**4. Dynamic characteristics of forest soil carbon**

#### **4.1 Evolution characteristics of soil carbon after afforestation**

Land degradation and desertification are pervasive in arid and semiarid regions, often resulting in emission of CO2 into the atmosphere as well as other environmental degradation [35]. Afforestation can increase sequestration of atmospheric carbon dioxide and hence attenuate global warming [36]. Farmland reclamation will exacerbate land degradation and desertification in arid and semiarid regions due to the special climate. Afforestation is the conversion of degraded farmland into vegetation in these regions and renovation without involving natural vegetation. This is similar to the method of Grain for Green Program (GGP) in central and western China [37]. In addition, we consider the choice of tree species to adapt to the arid climate is necessary. The contribution of afforestation to the C cycle has been estimated by many studies on a regional and global scale [6, 38]. Land use and land-cover changes have attracted increasing scientific interest in the past decades in relation to their contribution to potential impacts on soil carbon sequestration and soil nitrogen [39]. Liu et al. estimated the changes in SOC (a) and total nitrogen (TN) stocks (b) after afforestation of arid and semiarid regions using meta-analysis based on the dataset compiled from published studies (**Figure 2**) [40].

Afforestation on different land uses showed different impacts on SOC stock and TN stock. On average across all studies, afforestation significantly increased SOC stock and TN stock by 131 and 88%, respectively. SOC stock and TN stock decreased with different land uses in the following order: BF > CF > GF (CF: afforestation on cropland; GF: afforestation on grassland; and BF: afforestation on barren land); and they also reported significant increases in SOC stock as afforestation was observed for all tree species. SOC and TN accumulations in plantations with different tree species decreased in the following order: broadleaf deciduous > conifer > broadleaf

#### **Figure 2.**

*Changes in soil organic carbon (SOC) (a) and total nitrogen (TN) stocks (b) after afforestation as influenced by prior land-use type, planted tree species, and plantation age, respectively, cited from Liu et al. [40]. CF, afforestation on cropland; GF, afforestation on grassland; and BF, afforestation on barren land. Planted tree species were classified into three categories: broadleaf deciduous, broadleaf evergreen, and conifer. The ages of afforestation were divided into three groups: young age (*≤*10 year), middle age (>10,* ≤*30 year), and old age (>30 year).*

**45**

*Soil Carbon Biogeochemistry in Arid and Semiarid Forests*

evergreen. SOC stock significantly increased following afforestation from 114 to 183% with the increase in plantation age. Nonetheless, afforestation-induced changes in SOC stock did not differ significantly among plantation ages. Afforestation significantly increased TN stock by 84–100% for plantations with different ages, with the largest increase being found in plantation with old age. However, the differences in

These results suggest that in arid and semiarid regions, BF is a better option for accumulating C and N in comparison to CF, while GF is not recommended as a way to sequester C and N into soils. Korkanç's study showed that the 0–10 cm soil layer of lands afforested with Cedar, a coniferous tree, sequestrated more organic carbon than Black Pine in the central Anatolia region [27]. The inconsistency between the above two research conclusions also proves the necessity to evaluate afforestation efforts using different species of trees on semiarid degraded land as measured by soil SOC and selected soil properties. This also may be useful for determining which species of trees to plant in future afforestation efforts aimed at combating the impacts of global warming [27]. Although all estimates of soil C loss due to land degradation are speculative, the numbers are large (20–30 × 1015 g) [35]. Cole et al. through desertification control and adoption of recommended land use and soil management practices, this would amount to 12–20 × 1015 g over a 50-year period [41]. After afforestation, SOC and TN accumulations generally showed increasing trends with the increase of plantation age, and restoration age is an important factor to consider when estimating SOC stock and TN stock after afforestation in arid and semiarid regions. Korkanç also reports that SOC values of the afforested lands are generally higher than those in the bare land soils, and the highest SOC value was obtained from the 0 to 10 cm layer in the soils of the Cedar site (1.49%), and the lowest value was from the 10 to 20 cm soil layer in the bare land (0.44%) [27]. According to Lima et al., afforestation of degraded grasslands led to a rise in SOC accumulation in the semiarid regions for a period of 30 years [42].

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 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

changes in TN stock among plantation ages were not significant [40].

*DOI: http://dx.doi.org/10.5772/intechopen.87951*

**4.2 Soil carbon cycle process**

soil are closely related to inorganic soil particles [47].

### *Soil Carbon Biogeochemistry in Arid and Semiarid Forests DOI: http://dx.doi.org/10.5772/intechopen.87951*

*Applied Geochemistry with Case Studies on Geological Formations, Exploration Techniques…*

*Changes in soil organic carbon (SOC) (a) and total nitrogen (TN) stocks (b) after afforestation as influenced by prior land-use type, planted tree species, and plantation age, respectively, cited from Liu et al. [40]. CF, afforestation on cropland; GF, afforestation on grassland; and BF, afforestation on barren land. Planted tree species were classified into three categories: broadleaf deciduous, broadleaf evergreen, and conifer. The ages of afforestation were divided into three groups: young age (*≤*10 year), middle age (>10,* ≤*30 year), and old age* 

**44**

**Figure 2.**

*(>30 year).*

evergreen. SOC stock significantly increased following afforestation from 114 to 183% with the increase in plantation age. Nonetheless, afforestation-induced changes in SOC stock did not differ significantly among plantation ages. Afforestation significantly increased TN stock by 84–100% for plantations with different ages, with the largest increase being found in plantation with old age. However, the differences in changes in TN stock among plantation ages were not significant [40].

These results suggest that in arid and semiarid regions, BF is a better option for accumulating C and N in comparison to CF, while GF is not recommended as a way to sequester C and N into soils. Korkanç's study showed that the 0–10 cm soil layer of lands afforested with Cedar, a coniferous tree, sequestrated more organic carbon than Black Pine in the central Anatolia region [27]. The inconsistency between the above two research conclusions also proves the necessity to evaluate afforestation efforts using different species of trees on semiarid degraded land as measured by soil SOC and selected soil properties. This also may be useful for determining which species of trees to plant in future afforestation efforts aimed at combating the impacts of global warming [27]. Although all estimates of soil C loss due to land degradation are speculative, the numbers are large (20–30 × 1015 g) [35]. Cole et al. through desertification control and adoption of recommended land use and soil management practices, this would amount to 12–20 × 1015 g over a 50-year period [41]. After afforestation, SOC and TN accumulations generally showed increasing trends with the increase of plantation age, and restoration age is an important factor to consider when estimating SOC stock and TN stock after afforestation in arid and semiarid regions. Korkanç also reports that SOC values of the afforested lands are generally higher than those in the bare land soils, and the highest SOC value was obtained from the 0 to 10 cm layer in the soils of the Cedar site (1.49%), and the lowest value was from the 10 to 20 cm soil layer in the bare land (0.44%) [27]. According to Lima et al., afforestation of degraded grasslands led to a rise in SOC accumulation in the semiarid regions for a period of 30 years [42].
