**4.2 Soil carbon cycle process**

The soil carbon (C) pool includes organic carbon pools and inorganic carbon pools with carbon stocks of 1555 × 1015 and 1750 × 1015 g, respectively [43]. Inorganic carbon mainly refers to carbonate carbon existing in arid and semiarid soil. Carbonate can retain atmospheric CO2 during the formation process, and its formation and turnover have an important impact on the carbon cycle in arid and semiarid regions [2, 44]. Soil carbon cycle mechanisms in arid and semiarid regions include atmospheric pressure transport, carbonate dissolution, and soil waterin-gas percolation [45]. Li et al. showed that the evaporation in semiarid areas is greater than precipitation, forming an oasis landscape dominated by saline-alkali soils. Saline-alkali soil absorbs CO2 in the air at a slow rate, and the absorbed CO2 enters the underground saline layer; thus it is a huge potential inorganic carbon sink in the world (**Figure 3**) [46]. The SIC pool affects the SOC pool by affecting the status of soil aggregates, microbial activity, soil pH, and decomposition rate of organic matter. SOC is a very complex continuous mixture of residues of plants, animals, and microorganisms at all stages of decomposition. Many organic compounds in soil are closely related to inorganic soil particles [47].

Soil respiration consists of respiration by plant roots and respiration from catabolism by heterotrophy, mainly by soil microbes. Soil respiration is one of the major processes controlling the carbon budget of terrestrial ecosystems [48], the main export route of SOC and an important source of atmospheric CO2. Its dynamic changes will directly affect the global carbon balance [49]. Soil temperature is an

#### **Figure 3.**

*Schematic diagram of DIC (dissolved inorganic carbon) leaching and transport in a closed arid basin: Tarim Basin, as an example cited from Li et al. [46].*

important environmental factor, controlling a complex series of biochemical processes in soil respiration soil respiration rate sensitive to changes in soil temperature, soil temperature change will cause significant changes in soil respiration of terrestrial ecosystems carbon budget patterns have a significant impact [50]. Fierer et al. study have shown that with the increase of soil temperature, soil respiration rate of growth slowed, reducing sensitivity to temperature change, at lower temperatures, soil respiration mainly controlled by temperature changes [51]; when the temperature is high, soil respiration mainly affected by soil moisture and other factors. Soil moisture is a key limiting factor on soil respiration, soil moisture content in most ecosystems and soil respiration was significantly positively related to increased soil moisture will promote soil respiration [52]. Sponseller's study has shown that an increase in soil moisture accelerates the rate of soil respiration by affecting the vegetation's root metabolism and soil microbial activity [53]. After the soil temperature and moisture increase exceed a certain threshold range, microbial activity and soil permeability become lower, which will significantly inhibit soil respiration [54].
