# A Review on Carbon Source and Sink in Arabic Land Ecosystems
## 3 Main Research Methods of Carbon Sources and Sinks in Arable Land Ecosystems
### Mathematical Model of Carbon Sources and Sinks among Arable Land Ecosystems  
For the estimation of carbon budgets of arable land ecosystems in a region, the mathematical model estimation method is mainly used. It estimates carbon absorption and carbon emission with relevant statistical data such as crop sowing area, crop yield and agricultural-production input [26; 31]. The estimation of carbon absorption is mainly obtained by establishing mathematical models of the biomass and carbon absorption rate of various crops, while carbon emission is estimated by multiplying the use of various agricultural materials by their corresponding carbon emission coefficients [24; 28]. It is necessary to include carbon emissions from agricultural operations and inputs in the calculation, because changes in carbon emissions from agricultural operations can affect the net flux of carbon to the atmosphere by enhancing or reducing carbon sequestration [43]. In terms of index selection, chemical fertilizer, pesticide, plastic film, diesel, agricultural machinery and agricultural irrigation are usually selected [2]. The emission caused by crop growth is determined by the sowing area and its corresponding carbon emission coefficient, the carbon stocks of crop vegetation are mainly affected by planting area and carbon density, the planting area is affected by land use change and the carbon density is affected by crop biomass [74]. [PERSON] and [PERSON] [24] estimated the carbon-source and sink status of arable land ecosystems using the statistical data of crop yield and agricultural input from 1981 to 2001 in China's coastal areas, finding that reductions in agricultural planting area and increases in agricultural input weakened the carbon-sink function of arable land ecosystems. [PERSON] et al. [75] used a quantitative analysis method to estimate the NEP and NPP of arable land ecosystems in Virginia in the United States, concluding that these areas changed slowly from carbon-source to carbon-sink status. Since the carbon-sink function of crops played an important role in mitigating climate change [26], and there were differences in carbon-emission and carbon-absorption rates of different crops, quantifying the typical emissions of some food crops provides a starting point for exploring the potential for reducing the carbon emissions of food crops [76]. [PERSON] et al. [26] divided China into six typical regions, namely northeast, north, northwest, middle-lower reaches of Yangtze River, southwest and south, then collected and analyzed the carbon cost data of main crops, estimated the carbon-sink and source effects of arable land and quantitatively evaluated the carbon inputs and outputs of crop-production systems. The results showed that the major crop production was a net carbon sink of 236.32 Tg C yr\({}^{-1}\). The total annual net carbon sink of rice, wheat and corn was about 165.76 Tg C, to which rice was the highest contributor, accounting for 48.71%. In the study of [PERSON] et al. [77], optimization of crop-production structure by increasing the production area of soybean and reducing the production area of corn showed that 26% of nitrogen fertilizer use, 28% of active nitrogen loss and 19% of greenhouse gas emissions could be reduced. Some studies have found that the residue carbon input of rice straw and root system was higher than that of dry crops (rain-fed crops that mainly rely on natural precipitation), such as wheat, rape, cotton, corn and soybean [78,79]. [PERSON] et al. [32] also found that the SOC of rice crop fields was higher than that of soybean and significantly higher than that of sesame and cotton, indicating that rice was more conducive to carbon fixation than dry crops, and soybean was the most suitable dry crop for carbon fixation. The SOC content in paddy soil was higher than that in dry land, mainly because paddy soil is an anaerobic and low-temperature environment that inhibits the activity of microorganisms and reduces the mineralization rate of SOC, and is thus conducive to carbon sequestration [80].