Can decarboxylation be done under carbon dioxide? From Why is the decarboxylation of 2-hydroxybenzoic acid (and related compounds) done &#39;under nitrogen&#39;? I confirmed that nitrogen is used to exclude oxygen - in order to prevent oxidation of the material.

Can $\ce{CO_2}$ be substituted for nitrogen, or will the presence of carbon dioxide slow down the decarboxylation reaction (which is causing carbon dioxide to be released)?
Because nitrogen is the standard inert gas in lab practice. If you are "rich" You can use argon, but carbon dioxide is seldom used, because it won't work on alkaline solutions. 


  or will the presence of carbon dioxide slow down the reaction


First, what you presumably mean is influencing the equilibrium unfavourably, don't you? You should learn to discern kinetics and equilibrium. 

Answer: not really. Whether you have a carbon dioxide atmosphere or not will shift the decomposition temperature not more than some degrees.