# A Review on Carbon Source and Sink in Arabic Land Ecosystems
## 4 Main Factors Affecting the Dynamics of Carbon Sources and Sinks in Arable Land Ecosystems
### Effects of Natural Environmental Factors on Arable Land Carbon  
The rate of soil carbon loss is related to the soil environment, which is strongly controlled by climatic conditions [19]. Climate controls NPP above and below the ground, and thus the input of organic matter, while climate also contributes to carbon loss by driving the output of organic matter through microbial activity in the soil [82; 83]. A large number of studies have confirmed a negative correlation between SOC and temperature in arable land ecosystems. Low temperature may reduce the mineralization of SOC through thermodynamic mechanism [3; 20; 84; 28]. This conclusion was verified in arable-land ecosystems in different regions [85; 86; 87], and was also confirmed in forest ecosystems [82] and shrub ecosystems [30]. However, the situation was different in some low-temperature areas. Increasing temperature could stimulate the input of plant productivity [88], which was more conducive to the accumulation of SOC. On the Qinghai Tibet Plateau, [PERSON] et al. [89] concluded that the increase of annual average temperature had a positive impact on SOC density, which might ascribe to the increase of soil carbon input exceeded the carbon loss caused by elevated temperature. Therefore, the increase of temperature might make the soil accumulate carbon under warming conditions. In addition, the types suitable for crop growth will vary under different temperatures, leading to differences in photosynthetic rate and carbon absorption rate. For example, temperature led to differences in crop structure between South and North China, further resulting in differences in carbon sources and sinks in arable land ecosystems [24]. In the research of [PERSON] et al. [72], although the season length of maize (113 days) was 52% shorter than that of wheat (235 days), more than 55% of CO\({}_{2}\) emissions come from maize season, and the interaction of soil temperature and moisture better explained the variations of the ecosystem respiration and soil respiration  
Figure 2: The diagram depicts the flow of carbon between the soil carbon pool, the atmospheric carbon pool and the plantation carbon pool in arable land ecosystems.  
from the relatively colder and drier wheat growing season to the warmer and wetter maize growing season.  
The impact of precipitation on SOC cannot be ignored, because water is the basic driver of almost all chemical and biological processes, including plant growth and survival, photosynthesis, microbial activities and soil respiration [90]. Precipitation changes might directly (by promoting microbial growth and activity) and indirectly (by improving plant productivity and soil carbon input) affect the soil carbon cycle [21]. When the increase of organic matter input from biomass exceeds the increase of biological activity, SOC would be accumulated [85]. Precipitation is generally positively correlated with SOC, which can be explained that when water resource is limited, plants grow slowly and contribute less organic matter to the soil [86]. Drought inhibits ecosystem productivity and respiration. Since ecosystem productivity is usually more sensitive to drought than respiration, drought may significantly reduce the intensity of terrestrial ecosystem carbon sink and even convert it into a carbon source [6]. However, in low-temperature ecosystems, dewdrops could significantly supplement the water required for vegetation growth [91], so precipitation might not be the main factor affecting SOC accumulation. At present, one of the most severe challenges is the increase in the frequency, intensity and duration of extreme climate events, which is also one of the most significant features of global climate change [6]. The strong dependence on precipitation and temperature highlights the carbon sensitivity of arable land ecosystems under future climate change. It is necessary to further discuss how to better describe the response of arable land ecosystem to extreme climate.