2008). It is believed that due to these characteristics CASs can better simulate the dynamics of complex systems such as soil.

The study developed by Salvati et al. (2015) evaluated in detail changes in soil degradation level and sensitivity of land to desertification for 773 agricultural districts in Italy during the period of 1960–2010. They arrived at the conclusion that "fast" and "slow" factors were identified as underlying soil and land degradation according to the speed of change estimated for each of the studied indicators.

Studies, such as those performed by Salvati et al. (2015), Nicholson (2005) and Anyamba and Tucker (2005) reveal important strategies for identifying, understanding, and measuring soil and land degradation, which are steps that precede the formulation of mitigation action plans.

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Lake and Reservoir Management

In Developments in Water Science, 2005

Turbidity

Soil degradation, including erosion and loss of fertility, is considered the most significant environmental problem in developing countries ( Fig. 2.1). Increased erosion, particularly related to strong rains and runoff, causes turbidity with several consequences for lake and reservoir water quality, including:

•: Mechanical filling of the lake basin,
•: Increased organic matter input (clay particles contain organic matter subject to decay, both during suspension and after sedimentation),
•: Increased bacterial production and formation of clay–organic-bacteria aggregates (Lind and Davalos-Lind, 1999),
•: Increased phosphorus loads, representing the dominant part of the sum of all phosphorus sources from agriculture (on the one hand, the turbidity contributes to phosphorus concentrations in the water column; on the other hand, clay particles compete with plankton for phosphorus and can transport phosphorus to the sediments),
•: Increased heavy metal concentrations, depending on their presence in the soil because of the application of fertilizers and from other sources in the drainage basin,
•: Decreased light availability for phytoplankton photosynthesis, leading to water quality improvement, or at least compensating for photosynthesis resulting from increased phosphorus loads (Lind and Davalos-Lind, 1999),
•: Interference with trophic interactions in lake plankton, particularly decreasing the visual grazers of visual grazers (Cuker, 1987).

The general nature of suspended material in the aquatic environment is described by Eisma (1992).

Summer temperature and primary production profiles of clear and turbid lakes can be quite different. The surface temperature of a turbid lake is higher than that of a clear lake because the particles causing turbidity can adsorb the incoming solar radiation in the uppermost layers. Clean water allows a deeper penetration of solar radiation into the water column, resulting in a heat gain in the deeper water strata. In a turbid lake, oxygen is produced by photosynthesis within shallower water strata than in a clear lake. Water strata exhibiting no photosynthesis, therefore, are more extensive, extending the oxygen deficit. Oxygen conditions are worse in turbid waters because of the increased nutrient load, and the production of organic matter associated with particles, as well as decreased re-oxygenation associated with photosynthesis due to light limitation of primary production. Light penetration also is important for the distribution of macrophytes in lakes (Duarte and Kalff, 1987, 1990). The influence of turbidity on lake water quality is summarized in Table 2.4.

Table 2.4. Management consequences of turbidity related to inorganic particles

Positive effects:

•: Costs of producing high quality water are low, since particles are easily flocculated
•: Reduction of the potential for algal and submerged macrophyte growth
•: Increased fishery potential due to associated nutrient inputs

Negative effects:

•: Increased capacity to transport nutrients and toxic substances
•: Reduced storage capacity of reservoirs and impoundments, as a consequence of settling out of riverborne inorganic suspended material
•: Reduced fishery potential, due to high sustained levels of turbidity, which cause large zooplankton species to be grazed and restrict the littoral to a small vertical depth

(modified from Hart and Allanson, 1984)

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Volume 3

A. Cook , ... R. Watkins , in Encyclopedia of Environmental Health (Second Edition), 2011

The Role of Soils in Food Security

Croplands and pastures cover more than 40% of Earth's land surface, and their successful cultivation is largely dependent on climate as well as the physical and geochemical status of the underlying soils. Since soil characteristics differ as a result of geological variations, crop production varies widely throughout the world, with regard to both crop yield and type. Mollisols are the most productive among soils, and are found largely in the Midwest of the United States and Russia. The high native fertility of these soils has allowed high crop yields without initial addition of fertilizer, and they have been intensively cultivated without fallow periods or crop rotation. As a result, serious deterioration of soil structure and soil erosion have occurred. Intensive cropping has also required the application of heavy rates of manure, chemical fertilizers, and pesticides; thereby polluting nearby rivers and water supplies. Extensive areas in the humid tropical areas of Africa and Latin America are dominated by Oxisols or Ultisols; these highly weathered soils are relatively impoverished in plant nutrients, which have leached out of the soil profile with time. These soils were made quite productive through fertilization and liming, although there may be no readily available sources of either in nearby areas. In the semiarid tropics, the soils are not highly weathered, but crop production is limited by low and irregular precipitation. In fact, soil moisture deficiency is probably the most significant restraint on food production worldwide, notably in Australia, Africa, and South Asia.

Today soil degradation affects approximately 15% of Earth's ice-free land surface, and irreversible erosion has occurred in an area of approximately 430 million ha. In Asia, approximately 40% of the soils are classified as degraded. The availability and viability of soils for meeting global food requirements have a profound effect on the intake of both macro- and micronutrients, including trace elements, and, therefore, far-reaching consequences for health. Declines in the total area of soil suitable for production of foodstuffs arise from three principal processes:

1.: direct physical loss or physical transformation of soils, such as from erosion and desertification. Changes in the structure of the soil, such as soil compaction through the intensive use of farm machinery, also tends to inhibit plant growth and costs several billions of dollars per year in terms of lost yield;
2.: loss of soil fertility due to acidification, salinization, or nutrient deficiency as a result of overcultivation or failure to replace the nutrients that have leached away or have been removed through harvest (such as through the use of nitrogen-fixing legumes or fertilizers); and
3.: biological and chemical contamination of soils.

A major physical problem affecting soils is accelerated erosion, which occurs with the stripping of topsoil by wind and water, particularly once the vegetation cover is lost (Figure 1). The classic agricultural pathway of boom, erosion, and bust (or 'expansion then abandonment') has been repeated around the world, particularly in the past century. An extreme example occurred in the prairies of North America's Midwest during the 1930s. In the Great Plains States of Kansas, Oklahoma and Colorado, and the Palliser triangle in Alberta, Saskatchewan, and Manitoba, wheat was planted on a wide scale, particularly from ca. 1900, when sufficient rainfalls encouraged settlement and cultivation. However, insufficient time for regeneration and replenishment, unpredictable rainfall, and high numbers of windy days resulted in the Dust Bowl years of the 1930s. Millions of hectares were destroyed, and thousands of families were forced off the land in a mass exodus. In the 1950s, vast regions of the Kazakh and Russian steppe succumbed to the plow under Khrushchev's Virgin Lands Scheme, of which 25% has now been abandoned. From 1960, most of the agricultural conversion has occurred in the tropical areas of Africa, South America, and Indonesia.

At the extreme, complete desertification may arise, and can be driven by both natural processes (e.g., annual regional fluctuations from variable rainfall patterns) and human interventions. The United Nations Millennium Ecosystem Assessment estimates that desertification threatens the semiarid and arid areas that comprise over a third of the earth's total land area, on which 2 billion inhabitants are dependent. The African continent is highly vulnerable to this process, with more than 60% of its land area composed of deserts or dryland, and many regions are subject to severe droughts. Over a quarter of China is also affected by desertification.

High rainfall areas are also subject to massive soil loss. In the Philippines, much of the terrain is steep and prone to monsoonal downpours, and high rates of erosion can, therefore, occur even without human involvement. However, large-scale land clearing over the past century, to cater for an increasing rural population; the development of plantations; and the widespread exploitation by timber companies have significantly accelerated the process of soil loss.

Most of the world's soils exhibit depletion in one or more elements essential for plant growth – including zinc, boron, iron, molybdenum, and manganese – thereby reducing crop yields. Low levels of soil iodine, cobalt, and copper also affect the health of grazing livestock and are thus indirectly detrimental to human food supplies. A low degree of soil fertility may arise as a result of natural geological and geochemical processes, but anthropogenic interventions may often cause or accelerate the problem. For example, the process of land salinization, most commonly develops from poor irrigation practices, particularly when excessive water is added to soils allowing high rates of evaporation and the develpment of mineral hardpans (irrigation salinity). Such salinization affected the civilizations of Mesopotamia as long ago as 2500 BC, in which crops increasingly began to fail from progressive salt accumulation. This problem caused a conversion of wheat into barley, which is more salt-tolerant, until many lands (and the city states they supported) were abandoned altogether. Another form of salinity, dryland salinity ('salt creep'), results from changing vegetation cover. Following clearance of the original plants and replacement with shallow-rooted crops reduced evapotranspiration in cultivated lands, which causes the water table to rise and triggers the release of soluble salts.

As a result of the various processes of soil degradation, approximately 0.5% of the world's cropland is lost every year. Globally, more than 95% of food is obtained from areas of cultivation classified as 'at-risk' from degradation. In developed countries, the most likely mechanisms of ill-health from declines in food production are indirect; from socioeconomic deprivation and regional collapse as rural livelihoods are affected, coupled with rising food costs for consumers more generally. In developing countries, loss of soil viability, if sufficiently severe and protracted, results in malnutrition and starvation due to insufficient calorie and protein intake.

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Soil enzymes in a changing climate

Marta Jaskulak , Anna Grobelak , in Climate Change and Soil Interactions, 2020

25.2 Extracellular Enzymes in Soils—Synthesis and Functions

The soil degradation of organic matter is a process fundamentally mediated by soil microorganisms and their production of a vast range of EEs, which serve two main functions ( Table 25.1). The first one involves their ability to catalyze a multitude of chemical reactions associated with the degradation of organic matter (Adl, 2016). The second one regards the necessity to acquire energy and nutrients for producers of extracellular enzymes. Thus enzyme activity in soils is a predominant factor for the quality of soil organic matter including the availability of carbon and nitrogen (Wallenstein et al., 2011). The synthesis and secretion of such enzymes are immensely expensive in regards to the amounts of energy necessary to do so. Thus a range of specific ecological strategies and regulatory mechanisms had evolved to ensure its efficiency (Sadhu and Maiti, 2013). Nevertheless, it has to be pointed out, that soil is always a hostile environment for all extracellular enzymes as, once out of the cell, they are subjected to the degradation and denaturation that makes them particularly vulnerable to changing environmental conditions (Li et al., 2014).

Table 25.1. Main Soil Extracellular Enzymes and Their Roles (Kotroczó et al., 2014; Abad-Valle et al., 2017).

Enzyme	Role in Nutrient Cycling or OM Decomposition	Substrate Used for the Reaction
Urease	Creation of plant-available NH₄ ⁺	Nitrogen
β-Glucosidase	Energy source for microorganisms	Carbon
Phosphatase	Transformation of P into plant-available P	Phosphorus
Leucine aminopeptidase	Release of leucine	l-Leucine-7-amino-4-methyl coumarin
Amidase	Creation of plant-available NH₄ ⁺	Carbon and nitrogen
Sulfatase	Creation of plant-available S	Sulfur
Fluorescein diacetate	The energy source for microorganisms	Organic matter

Soil extracellular enzymes catalyze not only the degradation of plant and animal matter but also many potentially dangerous xenobiotics and other contaminants (Pathan et al., 2015). Enzymes produced by Phanerochaete chrysosporium, also known as white root fungus, possess over 90 genes encoding glycosyl hydrolases, more than a hundred of genes encoding ligninases and also a secretome containing approximately 800 proteins (Pragya et al., 2013). Its enzymes were shown to take part in the oxidative degradation of many organic contaminants including dioxins, pesticides, polycyclic aromatic hydrocarbons (PAHs), and pentachlorophenols. Hence, researchers (Cenini et al., 2015) currently extensively study the research of its feasibility to be used for such purposes.

The carbon-rich macromolecules derived from plants and other organic debris including pectin, lignin, chitin, tannin, cellulose and hemicellulose, all require a vast range of microbes and enzymes to be adequately degraded (Fekete et al., 2014). Moreover, many components of plants, animals, and microbes involve complex polymeric nitrogen and carbon compounds, and their degradation demands combined and vigorous activities of many taxonomical groups of microorganisms at the same time. Thus the most crucial agents for OM decomposition are the soil EEAs (Pathan et al., 2015). One of the primary functions of soil EEs is the degradation of most common plant polymers: lignin and cellulose, both of which rely on a large number of enzymes acting simultaneously that are produced by diversified bacterial and fungi communities (Zhang et al., 2017). As an example, research showed that the complete breakdown of a single plant leaf involves more than 50 different enzymes (Wang et al., 2013). Cellulose is mostly broken down by enzymes such as endo-1,4-β-glucanases, β-glucosidases, and cellobiohydrolases, produced by two taxonomic groups of fungi: Basidiomycete and Ascomycete. Trichoderma reesei is currently the best-known cellulose degrader and possesses more than 30 glycosyl hydrolases as well as multiple endoglucanases and a secretome containing approximately a hundred proteins (Abad-Valle et al., 2017). The second most abundant plant polymer, lignin, is a complex phenylpropanoid, and its degradation includes a vast range of oxidative enzymes such as lignin peroxidases, laccases, and manganese peroxidases (Kotroczó et al., 2014).

One of the most common soil extracellular enzymes is β-glucosidase, which plays an essential role due to its association with the degradation of β-glucosides found predominantly in plant and animal debris. Therefore it is one of the enzymes from a family able to hydrolase saccharides called glucosidases, but among them, β-glucosidase is the most abundant of its kind in soils. Its activity had been observed in many plant species including sorghum and maize, as well as in microbes such as Trichoderma spp., Lactobacillus plantarum, Penicillium urpougeum, and Flavobacterium johnsonae (García-Palacios et al., 2015). Its abundance and activity are often used as a soil quality bioindicator that can be used to assess past microbial activity, and soil capacity to stabilize the organic matter (Kubartová et al., 2015). Overall, its activity can show changes in soil organic carbon content before it is measurable by conventional, physiochemical methods. It is also sensitive to changes in soil pH; thus, changes in climate and soil management. Such properties allow it to be used as a selective indication of ecological changes (Sinsabaugh et al., 2014). Moreover, apart from differences in pH, it is also extremal sensitive to heavy metal contamination, including nickel, lead, copper, cadmium, and arsenic. One study showed that soil contamination with heavy metals shows a significant decrease and even a complete lack of β-glucosidase activities (Wallenstein et al., 2011).

Another crucial soil extracellular enzyme is urease, which catalyzes the hydrolysis of urea to carbon dioxide and ammonia. Its molecular weight is estimated at approximately 150,000–480,000 Da, and its structure contains nickel as an enzyme cofactor (Kulkarni and Kale, 2014). Urease is vastly distributed in plants and microorganisms. As it was observed to be quickly degraded in soil by proteolytic enzymes, extracellular urease is mostly seen stabilized and immobilized on soil colloids, both organic and mineral. Studies have shown that urease activity in soils increases after soil supplementation with organic fertilizers and decreases after soil tillage (Shaw and DeForest, 2013). It is also highly sensitive to changes in soil microbial population, physical and chemical soil, and climate. Thus its activity can be used as a bioindicator to check soil quality after various management practices (Kulkarni and Kale, 2014).

Organic phosphorus observed in the soil can contribute to plant nutrition of phosphorus after its hydrolysis and release of free phosphate fraction (Fig. 25.2). Such reactions are catalyzed by various phosphatases, secreted to soils when the content of free phosphorus is low via plants and microbes, or it can be released from decaying debris (Kulkarni and Kale, 2014). Overall, phosphatases are believed to play a crucial role in P cycling, and thus, they are strongly correlated with plant growth. Plant species possess several mechanisms that allow them to tolerate and survive at low phosphate availability (Sadhu and Maiti, 2013). One example of such evolutionary adaptation is the plants' ability to produce and secrete phosphatases from roots to the soil after exposure to P deficiency (Yokoyama et al., 2017). Phosphomonoesterases are the most studied kind of phosphatases and are characterized by low molecular weight. On the other hand, studies on phosphodiesterases are significantly less abounded which creates a major oversight since they are involved in the degradation of phospholipids and nucleic acids, which are the most significant sources of organic phosphorus in soil (Su et al., 2015). Species of fungi that synthesize soil phosphates belong mostly do the Penicillium and Aspergillus genera. Species of bacteria that produce phosphatases included mainly Pseudomonas and Bacillus genera (Yokoyama et al., 2017).

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Soil Environment

M.K. Doula , A. Sarris , in Environment and Development, 2016

4.2.8 Biodiversity

Rapidly advancing soil degradation is severely threatening soil biodiversity, eventually leading to the extinction of species yet to be discovered and fully studied. Implications for human health of the degradation of the soil ecosystem need still to be fully understood.

Hence, soil degradation by erosion, contamination, salinization, and sealing all threaten soil biodiversity by compromising or destroying the habitat of the soil biota. Management practices that reduce the deposition or persistence of organic matter in soils, or bypass biologically mediated nutrient cycling, also tend to reduce the size and complexity of soil communities. It is, however, notable that even polluted or severely disturbed soils still support some level of microbial diversity.

Little is known about how soil life reacts to human activities, but there is evidence that soil organisms are affected by SOM content, the chemical characteristics of soils (eg, pH, the amount of soil contaminants or salts), and the physical properties of soils such as porosity and bulk density, both of which are affected by compaction and sealing.

A limited number of data concerning the dynamics of soil biodiversity are available and these generally refer to a few groups of soil organisms. Mushrooms, for instance, are a group of soil organisms for which a relatively long history of records exists. From this type of data set, it has been possible to show mushroom species decline in some European countries. For example, a 65% decrease in mushroom species over a 20-year period has been reported in the Netherlands, and the Swiss Federal Environment Office has published the first-ever "Red-List" of mushrooms, detailing 937 known species that face possible extinction in Switzerland [88].

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Tenets of Soil and Landscape Restoration

Rattan Lal , in Land Restoration, 2016

2.1.7 Landscape Restoration and Ecosystem Services

Landscape and soil degradation involve a reduction in ecosystem functions and services. Thus, decisions about sustainable landscape management must consider the restoration of essential ecosystem services ( Forouzangohar et al., 2014). It is pertinent to identify the hot spots of important ecosystem services within a landscape and to prioritize these areas. Important among these services, with numerous co-benefits, are soil/ecosystem carbon sequestration (Lal et al., 2013), renewable water supply, and biodiversity. There is a close interaction between ecosystem services and biodiversity, which are mutually beneficial or even symbiotic (Schneiders et al., 2012). Conversion of natural lands to agroecosystems reduces biodiversity with adverse effects on the latter. Therefore, restoring biodiversity within agricultural landscapes is important to improving ecosystem services while sustaining agronomic productivity. Land sharing, rather than land separation can enhance farmed environments, and involves biodiversity-based agricultural practices (Benayas and Bullock, 2012). Afforestation of agroecosystems to enhance biodiversity has the co-benefits of generating another income stream for carbon credits (Perring et al., 2012; Schneiders et al., 2012). Thus, the economic analysis of landscape restoration must consider all co-benefits and tradeoffs.

The flow of ecosystem services and their contribution to the well-being of humans and nature must be included in any economic assessment (Costanza et al., 1997, 2014; Bateman et al., 2011; Zhang et al., 2013). Generating carbon credits is a by-product of one of the most important ecosystem services: carbon sequestration provided through landscape/soil restoration (Lal, 2004; Lal et al., 2004; Curran et al., 2012). It is also an example of the multiple outcomes possible through landscape restoration (Perring et al., 2012). However, enhancing one ecosystem service can potentially suppress another. For example, advancing food security by intensification and monoculture can decrease biodiversity. Similarly, removing crop residues for biofuel production can reduce SOC concentration and the SOC pool. Therefore, anticipating and managing future tradeoffs and complementarities between ecosystem services is an important guiding principle of landscape restoration (Reed et al., 2013).

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Socioecological soil restoration in urban cultural landscapes

Loren B. Byrne , in Soils and Landscape Restoration, 2021

14.7 Working with biota as restoration partners and foes

That physicochemical soil degradation affects soil organisms is well-established. In contrast, how plants and soil organisms affect urban soil properties and restoration outcomes has been investigated much less. As noted in Section 14.5, plants and microbes can provide "nature-based" pollution remediation in urban soils (Song et al., 2019), and studies identifying which species are more effective bioremediators generate insights for successful use of this approach (e.g., Jensen et al., 2009; Dadea et al., 2017; Liu et al., 2018; Shuttleworth et al., 2018). Favorable restoration outcomes can be achieved with such restoration partners but "restoration foes" can interfere with restoration goals.

Simply letting nature take its course (naturalization) is one possible management decision for degraded urban lands, especially in the context of financial and expertise constraints, and possibilities that interventions will negatively affect the functions of soil biota (for instance, liming was found to reduce the remediation abilities of bacterial communities: Hesse et al., 2019). Even in polluted and constructed soils (e.g., covering landfills), plants, including native ones, and soil biota, including mycorrhizal fungi, can establish and survive well (e.g., Handel et al., 1997; Fischer et al., 2013; Pregitzer et al., 2016; Everingham et al., 2019; Singh et al., 2019). Spontaneous, unmanaged vegetation can stabilize the soil, preventing erosion and movement of pollutants (Song et al., 2019), though Setälä et al. (2017) observed differences between deciduous and coniferous tree communities in their levels of water and heavy metal retention. Naturalization may be a good option for other degradation such as compaction from human traffic; for example, in an urban forested park Millward et al. (2011) found that allowing understory plants to grow without management (and keeping humans out) for 6 years reduced soil bulk density and increased water infiltration. As such, an important lesson for ecological restoration is that, when humans do nothing, wild and weedy restoration partners can sometimes provide ecosystem services that advance restoration goals.

When restoration goals necessitate intentional plant installation, existing urban soil properties can affect successful establishment, leading to the recommendations that plants should be chosen that match the local conditions—an important aspect of identifying restoration partners (Hitchmough, 2008; Haan et al., 2012). After installation, plants can improve soil conditions (in addition to that caused by initial soil preparation) by adding OM to the soil via roots and detritus (e.g., Vannucchi et al., 2015) and reducing bulk density and creating channels for water flow via root growth (e.g., Bartens et al., 2008). In restored prairies created in urban lawns, Johnston et al. (2016) found trends of improved soil conditions after 15 years (but see Yost et al. (2016) for more complex outcomes). Installed plants also influence restoration of soil microbial communities, such that they can become more like reference ecosystems (Gellie et al., 2017).

In addition to plants, soil microbes and animals, should be considered as potential restoration friends. For example, reestablishing microbial root mutualists (i.e., nitrogen-fixing bacteria, mycorrhizal fungi) that can improve host–plant establishment and health may be aided by inoculations to plants before installation (Fini et al., 2011; Bashan et al., 1999). Animals, such as arthropods and ecosystem engineers such as mammals and earthworms (even if they are not native), may also help improve soil conditions; thus, management to conserve or restore them could be an effective strategy for urban soil restoration (Byers et al., 2006; Snyder and Hendrix, 2008). An example is from Australia where bandicoots inhabiting urban landscapes dig into soil for food such as mycorrhizal fungi fruiting bodies; through their bioturbation and dispersal of fungal spores in their scat, bandicoots might aid in the restoration of a declining Eucalyptus species in degraded urban forest remnants (Tay et al., 2018). Though fascinating, such mutualistic animal–plant–soil–restoration relationships have rarely been examined in urban contexts and deserve much more attention.

Certain organisms can also create ecosystem disservices that impact urban restoration efforts. For instance, nonnative invasive species (e.g., plants, insects, worms), which are often especially successful in urbanized landscapes, are a form of "biological pollution" that can degrade soil conditions including bulk density, biodiversity and biogeochemical cycles (Szlavecz et al., 2006; Heneghan et al., 2009; Vilà et al., 2011; Ferlian et al., 2018). In turn, invasive-degraded soils may facilitate continued invasion and persistence of invasive species, with soil legacies (e.g., seed banks, altered structure and nutrient levels) remaining even after invasives are removed (e.g., Heneghan et al., 2009; Corbin and D'Antonio, 2012; Overdyck and Clarkson, 2012). Along these lines, studies have found that restoration interventions do not prevent regrowth of nonnative weeds (e.g., due to dispersal via birds from surrounding landscapes; Sullivan et al., 2009) and might actually promote them in some contexts (e.g., compost amendments can increase nonnative seedling recruitment: Doroski et al., 2018). Aboveground native animals may also interfere with achieving soil restoration goals because they can alter soil conditions alone and in combination with invasive species. For example, exclosure experiments revealed many direct and indirect negative effects of white-tailed deer (which are overpopulated in many U.S. urbanized landscapes) on variables relevant to urban soil restoration, including compaction, native plant survival (Shelton et al., 2014) and earthworm densities and biomass (Dávalos et al., 2015; Mahon and Crist, 2019). On the positive side, results from these studies suggest that when deer are excluded, soil conditions can "restore" themselves with potentially positive outcomes for native plants. This reflects the need for soil restorationists to consider a wide range of variables and management practices (not just soil and plant ones) to facilitate restoration success in urban landscapes.

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DEGRADATION

C.J. Ritsema , ... S.M. de Jong , in Encyclopedia of Soils in the Environment, 2005

Approaches

The status of soil degradation can be assessed in a qualitatively broad manner or in a more detailed quantitative manner. The former generic approach is better suited for small-scale assessments, such as for entire countries, continents, or global overviews. A quantitative approach is required for more specific and detailed assessments, e.g., to determine the erosion status for a watershed or the pollution status for a province. Qualitative assessments are based on expert judgement and hence more liable to subjectivity than quantitative methods. A method does not have to be fully qualitative or quantitative, mixtures may occur. Some frequently used methods or tools are:

1.

Expert opinion: Qualitative assessment on a controlled mapping base and semiquantitative definitions, as employed for instance in the Global Assessment of Human-induced Soil Degradation (GLASOD) survey. GLASOD and related methods are based on an assessment of land suitability by national experts that use defined, semiquantitative class limits on a given mapping base. Its major disadvantage is the inevitable degree of subjectivity. Its major advantage is its capacity to produce results, such as achieving complete world coverage (Figure 2), in a short time and on a small budget. Costs per unit area are relatively low. In Figure 3 an integrated global soil degradation severity map is shown, indicating areas with different degradation rates;

2.

Remote sensing: Analysis of low- and high-resolution satellite data and airborne imagery (e.g., analysis of composite indices such as the Normalized Difference Vegetation Index (NDVI)). Remote sensing always includes linkages with ground observations. The basis of this method is comparison of remotely sensed imagery of different dates, for regional coverage, mainly low-resolution imagery; and, specifically, comparison of the NDVI, derived from imagery collected by the sensor aboard the National Oceanographic and Atmospheric Administration (NOAA) satellite, and more detailed imagery. This method was tested amongst others in Saudi Arabia and shows areas where vegetation response to rainfall is decreasing (degradation of resources) or increasing (rehabilitation of resources). It has been applied particularly to early warning systems. For longer-term comparisons, some form of calibration for preceding rainfall is needed. Costs are relatively low. It is recognized that remote sensing cannot be used alone.

Spectral mixture analysis (SMA): Since 1985 hyperspectral remote sensing has been developed, opening new methods to survey and assess degradational state of the soil surface. Hyperspectral remote sensing refers to the collection of images in the solar spectrum, with many narrow spectral bands allowing the collection of very accurate spectra of objects and the earth surface and identification of absorption features of plants and of soil minerals in these spectra. SMA is a technique to unravel the spectral information in the remote-sensing images by assuming that the spectral variation is caused by a limited number of surface material (green vegetation, senescent vegetation, a number of soil types, and water). A reference library of these surface materials collected in the field or in the laboratory yields the basis for SMA of the remote-sensing images. This approach has been applied successfully in a number of case studies to survey soil conditions and to identify classes of degradation. The SMA approach normally improves on results using the NDVI but requires more spectral bands: SMA is successfully applied to separate, in images, bare soil surfaces from senescent vegetation and yellow vegetation from green vegetation. These three factors are important inputs in soil-erosion models because they act differently with respect to raindrop interception.

3.

Field monitoring: Stratified soil sampling and analysis, and field observation of vegetation and biodiversity under certain land-use or management practices and climate variability. To date, soil monitoring has been applied mainly in developed countries, and tests are needed of its cost-effectiveness in developing countries. In areas where baseline studies have been established, monitoring of changes will be undertaken; in other areas, establishment of a baseline will be a priority. Stratified soil-sampling with analysis, and/or benchmark sites, repeated over 5- to 10-year intervals, has been advocated as a basic activity for national soil survey organizations. Examples of application to date (2003) are few, but successful: the method has been applied to 20 000 sites over a 25-year period in Japan; is currently being used for a national 16-km-grid in France; and has been started in Denmark and Switzerland. The same approach has been applied to field observations of vegetation, along transects or in sampling plots, and to biodiversity. Costs per unit area are relatively high, but could be reduced by application to priority areas only, on a stratified sampling basis.

4.

Productivity changes: Observation of changes in crop yields, biomass production, and livestock output, which directly apply to the definition of land degradation in terms of lowered productivity, although they are influenced by many other factors. There is a range of possibilities: At national level, use might be made of national yield statistics (of which the reliability is still under debate), adjusted for fertilizer use and climate. At local level, yield monitoring is possible by comparisons with a standard crop, either without fertilizer or with standard fertilizer and management. Substantial problems arise in that productivity decline could be due to factors other than land degradation, e.g., removal of fertilizer subsidy or civil strife. The same cost constraints apply as for soil monitoring.

5.

Sample studies at farm level, based on field criteria and the expert opinion of land users. Even at national level, such detailed studies are essential on a sample basis, to obtain grass roots views both of the severity of degradation and its causes, together with practicable remedies (Stocking and Murnaghan, 2001). Field indicators of soil degradation were developed about 20 years ago, and could be extended to condition of vegetation. Talking with farmers means getting the views of farmers, and other land users, on whether things have got worse – which are of course, subjective and perhaps systematically biased, but still essential to get grass roots view at local level. The method is clearly applicable only at a local scale, and thus on a selective sampling basis. Observations of the state of the land can be combined with assessment of driving factors and impacts.

6.

Modeling: Based on data obtained by other methods, modeling can be used in many ways, such as: (1) prediction of degradation hazard; (2) operational definition of degradation in terms of unfavorable changes in plant productivity, soil properties, and hydrology; (3) design of conservation measures using climatic data with a specific return period (worst-case scenario modeling); (4) extending the range of applicability of results; (5) integrating biophysical with socioeconomic factors. Much research has been put into devising models for the prediction of soil-erosion hazard. There are established methods for the modeling of both water and wind erosion, which have been widely applied, in part because it is vastly cheaper than any form of field observation. The modeling approach is mainly relevant to degradation hazard, but can be applied to actual degradation first, as a means of calibration of the model to the specific requirements of an area, optimizing sampling design, or to extrapolate the applicability of results obtained on a sampling basis. Risk reflects a potential development in the future, while status reflects the development to date. Models vary widely in complexity and data requirements, depending on the type of degradation they are addressing and the size of the area under investigation. Models are useful to learn and understand degradation processes, but both the model and input data are a simplification of reality, hence extrapolation of models should be done with care. Very often models are developed for experimental plots or pilot zones of a restricted size and under more or less controlled conditions, which should be taken into account when applying the model elsewhere. The data requirements and structure of a model, and the type of processes included in it, depend on many things: (1) the temporal scale of the research objectives: Is an annual, daily, or event-based result required? (2) the spatial scale: Are predictions needed for a single plot, a field, complex spatial catchment, or an entire region? (3) Is the emphasis on the on-site effects of land degradation (e.g., soil erosion or crop yield changes) or on the off-site effects such as water sediment levels and pollution? Spatial and temporal scales are often linked, as, for example, is the case for physically based spatial erosion models (Figure 4) that simulate single events for first-order catchment with a high level of detail. They can be used to answer subtle questions about the effects of specific land-use changes or soil and water conservation measures in the catchment upon reducing runoff and erosion (Figure 5). On the other hand there are less-complex empirical models that can simulate continuous periods mostly for fields or hillslopes, but they can only show the change in annual erosion or soil loss.

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Carbon Sequestration, Terrestrial☆

R. Lal , in Reference Module in Earth Systems and Environmental Sciences, 2013

Restoration of Degraded Ecosystems and Terrestrial Carbon Sequestration

Accelerated soil erosion, soil degradation by other degradative processes (e.g., salinization, nutrient depletion, elemental imbalance, acidification), and desertification are severe global issues. Accelerated erosion by water and wind is the most widespread type of soil degradation. The land area affected by water erosion is estimated at 1094 million hectares (Mha), of which 751 Mha is severely affected. The land area affected by wind erosion is estimated at 549 Mha, of which 296 Mha is severely affected. In addition, 239 Mha is likely affected by chemical degradation and 83 Mha by physical degradation. Thus, the total land area affected by soil degradation may be 1965 Mha or 15% of the earth's land area. With a sediment delivery ratio of 13–20%, the suspended sediment load is estimated at 20 × 10⁹ metric tons (Mg)/year. Soil erosion affects SOC dynamics by slaking and breakdown of aggregates, preferential removal of C in surface runoff or wind, redistribution of C over the landscape, and mineralization of displaced/redistributed C. The redistributed SOC is generally light or labile fraction composed of particulate organic carbon (POC) and is easily mineralized. As much as 4–6 Pg C/year is transported globally by water erosion. Of this, 2.8–4.2 Pg C/year is redistributed over the landscape and transferred to depressional sites, 0.4–0.6 Pg C/year is transported into the ocean, and 0.8–1.2 Pg C/year is emitted into the atmosphere. The historic loss of SOC pool from soil degradation and other anthropogenic processes is 66–90 Pg C (78 ± 12 Pg C), compared with total terrestrial C loss of 136 ± 55 Pg C. Of the total SOC loss, that due to erosion by water and wind is estimated at 19–32 Pg C (26 ± 9 Pg C) or 33% of the total loss.

There is also a strong link between desertification and emission of C from soil and vegetation of the dryland ecosystems. Desertification is defined as the diminution or destruction of the biological potential of land that ultimately can lead to desert-like conditions. Estimates of the extent of desertification are wide-ranging and are often unreliable. The land area affected by desertification is estimated at 3.5–4.0 billion ha. The available data on the rate of desertification is also highly speculative and is estimated by some to be 5.8 Mha/year.

Similar to other degradative processes, desertification leads to depletion of the terrestrial C pool. The historic loss of C due to desertification is estimated at 8–12 Pg from the soil C pool and 10–16 Pg from the vegetation C pool. Thus, the total historic C loss due to desertification may be 18–28 Pg C.

Therefore, desertification control and restoration of degraded soils and ecosystems can reverse the degradative trends and sequester a large fraction of the historic C loss. The potential of desertification control for C sequestration is estimated at 0.6–1.4 Pg C/year. Management of drylands through desertification control has an overall C sequestration potential of 1.0 Pg C/year. These estimates of high C sequestration potential through restoration of degraded/desertified soils and ecosystems are in contrast to the low overall potential of world soils reported by some.

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