Sources of $\ce{CO_2}$ during Carboniferous period
The terrestrial vegetation makes ca. 500 PgC (i. e. $500 \times 10^{15}$ grams of Carbon) which is rather small compared to the carbon stored in the ocean of ca. 37 000 PgC, see Fig. 6.1 in Ciais et al. 2013 1.
Even if we assume that the fossil fuels reserves are the remainders of the Carboniferous period, they only add up to no more than ca. 2 000 PgC. 

The terrestrial biosphere only makes use of a very small portion of the total available carbon.


1 Ciais, P., C. Sabine, G. Bala, L. Bopp, V. Brovkin, J. Canadell, A. Chhabra, R. DeFries, J. Galloway, M. Heimann, C. Jones, C. Le Quéré, R.B. Myneni, S. Piao and P. Thornton, 2013. Carbon and Other Biogeochemical Cycles. In T. F. Stocker et al. (Eds): Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.