It's not the only net source of CO2; land use in livestock is a meaningful part of it. (scientificamerican.com)
Thus, on balance, the terrestrial ecosystems appear to have been a comparatively small net source of carbon dioxide during this period. (rabett.blogspot.ru)
The carbon balance of tropical ecosystems remains uncertain, with top - down atmospheric studies suggesting an overall sink and bottom - up ecological approaches indicating a modest net source. (science.sciencemag.org)