Assessment of Land Degradation Factors

Abstract

Land degradation is a phenomenon that threatens food security and ecosystem balance observed on a global scale. At the beginning of the 20th century on a global scale, its importance was not yet understood due to low climate change, population growth, and industrialization pressure, but today, with the increasing effect of these factors, it has affected more than 25% of the world’s terrestrial areas. Land use/cover change, destruction of forest areas, opening to agriculture, or conversion of forest areas to high economic plantations are the main factors of land degradation. Population growth and increasing demand for food, water, and energy are increasing pressure on natural resources, primarily agricultural and forest land. Due to its dynamic relationship with the climate change, land degradation creates more pessimistic results in arid and semi-arid areas that are more vulnerable and have a high population density. Despite the intergovernmental meetings, commissions, and decisions taken, land degradation continues on a global scale and the human-climate change dilemma creates uncertainties in achieving the targeted results.

1. Introduction

Land degradation (LD) is the physical, chemical, and/or biological degradation of soil by natural or anthropogenic processes that result in the reduction or destruction of essential ecosystem functions. The leading causes of land degradation and, consequently, the main threats to its ecological functions are erosion, reduction of organic matter, loss of biodiversity, soil compaction, framing, point and diffuse pollution, and salinization [1].

LD is a widespread and global phenomenon that affects food security, ecosystem services, and human well-being [2]. Land degradation is a major threat to the food security of a rapidly growing global population, especially those living on limited land resources. Approximately 15% of the world’s population is currently living as being deprived of food security [3, 4]. It seems inevitable that the problem of food security will be exacerbated by land degradation as the global population is projected to reach nine billion by 2050 [5].

While LD was approximately 36,108 ha worldwide in the early 1900s [6], today, more than 25% of the world’s land area (37.25 million km²) is affected by LD [7]. It is estimated that an area of about 5 to 10 million hectares is added to these areas each year [8]. In these areas, there is a decrease or degradation of soil quality and a decline in biological and economic productivity due to erosion and physical and chemical changes [7]. This negative change is observed mainly in arid and semi-arid areas [9].

About 40% of LD is observed in developing countries. Projections show that these areas will increase to 78% by 2100, covering 50% of population growth [10]. Climate change appears to be a significant risk to agriculture, biodiversity, and livelihoods in developing countries [11].

Population growth and increasing demand for food, water, and energy are increasing pressure on natural resources, primarily agricultural land. It is estimated that demand for food will increase by 50% and demand for water and energy by 40% by 2030 compared to current levels [12]. The increase in natural resource consumption and industrialization to meet growing population needs has led to conditions that threaten food security today due to climate change.

Although it is a global phenomenon, soil degradation (SD) is much more pronounced in drylands, where land is highly vulnerable to degradation due to drought and water scarcity than in non-drylands [13]. The percentage of global soils vulnerable to severe drought was more than doubled between the 1970s and the early 2000s [14]. It is estimated that drylands will increase by 10% by the end of this century [15]. The expansion of drylands combined with increasing aridity due to climate change has direct implications for desertification [16]. Vegetation change and LD have made these areas much more vulnerable in terms of agricultural production and sustainable management.

Drylands, home to 38% of the world’s population (2.7 billion people), account for 44% of the world’s cropland and 50% of its livestock [9], as well as about 30% of the global carbon [17]. Drylands are, therefore, central to sustaining the habitats, crops, and livestock that support most of the world’s population [13].

Given the importance of soil carbon sequestration and storage, preventing, mitigating, and reversing land degradation can provide more than one-third of the most cost-effective greenhouse gas mitigation needed until 2030 to keep global warming below the targeted threshold of 2°C. The Paris agreement (2016) on climate change seeks to increase food and water security and help prevent conflict and migration (Intergovernmental Science and Policy Platform on Biodiversity and Ecosystems (IPBES)).

2. Assessing land degradation

After desertification was officially recognized as a severe problem at the United Nations Conference on Desertification (UNCD) [18], Action Plans to Combat Desertification (PACH) studies were conducted in 1979 and 1991. The GLASOD project of the Global Assessment of Soil Degradation of the Soil Degradation Database, initially led by ISRIC and funded by the United Nations Environment Program (UNEP) in the 1990s, produced the first world map of human-induced land degradation at a scale of 1:10,000,000. Subsequently, awareness of LD problems was raised by the map, which was based on the methodology developed by experts (200 soil scientists and 21 environmental experts from around the world) at the United Nations Conference on Environment and Development (UNCED) Summit in Rio de Janeiro in 1992. The assessment and monitoring of situations and processes of desertification at local, regional, or larger scales were carried out in the 1980s by the food and agriculture organization (FAO), and the United Nations Environment Program (UNEP) developed the methodology for assessing and mapping desertification. In this context, many studies have been conducted worldwide to measure desertification, monitor the implementation of the desertification vonvention, and determine indicators for improvement.

Common to all studies is that LD is one of the world’s most pressing environmental problems. The situation will inevitably deteriorate if precautionary measures are not taken, as it is related to releasing greenhouse gasses into the atmosphere. Globally, about 25% of all land is degraded, and more than 1.5 billion people live in these areas. It is estimated that 24 billion tons of fertile land are lost annually due to unsustainable agricultural and industrial practices to increase efficiency, and if these aberrations continue, 95% of the world’s land could be degraded by 2050 [19].

It seems ordinary that LD is most prevalent in arid, semi-arid, and low-yield areas along with the pressure of population increase. Globally, half of the total land area of the European Union (4.18 million km²) is degraded annually, with Africa and Asia being the most affected [19]. On the global scale, smallholder farmers living in arid and semi-arid areas are the most affected by LD. Because the process is slow and imperceptible, people in the region usually become aware of it too late, which increases the severity and negative consequences of LD. It is estimated that, by 2050, LD and climate change will lead to a decline in global crop yields by about 10%. Most of this will occur in India, China, and sub-Saharan Africa, where SD could halve crop production [16]. It is estimated that the world population’s growing demand for agricultural products, including food, feed, fuel, and manufactured goods will increase by about 35% to 9.8 billion people by 2050 [20]. However, pressure on global land resources is also increasing as agricultural production systems become less resilient due to biodiversity loss and natural factors, such as climate change and extreme weather events. It is estimated that up to 700 million people will be displaced by 2050 due to issues related to scarce land resources. This number could reach 10 billion by the end of this century [19].

The components of desertification are the increase in population and food demand, increase in industrial raw materials, overconsumption, waste, and LD due to misuse. In addition to increasing food demand, food waste leads to increased consumption of natural resources. Globally, about 14% of food produced is lost between harvest and retail, while an estimated 17% of total global food production is wasted (11% in households, 5% in restaurants, and 2% in retail). Lost and wasted food accounts for 38% of total energy consumption in the global food system [20].

Because land degradation processes reduce carbon sequestration and increase greenhouse gases emissions (GHG), GHG reduction targets are unlikely to be met unless action is taken. Moreover, global food security targets will be missed if land degradation is not successfully addressed, as land degradation leads to productivity losses and thus reduced food supply [8, 21].

Human-induced accelerated erosion is directly influenced by land use/cover change (LU/LC). Overall, human activities have increased soil erosion to levels between 8% and 90% in many parts of the world. Globally, soil erosion caused by human activities has increased by about 60% during this period [22]. At the beginning of the twenty-first century, the global average value of potential erosion is 10.2 tons ha⁻¹ per year and the global soil loss due to erosion processes ranges from 24 to 75 billion tons of fertile soil [8, 12, 14]. Furthermore, it is estimated that there is an economic loss of about $ 400 billion in crop production [23].

Stopping or controlling LD to protect natural resources and reclaim degraded land has become a global phenomenon to ensure food security, and solutions have been sought at the global level. To this end, in the sustainable development goals (SDGs) adopted by world leaders in 2015 (Goal 15.3), a consensus was reached on combating desertification, restoring degraded lands and soils, including lands affected by desertification, drought, and floods and achieving a balanced world of LD by 2030 [2]. Therefore, countries are expected to develop implementation strategies by setting their targets to reduce poverty, ensure food security, improve nutrition, and reduce land degradation over the following decades.

On the other hand, efforts to offset LD by rehabilitating degraded land alone may not yield sufficient results. LD, which develops slowly and takes time to show results, needs to be monitored spatially and temporally on a global scale because of its dynamic relationship with climate change.

3. Land use/cover changes

With rapid population growth and evolving technology, human pressure on natural resources is increasing, causing these resources to lose their regenerative power. Appropriate management systems must be developed to protect natural resources and ensure their sustainability by preventing SD. For sustainable land management, it is crucial to ensure the effective management of natural resources by considering the balance between protection and use. LD and loss of productivity are mainly due to human misuse of land.

Land use /Land cover Cchange (LU/LCC) is a phenomenon that directly affects the sustainable use of natural resources, such as climate change, biodiversity loss, LD, desertification, and carbon sink areas. The increasing demand for food and energy that has accompanied population growth has resulted in land being used beyond its capacity. Humans have altered about three-quarters of the world’s land in the last millennium [24, 25]. LUC is estimated to have impacted nearly one-third (32%) of global land area over the past six decades (1960–2019), about four times greater than estimates from long-term land change assessments [26].

Humans cause 60% of land use change in the world’s land areas, and 40% is caused by the effects of climate change [27]. The majority of human-caused land change appears as the opening of forest land for agriculture, human settlement, and industry. Land use change for more productive purposes increases global sustainability problems. Critical sustainability components such as biodiversity loss, habitat destruction, and degradation of carbon sinks are directly affected by land use/land cover change.

Unfortunately, despite this importance, there are still large spatial and temporal uncertainties related to LU/LCC [28, 29]. The vast majority of studies using satellite imagery, etc., show that forest areas have decreased and agricultural areas have increased [26]. Results obtained with multidimensional maps show that forest areas are decreasing worldwide [30]. Another study found that forest land increased in 2016 compared to 1982 [27], but it is unclear from which LU this land originated or whether it is a forest or industrial land. For example, 66,000 hectares of land in West Sumatra, Indonesia, were put into operation for logging and 33,000 hectares of land that were destroyed after the contract expired were given to private companies for palm plantations [31]. It appears that these uncertainties will continue with normalized difference vegetation index (NDVI) imagery using satellite technologies that are not supported by ground reality on a global scale. LU/LC algorithms that classify global land cover into different types may not be suitable for producing scientific approaches and realistic values [32]. Global and national LU/LC data produced using different approaches are inadequate or inappropriate for land use management and reporting [33, 34].

Common to all studies that address LU/LCC is that LU/LCC in arid and semi-arid areas is negatively evolving due to increasing population pressure. The basis of decision made on Climate Change is based on taking place in the scope of Paris agreement about land use-change [25, 35]. These areas most affected by LU/LCC are naturally arid and semi-arid. Due to their fragile nature, these areas are extremely sensitive to changes in LU and LC. Balancing LD in these areas under the control of meteorological conditions requires a very long-term process. On the other hand, changing LU/LC in these areas degrades carbon sinks and triggers climate change through greenhouse gasses released into the atmosphere.

Forest areas are affected the most by LU/LCC. In the last two decades (2000–2020), forest area has decreased by 1 million km², or 2.4% of the forest area in 2000. Restoration of forest areas or afforestation activities has increased globally only in the European continent by 1.7% (64,000 km²). In other continents, restoration activities are not sufficient to prevent losses. In Africa and South America, this rate constitutes 30% of losses [32].

LU concepts and policies that lead to LD may make agroecological systems even more vulnerable to climate change [36]. Although replacing destroyed or degraded natural forest areas with plantations of high commercial value is considered beneficial in economic terms, these plantations that replace the natural ecosystem can lead to biodiversity loss [37] (e.g., the process of deforestation). The destruction of forested areas and their conversion into plantations with commercial value is one of the severe problems associated with LU/LCC today. Although the climate is significant for land productivity, the human factor is still the most important factor for climate change [2, 27, 38].

Short-term fluctuations in primary production make it extremely difficult to distinguish long-term fluctuations resulting from human-induced LD from the effects of periodic droughts [39, 40, 41, 42]. Human influences are further masked by topography, soil types, vegetation types, and spatial variability in land use.

4. Climate

Climate change is recognized as one of the most critical factors contributing to LD, as defined by the UNCD. Changes in the surface energy budget resulting from land surface change have a major impact on the Earth’s climate. Addressing climate as one of the factors leading to LD is more important than addressing the consequences of LD [43].

Precipitation and temperature are the main factors that determine the Earth’s climate and thus the distribution and density of vegetation. The interaction of human activities on the distribution of vegetation through land management practices and precipitation events makes the land more vulnerable to degradation. This vulnerability may become a severe threat as climate change continues.

Looking at the relationship between LD and vegetation/vegetation in general, it is known that as vegetation declines, land degradation continues to increase through feedback from the land surface to the atmosphere. Therefore, the decrease in vegetation is thought to reduce evaporation and increase radiation reflected into the atmosphere (albedo), which decreases cloud formation, that is, precipitation. Precipitation is the most important climate factor in identifying areas at risk of LD and potential desertification [44].

More important than the increase in precipitation has been the increase in heavy precipitation [45]. Climate models conclude that heavy precipitation will increase in the 21st century [46]. Soil erosion is a consequence of varying precipitation amounts and intensities. On the other hand, the intensity of precipitation and its energy are more critical than the amount of precipitation. The studies found that every 1% increase in the total amount of precipitation without any change in the intensity of precipitation increases the erosion rate by 0.85%. It has been shown that rainfall and intensity change together, and erosion increases by 1.7% for each 1% increase in rainfall amount [47].

In recent years, how climate change may affect temperature and precipitation has been a matter of debate considerably. However, there is still no extensive research conducted on the impacts of climate change on the wind. Changing wind speed has a direct and indirect impact on wind energy, climate fluctuations, storm events, and social life (maritime, etc.) [48, 49]. Wind speed is also a significant factor in the revised wind erosion equation model, which is calculated by considering the soil properties, vegetation cover and height, land roughness, soil moisture content, and land use changes [50]. In addition, the change in wind speed has a significant impact on soil moisture, evaporation, and water resources in arid and semi-arid regions. The removal of the topsoil, which is rich in prolific mineral and organic matter, through the wind causes a decrease in agricultural production potential and, consequently, contributes to land degradation [51].

LD and associated climate change must be controlled to protect vegetation, biodiversity, and ecological balance in land areas. LD and climate change are evolving as two phenomena that interact due to their dynamic behavior. According to IPBES, the contribution of climate change to LD is 10% of human-induced greenhouse gas emissions and forest land destruction. After the industrial revolution, global carbon emissions are estimated at 270 ± 30 gigatons (Gt) from fossil fuel combustion and 136 ± 5 Gt from LU and tillage changes. Most importantly, greenhouse gas emissions stored in the soil are released into the atmosphere through SD, triggering climate change. In this way, 4.4 billion tons of CO₂ were released into the atmosphere between 2000 and 2009 [52, 53]. Although CO₂ emissions into the atmosphere are proportionally decreasing due to intergovernmental meetings and treaties, they continue to increase and reached 418.2 parts per million (ppm) in 2021. This rate is around 50% higher than when the industrial revolution began [54].

According to the European Commission report (EC), 2011–2020 were the warmest 10 years on record, and the global average temperature rose 1.1°C above pre-industrial levels in 2019. Human-caused global warming is currently increasing by 0.2°C per decade. Compared to pre-industrial times, an average annual increase of 2°C will lead to dangerous and potentially devastating changes on a global scale. The world could face unexpected and far worse consequences than predicted. To control and prevent this pessimistic picture, the international community has accepted the need to keep warming below 2°C and limit it to 1.5°C [55].

Climate change and related meteorological conditions combined with LD seem to be a paradox that needs to be resolved quickly. The warming of the world with the effects of increasing human-induced greenhouse gasses, such as LD and industrialization, has increased about twofold in decades after the 1980s, considering past years (1880), reaching 0.18°C [56]. The change in precipitation regime and temperature increase with warming will affect all soil’s physical, chemical, and biological properties. Increasing soil fragility, especially in arid and semi-arid areas, increases the possibility of fundamental problems, such as erosion, loss of organic matter, compaction and decrease in water retention, loss of plant nutrients, and decrease or extinction of biodiversity [57].

SD intensely impacts a large part of the European continent. 45% of the continent’s soils contain little or very little organic matter [58]. Loss of natural soil functions due to drought, fire, and erosion leads to a significant increase in desertification risk in these areas [57]. Stabilizing LD and improving soil quality are linked to mitigating climate change [59].

The largest contributor to global warming is CO₂ released into the atmosphere by human activities. Atmospheric CO₂ concentrations in 2020 are 48% above pre-industrial (pre-1750) levels. Natural causes, such as solar radiation or changes in volcanic activity are estimated to have contributed less than plus/minus 0.1°C to the total warming between 1890 and 2010 [60].

Resulting in a temperature increase of 1 to3 °C in arid regions, the CO₂ concentrations are predicted to reach 700 ppm by 2050. This increase will increase the global potential evapotranspiration rate by 75–725 mm. Climate models predict that 50% of the world will experience regular droughts throughout the 21st century [59]. Higher temperatures, changing precipitation and regimes, and extreme weather will trigger LD, leading to water and wind erosion and further soil loss. Prolonged droughts could have devastating consequences for the 2.7 billion people living in semi-arid areas beyond meeting growing food needs.

Climate factors can lead to the degradation of clean water resources and soil quality. Considering that 10 countries are located in the region that is globally considered a risk zone with an elevation of only 10 m above sea level, the proportion of the population living in these areas varies from 38% (Gambia) to 88% (Bahama), and 634 million people live in these 10 countries [61], the dichotomy of LD and climate change is becoming more apparent. According to the February 2007 intergovernmental panel on climate change (IPCC) report [62], the sea level is projected to rise 59 cm by the end of the 21st century if fossil fuel consumption and economic growth continue unabated. If no further changes are made, a 38 cm rise in sea level is estimated to increase the number of people exposed to storm surges fivefold [63]. The potential sea level rise due to climate change may render clean water resources in these countries unusable due to salinization.

Temperature changes alter evapotranspiration, soil moisture, and infiltration. Because these potential changes alter surface runoff and infiltration rates, they can alter groundwater’s quantity and storage capacity. Climate change projections predict that soil moisture content will decrease significantly during extremely dry periods in the summer, while increases in excessive precipitation during these periods will increase erosion.

A significant portion of the climate change observed over the past 50 years is due to human activities [45]. In particular, the increase in average temperatures is the most apparent human-induced meteorological change. On the other hand, meteorological components are interrelated in many ways. The change in average annual precipitation amount, time, intensity, and the length of dry periods are the most prominent examples of the multiple dynamic relationships of meteorological factors. According to the IPCC assessment, the rise in temperature and changes in other climate parameters during the twentieth century are the result of radiative forcing from human greenhouse gas emissions [64].

The role of temperature in land productivity trends in recent years is explained by the 0.6 C average increase in air temperature observed globally between 1960 and 2006 [65]. About 9% of the decline in land productivity is due to temperature alone [2, 66]. A study on desertification factors in China found that the value of net primary productivity (NPP) decreased by 35.2% due to temperature increases and precipitation decreases [67].

Climate change and human activities are the main factors affecting vegetation dynamics, especially in high mountain regions characterized by highly fragile ecosystems [68]. Studies examining the effects of factors, such as climate change, temperature change, precipitation change, and solar radiation change [68] found an increasing trend compared to long-term averages. Studies show that temperature increases, especially in winter, compared to long-term averages [69, 70].

Climate conditions were found to be more important than human factors in affecting vegetation [68]. In studying the effects of climate change and human activities on desertification, it was found that desertification increased by 55.8%, with 70.3% of this rate caused by humans and 21.7% caused by climate change. On the other hand, stabilization or recycling of LD was found to be 42.1% caused by humans and 48.4% caused by climate change [67].

Most studies on desertification have produced mixed results regarding climate change and human impacts causing desertification. In fact, some study results indicate that the leading cause of desertification is long-term overgrazing, deforestation, and conversion of grasslands to cropland, while human-induced factors are more effective [71, 72, 73]. Other studies explain climatic conditions, such as drought, strong wind, and water erosion, temperature fluctuations, and winter precipitation as the main causes of desertification [74, 75].

The IPBES report notes that LD is a major contributor to climate change, and that deforestation alone is responsible for about 10% of all anthropogenic GHG. Another major contributor to climate change is the release of carbon previously stored in the soil, and LD was responsible for annual CO₂ emissions of up to 4.4 billion tons between 2000 and 2009. LD is a significant contributor to climate change, and climate change is projected to be the primary cause of biodiversity loss. By 2050, LD and climate change will reduce crop yields by an average of 10% globally and up to 50% in certain regions.

5. Soil organic matter

Soil organic matter (SOM) is an essential component of the terrestrial ecosystem. Any change in its quantity and composition in the soil significantly impacts soil and air conditions. Terrestrial areas, which contain the most organic carbon after the oceans, are much more unstable and open to short-term changes compared to the ocean and atmospheric conditions. The carbon balance in terrestrial ecosystems can change significantly under human activities.

High temperatures and low precipitation in drylands generally result in low organic matter (OM) production and rapid oxidation. Low OM leads to poor aggregation and low aggregate stability, which means a high potential for wind and water erosion. Loss of natural soil functions due to drought, fire, and erosion leads to a significant increase in desertification risk in these areas. The risk of desertification is most likely to occur in areas where precipitation is decreasing, dry periods are increasing in the summer months, and mis-intensive LU is occurring. The increase in temperature negatively affects carbon accumulation in the soil, leading to a decrease in organic carbon and an increase in the amount of carbon in the atmosphere [76, 77].

Land management will continue to be the most important determinant of SOM content and susceptibility to erosion in the coming decades. However, changes in vegetation due to short-term weather conditions and short-term climate changes will significantly affect soil organic matter dynamics and erosion, especially in semi-arid regions.

Soil carbon stores have a major impact on global climate change, and LD due to natural conditions or human activities is one of the leading causes of changes in soil carbon storage [78]. In a study conducted in semi-arid steppe areas [79], it was found that a sudden change in soil moisture due to high inter-annual rainfall variability causes about 65–80% of the total carbon loss in soils with different vegetation [80].

About 1550 Gt of soil organic carbon and 950 Gt of soil inorganic carbon constitute the global carbon source (2500 gigatons, Gt). Soil carbon source is 3.3 times greater than atmospheric carbon (760 Gt). Soil organic carbon varies from 30 tons ha⁻¹ at 1-meter soil depth in semi-arid climates to 800 tons ha⁻¹ in organic soils in cold regions and plays a vital role in the global carbon cycle and balance [81, 82]. The amount of soil organic carbon (SOC) is in a dynamic balance between storage and loss [75]. Even small changes can significantly impact climate and ecosystem stability, as organic carbon plays a critical role in soil-atmosphere carbon exchange and plant growth and food production in SOC [83, 84]. It is an indicator of the importance of soils in reducing the effects of global warming by retaining carbon dioxide in the atmosphere.

SOC depletion is a specific form of degradation that causes a decrease in soil quality and fertility [85]. It has been reported that some croplands have lost half to two-thirds of their SOC pools following LU/LCC, with a cumulative carbon loss of 30–40 tons ha⁻¹ [82]. Soil erosion results in the loss of a significant portion of SOC from the upper soil layer, where terrestrial ecosystems have more biological activity and organic matter. Regardless, lower concentrations of SOC reduce soil quality and productive capacity [84]. Therefore, understanding the spatial-temporal changes of SOC and the associated driving factors is crucial for assessing the feedback and maintaining ecosystem functions between the terrestrial carbon cycle and climate change [86, 87].

Human-induced desertification seems to be the main reason for the rapid release of SOC into the atmosphere. Due to the fragile ecosystem structures, especially in arid and semi-arid regions, unsustainable LU leads to increased carbon emissions released from the soil into the atmosphere [78]. As the SOC pool is depleted, 78 ± 12 Gt of carbon enters the atmosphere, which is about 1/3 of the acceleration of LD and erosion. The remaining 2/3 is mineralized. LD exacerbates CO₂ driven climate change by releasing CO₂ from cleared and dead vegetation and reducing the carbon storage potential of degraded land.

The slow decomposition of dead biomass (leaves, plant stems, and plant roots) in areas with low temperatures and adequate humidity leads to the accumulation of organic matter. Climatic conditions significantly impact the formation and storage of soil organic carbon. As temperature rise increases the decomposition of organic residues, it also increases carbon dioxide and methane gasses released from the soil. In the process of LD, the decrease of soil organic carbon content and the decrease of vegetation may cause a more potent greenhouse effect due to greater warming of the surface soil [78, 88]. Today, the area affected by desertification worldwide is about 3.6 billion hm² [82].

Whether the soil C pool acts as a source or sink of atmospheric CO₂ is primarily controlled by changes in climate and soil water content (SWC) [89, 90]. The variability in precipitation associated with climate change leads to changes in SOM. While plant growth and C storage are enhanced by precipitation [91, 92], the opposite is true in areas with insufficient precipitation and high temperatures [93, 94]. On the other hand, an increase in soil temperature can cause a decrease in SOM despite increased precipitation [95, 96]. High air temperatures and adequate moisture can promote microbial activity in the soil, leading to faster decomposition of SOC [97]. For example, while an expected 3.3 ⁰C increase in air temperature will result in a loss of 11–16% SOC in Europe, an average increase of 1°C in surface air temperature will result in a net loss of 5% in the worldwide SOC pool. Temperature increase alone leads to SOC losses in all soil moisture contents [80, 83, 96, 98].

6. Soil quality

Soil quality (SQ) stands out regarding the sustainable development of the global biosphere due to the fact that it is one of the most important terrestrial ecosystem functions. SQ is an indicator as a comprehensive reflection of the soil physically, chemically, and biologically. Revealing the dynamics of soil conditions, this sensitive indicator may change with the effect of different land uses and ecological restoration measurements [99, 100, 101]. Changes in land use have an impact on the physical and chemical properties of the soil, biological processes, and land productivity, ultimately leading to a change in SQ (Figure 1) [102, 103]. The impacts of the factors and their interaction on the soil quality were visualized in Figure 1.

Figure 1.

Basic soil quality flow chart.

In recent years, intensive efforts have been made on the concept of SQ and a reliable method to be used in measuring SQ. Some researchers define soil quality as the soil’s capacity and suitability for use based on its functions. Capacity is defined as a common function of properties including climate, topography, vegetation, and parent material while suitability is associated with land use and management as a dynamic concept influenced by humans. However, it is known that there are some losses in the physical, chemical, and biological properties of the soil affected by land use and land management measures worldwide [104, 105]. In summary, SQ is how well the soil promotes and enhances plant and animal productivity and maintains or improves water and air quality. Maintaining and promoting SQ, ensuring ecosystem sustainability, and making rational management plans are fundamental requirements [106]. Accordingly, the welfare of a society largely depends on the productivity power of the land and the sustainable use of this power. In the opposite case when these soil conditions are not considered, it is inevitable that the land degradation will continue and it will lose its functionality within the ecosystem as a result of the disruption of its productivity and fertility parameters, as in other natural resources.