Volcanoes - myth, emissions are tiny.
http://www.geology.sdsu.edu/how_volcanoes_work/climate_effects.html
Oceans - they emit a lot, but it's almost a wash, they both emit a lot and absorb a lot.
http://environment.newscientist.com/channel/earth/climate-change/dn11638
graph in upper right.
Decaying vegetation is the same deal. The CO2 released was taken out of the air recently, so it's a wash:
" Terrestrial (land) biology consumes about 110 units per year, mostly in support of terrestrial plant growth. About 50 units of that goes back into the atmosphere in the decay process, and about 60 units goes into the soil. The soil, it turn, releases about 60 units back to the atmosphere, resulting in a balance of fluxes in and out of the natural terrestrial biological system."
http://www.iitap.iastate.edu/gccourse/chem/gases/gases_lecture.html
The recent _increase_ in atmospheric CO2 is due to human industry. The natural carbon cycle buried that carbon a very long time ago over thousands of years. We're digging it up and burning it, real fast.
Scientists can prove the CO2 increase is due to industry by measuring the isotopic ratio of the carbon. Details here:
http://www.realclimate.org/index.php?p=87
So, while there are other sources of CO2, human industry is responsible for the increase of CO2 seen on the last graph. The little teeth are the natural cycle; plants reducing it in summer, it going up a little in winter. The huge move upwards is us, burning fossil fuels.
http://scrippsco2.ucsd.edu/graphics_gallery/mauna_loa_record/mlo_record.html
We're messing up nature big time. And that threatens very bad things for coastal flooding and serious damage to our agriculture. We need to fix it.