Based on the characteristics that plants can take advantage of carbonic anhydrase enzyme to catalyze the bicarbonate ions into carbon dioxide and water,which can serve as substrates for photosynthesis,two sodium bicarbonate whose delta~(13)C_(PDB) value differences were greater than 10 were used as the carbon isotope labeling and to bidirectional water culture Orychophragmus violaceus and Brassica juncea lasted for 24 h in this study,respectively.The daily mean stable carbon isotope compositions in two different times and environments were investigated and analyzed according to isotope ratio mass spectrometry (IRMS) technique.In particular,Environment 1 represented adding some sodium bicarbonate to solutions to get different concentrations of bicarbonate solution and Environment 2 represented adding some polyethylene glycol to solutions to get different concentrations of PEG solutions.These results showed that delta_(Ca) values in the Environment 1 were related to the concentrations of added exogenous sodium bicarbonate to solution,but these in the En- vironment 2 had no significant correlation with PEG concentration in the treatment solution,these results in this study suggested that the daily mean stable carbon isotope composition of atmospheric carbon dioxide had no relationship with the cultured plant species,but was related to the concentration of exogenous bicarbonate ion added to the culture solution and growth rate of the two plants.The data obtained with good reproducibility and reliability,accurately determined the stable carbon isotope composition of atmospheric carbon dioxide in test environments.This study confirmed that the method was a very powerful tool for monitoring the carbon isotope composition and sources of atmospheric carbon dioxide in different times and places for further.