Due to the limitations in methodology it has been a difficult task to measure rhizosphere respiration and original soil carbon decomposition under the influence of living roots. 14C-labeling has been widely used for this purpose in spite of numerous problems associated with the labeling method. In this paper, a natural 13C method was used to measure rhizosphere respiration and original soil carbon decomposition in a short-term growth chamber experiment. The main objective of the experiment was to validate a key assumption of this method: the 6~3C value of the roots represents the 613C value of the rhizosphere respired CO2. Results from plants grown in inoculated carbon-free medium indicated that this assumption was valid. This natural ~3C method was demonstrated to be advantageous for studying rhizosphere respiration and the effects of living roots on original soil carbon decomposition.