Although labeling plant-soil systems with carbon isotopes has been used in a range of experiments since the 1960s, the application of continuous labeling has been much limited because of critical drawbacks of the existing methods. This presentation introduces a novel continuous labeling method using a 13C-depleted carbon dioxide source primarily produced from natural gas. Both theoretical and empirical results indicate that the CO2 concentration and its 13C abundance in a chamber (or in a greenhouse) can both be controlled at constant levels by using our method of injecting CO2-free air at a certain rate and injecting 13C-depleted CO2 as needed by a automated control system. This new method does not involve any radiation safety issues as does with any 14C labeling. The use of natural gas-derived 13C-depleted CO2 replaces the need for expansive 13C-enriched CO2 that is necessary in any 13C-enrichment labeling. The main limitation of this new labeling method is its relatively low sensitivity in partitioning different carbon sources.