# A Review on Carbon Source and Sink in Arabic Land Ecosystems
## 2 Relevant Research on Carbon Sources and Sinks in Arable Land Ecosystems  
### Related Concepts of Carbon in Ecosystems  
The United Nations Framework Convention on climate change defines a \"carbon source\" as the process, activity or mechanism that releases greenhouse gases, aerosols or their precursors into the atmosphere, while \"carbon sink\" refers to the activity, process or mechanism of removing greenhouse gases, aerosols or their precursors from the atmosphere [33]. The knowledge acquired through the definition of carbon source and sink is that they are relative concepts. \"Carbon source\" refers to the matrix that releases carbon to the atmosphere in nature, and \"carbon sink\" refers to the deposit of carbon in nature.  
Many scholars use two indicators, net primary productivity (NPP) and net ecosystem exchange (NEE) of vegetation, to describe the carbon sources and sinks of ecosystems [22; 27; 34; 35]. NPP refers to the amount of organic matter in photosynthetic products fixed by plants per unit time and unit area after deducting the part consumed by the plants respiration [36]. It is an important index to evaluate the production capacity of the plant community under natural environment conditions and to measure the carbon sequestration capacity of vegetation [34; 35]. [PERSON] et al. [37] evaluated the temporal and spatial variation characteristics of the NPP of arable land ecosystems in China from 2001 to 2010 by combining MOD17A3 NPP data and GIS techniques, and observed that only 22% of NPP was significantly correlated with precipitation, and only 7% with temperature, indicating that arable land ecosystems were greatly affected by human activities. NEE is an important indicator to measure the carbon balance of ecosystems. It is the result of the balance between the total photosynthesis and total respiration of an ecosystem [38]. [PERSON] et al. [39] used a novel geospatial agricultural modeling system to calculate the NPP of crops, so as to estimate the NEE of arable land ecosystems. A positive value indicated that an area was a carbon source, while a negative value denoted a carbon sink. [PERSON] et al. [40] found that net radiation directly affected the seasonal variation of evapotranspiration and NEE in a winter wheat- summer maize system. [PERSON] et al. [41] observed that in a rice-wheat system, seasonal variation in daily NEE and daytime NEE was directly affected by crop vegetation growth, and nighttime NEE and soil temperature at 10 cm during the wheat season exhibited a significant exponential relationship when accounting for grain removal and the return of straw to the field, indicating that the system was a weak carbon sink.  
In addition, when calculating the carbon sources and sinks of arable land ecosystems, carbon absorption and carbon emission are usually estimated separately [2; 26]. Carbon absorption is estimated based on crop-yield data, economic coefficient and carbon absorption rate, while carbon emission is estimated based on different carbon-emission pathways and combined with the carbon conversion coefficient [24]. The following parameters are often used [42]: biological yield is the total amount of dry matter (mostly above ground) harvested from crops per unit area of land; economic yield refers to the dry matter weight of grains or other organs of crops harvested for food or other uses per unit area of land; and economic coefficient is the ratio between economic yield and biological yield, varying with plant species, varieties, natural environment and cultivation measures. In general, the data of economic yield can be obtained from national or regional statistical data [24; 26], but it is difficult to obtain the small-scale data [28]. Most of the carbon-emission coefficients of production activities are directly quoted from the research results of West (Oak Ridge National Laboratory) [43], Lal [44] and IPCC.