Organic carbon (C6H7O2) and carbonate carbon (CaCO3) with different carbon concentration were selected as the model samples to perform laser induced breakdown spectroscopy experiments. The excitation characteristics of carbon with different forms were analyzed in air atmosphere. The results showed that carbon atomic emissions could be detected from carbonate carbon, which had a good linear relationship with the carbon content. But it is difficult to detect the molecular carbon. For organic samples, not only the atomic carbon could be detected, but also can detect the molecular carbon (CN and C2), and the molecular emissions had a good correlation with carbon content. The results indicated that the formation of molecular carbon is related with the organic form. After mixing the two forms of samples, it is found that there is a good correlation between the molecular CN and the organic carbon content, which indicated that the molecular CN could be used to realize the identification of organic carbon form. On the other hand, according to the formation route of molecular CN, a certain amount of nitrogen was added to the mixed samples with two forms of carbon, performed under the Ar atmosphere, it was also found that there exist the good linear relationship between the molecular CN and the organic carbon content. All the measurements showed that either the nitrogen from the air or from the sample itself could be both used for the identification of carbon from the organic forms.