An empirical method to measure respiratory CO₂ recycling using a fat growing agricultural cover crop as a model system was tested and compared with a theoretical method. Both methods gave values which were high and similar to each other. The theoretical method gave a value of respiratory based CO₂ recycling of 0.41, while the empirical method gave value of 0.49.Therefore 41 to 49% of the respired CO₂ is re-fixed during daytime photosynthesis in this densely planted cover crop. Refixation of respired CO₂ during the day should lead to an isotopic enrichment of the remaining respired CO₂ leaving canopy of the cover crop. Therefore, calculations of gross respiration and photosynthesis using isotopic mass balance equations that do not take the isotopic fractionation into account could be in error. We tested this premise but using isotopic mass balance equations to measure average gross photosynthesis and respiration in this cover crop under two scenarios: 1) no recycling and 2) recycling of respired CO₂ .Values of gross photosynthesis and respiration were unrealistically low when it is assumed that no recycling occurs. On the other hand realistic values similar to previous publications were observed when recycling is taken into account.