# A Review on Carbon Source and Sink in Arabic Land Ecosystems
## 2 Relevant Research on Carbon Sources and Sinks in Arable Land Ecosystems
### Research Status of Carbon Source and Sink Activities in Arable Land Ecosystems  
Global terrestrial SOC stocks are about 1400-1500 PgC within 1 m depth soils, and are the largest carbon pool on the earth's surface, 2-3 times greater than that of terrestrial vegetation and more than twice that of the atmospheric carbon pool [45]. Small changes in terrestrial stocks cause changes in CO\({}_{2}\) concentrations in the atmosphere, thus affecting global climate [46]. SOC in arable land accounts for 10% of total organic carbon in soils [47]. Therefore, SOC has a certain function in the regulation of atmospheric CO\({}_{2}\) concentrations. Whether an arable land ecosystem is a carbon source or sink largely depends on thebalance between the fixation of arable land SOC and the release of greenhouse gases [31]. Therefore, research on the arable land SOC pool has gradually become the focus of the international community. Many countries had completed the estimation of their SOC stocks on national or regional scales [48]. SOC stocks were mainly estimated by soil types, vegetation types or model methods, and the determination of relevant estimation factors was mainly obtained by collecting historical data and satellite images and through hyperspectral remote sensing technology. [PERSON] et al. [49] estimated that topsoil SOC stocks were about 5.1 PgC, based on the second soil survey data from 1979 to 1982 in China. Considering the entire arable land category, [PERSON] et al. [50] estimated that the average SOC stock in the topsoil (30 cm) in Italy was 52.1 \(\pm\) 17.4 Mg C ha\({}^{-1}\), which was similar to that reported by other European countries. [PERSON] et al. [51], combining SOC data with arable land area data, estimated that the SOC stocks of arable land was about 49,000 tons in Belgium. In France, [PERSON] et al. [52] estimated SOC stocks at 0-30 cm soil depth according to land use and soil type using data from geo-referenced databases. The results showed that SOC stocks were 15-40 Mg C ha\({}^{-1}\) in central France, and SOC stocks were 40-50 Mg C ha\({}^{-1}\) in northern and southwestern regions. Due to the strong carbon sequestration capacity of soil, a large number of studies on estimating soil carbon sequestration potential and on methods to achieve higher carbon sequestration have emerged. Some countries with large areas of crops and forestland have the potential to offset large greenhouse gas emissions by sequestering carbon in aboveground biomass and soil [53]. [PERSON] [54] estimated that the total carbon sequestration potential of global arable land was 0.75-1.0 Pg C yr\({}^{-1}\). In the United States, it was estimated the arable land has the potential to store 75-208 million metric tons of carbon equivalence per year, up to 8% of emissions [55]. In China, [PERSON] [56] estimated the soil carbon-sequestration potential of agricultural and forest soils and concluded that the soil carbon-sequestration potential of China can offset about 25% of the annual emissions of fossil fuels. [PERSON] et al. [57] predicted the carbon-sequestration potential of arable land soil under three management measures (nitrogen fertilizer application, straw returning and no tillage measures) in four agricultural regions of China using the existing field test survey results, which were about 12.1, 34.4 and 4.6 Tg C yr\({}^{-1}\), respectively. Among the three measures, straw returning had the greatest carbon sequestration potential, with the capacity to offset 5.3% of China's CO\({}_{2}\) emissions from fossil fuel combustion in 1990. If more incentive policies could be formulated and implemented, China's arable land soil carbon sequestration would be increased approximately twofold [57]. The 21 st United Nations climate change conference proposed to increase the global soil organic matter (SOM) by 0.4 percentage points, pointing out that under the best management practices, this goal was expected to be achieved or even exceeded.