**3.3 Effect of elevated CO2 on soil carbon**

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

and Db/chemical properties had a stronger effect on OC, than did physical properties, and goodness-of-fit indices for the SEM are all acceptable. We should note the independent effects on OC content between physical properties and chemical properties. The above case studies were conducted mainly in semiarid Mediterranean regions. It is easy to speculate that the conventional relationship between OC and influencing factor in soil is influenced on a global scale, such as that found commonly in northwestern China, western America, and Midwestern Australia. Land degradation and desertification are pervasive in arid and semiarid climate, lands are especially threatened by erosion phenomena, and the restoration of these regions needs afforestation, which inhibits these land degradation phenomena and enhances soil carbon sequestration and soil fertility. Korkanç's study also concluded that afforestation increased the SOC budget, and this situation improved some soil properties, such as increasing water holding capacity (WHC) and total porosity (TP) and

*The estimated parameters of the model predicting SOC in clayey soils (a) and sandy soils (b), respectively, cited* 

reducing Db and dispersion ratio (DR) over a period of 15 years [27].

**42**

**Figure 1.**

*from Brahim et al. [22].*

The changes in the amount of carbon sequestered by soils are closely related to the increase or decrease in the amount of CO2 accumulation in the atmosphere. Elevated atmospheric CO2 frequently increases plant production and concomitant soil C inputs, which may cause additional soil C sequestration [28]. While the processes of C sequestration are ultimately regulated at the molecular level, atmospheric CO2 concentration can greatly affect the way in which terrestrial ecosystems sequester C [29]. Niklaus et al. reported that the increases in leaf litter production at elevated CO2 may exceed the response in standing biomass [30]. In addition, elevated CO2 may also induce greater C fluxes from the growing plants to the soil through increasing rates of leaf litter and root material deposition [31]. Thus, elevated atmospheric CO2 will likely affect soil carbon cycle through its indirect impact on photosynthesis. If C input into the soil is increased, and given that elevated atmospheric CO2 increases plant production and allocation of photosynthate to below ground components, soil carbon sequestration would be expected to increase [32].

Diaz et al. found that increased C inputs under elevated CO2 stimulated competition between the soil microbial biomass and plants for soil N, leading to a decline in soil N availability [33]. Hu et al. suggested that elevated CO2 reduces the amount of N available to microbes through enhanced plant growth [34]. This could result in enhanced C accumulation in grassland soils at elevated CO2. However, it remains unclear how initial increases in soil C input under elevated CO2 affect microbial N transformation processes [28].
