# A Review on Carbon Source and Sink in Arabic Land Ecosystems
## 3 Main Research Methods of Carbon Sources and Sinks in Arable Land Ecosystems
### Direct Observation of Carbon Sources and Sinks among Arable Land Ecosystems  
#### 3.1.1 Chamber Method  
The chamber method calculates the exchange rate of CO\({}_{2}\) between ground and air by using a chamber of a certain volume to cover the surface to be measured. This isolates the air inside the chamber, and the exchange rate of CO\({}_{2}\) is measured in the chamber. It is a direct method for measuring carbon flux [59]. Trace gas flux could be a useful indicator of ecosystem health, because it measures the material balance of vegetation and soils and the manner in which ecosystems respond to environmental pressure [60]. This method is currently the most popular technology in small-scale research [59]. [PERSON] et al. [61] measured the annual CO\({}_{2}\), N\({}_{2}\)O and CH\({}_{4}\) dynamics of two organic agricultural soils with different soil characteristics by static-chamber method. In Malaysia, [PERSON] et al. [62] used the closed-chamber method to measure the CO\({}_{2}\) flux of forest, sago and oil palm ecosystems, which showed that the relative humidity of the forest, 5 cm soil temperature of sago and water-filled pores of oil palm were correlated with CO\({}_{2}\) flux, indicating that land use affected the CO\({}_{2}\) exchange between tropical peatlands and the atmosphere. In the Sanjiang Plain of Northeast China, [PERSON] et al. [22] used the chamber method to select paddy fields and dry fields to study the seasonal variation of NEE, and quantitatively expressed the temporal variation law of arable land carbon sources and sinks. The results showed that the two types of arable land showed a weak carbon source in the non-growing season and a carbon sink in the growing season. In natural ecosystems, carbon sequestration during plant growth can promote an increase in carbon storage in soil [63]. However, for arable land ecosystems under anthropogenic management, there was evidence that carbon sequestration during crop growth cannot really increase the content of organic carbon in soil or improve the quality of organic carbon [22].  
Since the chamber method can only measure the gas flux on the soil surface, it cannot obtain the gas flux from soil or from soil to atmosphere, which hinders understanding of the relation between soil depth and gas generation and movement in the soil profile [64; 65]. Therefore, [PERSON] [66] proposed a gradient method, in which [PERSON]'s law was used to measure the gas flux in the soil profile based on the gas concentration gradient in the soil profile. [PERSON] et al. [64] compared the chamber method and the gradient method to  
Figure 1: Advantages and disadvantages of research methods of carbon source and sink in arable land ecosystems.  
measure N2O and CO2 fluxes in onion fields and corn fields. The results showed that the gradient method could be used to measure N2O flux (excluding high flux) and understand the seasonal variation law of CO2 flux. [PERSON] et al. [67] combined these two methods to measure the greenhouse gas emissions and underground flux of 0-115 cm soil in the corn dry-farming system for two years in northern China, which showed that the 0-40 cm soil layer was the main site of CO2 production and CH4 absorption in arable soil. This discovery improved understanding of the processes of gas production and consumption in the soil-atmosphere system. However, the estimation of flux was very sensitive to changes in the soil gas diffusion coefficient. Accurate determination of the diffusion coefficient was also a challenge [68]. Since it was difficult to accurately estimate the diffusion coefficient through either modeling or experiment, this became the main source of error in the gradient method [25].