**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

#### *Assessment of Land Degradation Factors DOI: http://dx.doi.org/10.5772/intechopen.107524*

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 <sup>0</sup> C 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].
