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Is the sign of the NEE exchange defined as carbon sink or source? 

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The sign of the NEE exchange can be either a carbon sink or a carbon source, depending on the specific ecosystem and conditions. The NEE is defined as the net vertical exchange of CO2 between a specified horizontal surface and the atmosphere above it over a given period of time . In some cases, ecosystems act as carbon sinks, meaning they remove more CO2 from the atmosphere than they release, resulting in a negative NEE value. This is observed in saline lakes of the Tibetan Plateau, where significant CO2 uptake occurs during the freeze period . On the other hand, ecosystems can also act as carbon sources, releasing more CO2 than they absorb, resulting in a positive NEE value. For example, urban lawns in Beijing were found to be a CO2 source under control conditions . Therefore, the sign of the NEE exchange depends on the specific ecosystem and its carbon exchange processes.

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The sign of the NEE exchange is defined as a carbon sink under soil warming treatment and a carbon source under the control in the temperate urban lawn.
The paper states that eastern North America, Amazon, Congo Basin, Europe, boreal forests, southern China, and Southeast Asia are carbon sinks, while western US, African grasslands, Brazilian plateaus, and parts of South Asia are carbon sources. Therefore, the sign of the NEE exchange can be both a carbon sink or a carbon source depending on the region.
The paper states that the "large lake" and the "land station" have CO2 uptake, indicating that they act as carbon sinks. However, it does not mention the sign of the NEE exchange for the "small lake."
The sign of the NEE exchange is defined as a positive value representing emissions (a land source) and a negative value representing removals (a land sink).

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