A carbon pool (reservoir) can be a source of carbon to the atmosphere if less carbon is flowing into it than is flowing out of it. It is the opposite of a sink.
IPCC, as quoted in Global Canopy Programme, "Glossary of Terms" - "The Little REDD Book. A Guide to governmental and non-governmental proposals for reducing emissions from deforestation and degradation."(Global Canopy Foundation, November 2008.) IPCC. IPCC Special Report: Land use, Land-use Change, and Forestry. IPCC. 2000. http://www.globalcanopy.org/themedia/file/PDFs/LRB_lowres/lrb_en.pdf
A compartment within the Earth system that releases carbon to the atmosphere.