Some substantiation:

"Even without the massive burning, the popular conception of the Amazon as a giant oxygen factory for the rest of the planet is misguided, scientists say. Left unmolested, the forest does generate enormous amounts of oxygen through photosynthesis, but it consumes most of it itself in the decomposition of organic matter"

This is speciffically about the Amazon, but as it is mentioned in the article, I think it is relevant.

Link: http://articles.latimes.com/2005/jun/08/world/fg-amazon8