Answer:
In the context of the carbon cycle and Net Ecosystem Exchange (NEE), a carbon source refers to any process that releases more carbon into the atmosphere than it absorbs, such as the burning of fossil fuels or deforestation. On the other hand, a carbon sink refers to any process that removes more carbon from the atmosphere than it releases, such as photosynthesis in plants or the absorption of carbon by the oceans.
NEE is a measure of the net exchange of carbon dioxide (CO2) between the land surface and the atmosphere. A positive NEE value indicates that the ecosystem is a carbon source, releasing more CO2 than it absorbs, while a negative NEE value indicates that the ecosystem is a carbon sink, absorbing more CO2 than it releases.
Therefore, the difference between a carbon source and sink with regards to NEE is that a carbon source releases more carbon into the atmosphere than it absorbs, leading to a positive NEE value, while a carbon sink absorbs more carbon from the atmosphere than it releases, leading to a negative NEE value.