[en] Labelling of actively growing plants in small field plots by pulse introduction of ¹⁴CO₂ into the plant canopy provides plant material suitable for a variety of investigations of carbon transfer in any agricultural ecosystem. In this experiment, maize plants were labelled in a sealed plastic tent at three dates during active growth. Measurements of CO₂ at select times and the degree of radioactivity carried by this gas were used to assess the quantity taken up by photosynthetic fixation and that respired by the roots. At three different stages of growth, the photosynthetic fixation of ¹⁴CO₂ introduced into the canopy utilized all the labelled gas in less than 1 hour, indicating a fixation rate of about 1 g CO₂·m^-2·h^-1. About one-third of the label that was fixed became incorporated into the root biomass. Production of roots and the label they carried increased with time until the ear set stage. Thereafter, root biomass and the associated activity remained relatively constant. Significant transfer of ¹⁴CO₂ to the soil atmosphere by the roots was observed at 3 hours after introduction of a pulse label into the plant canopy. The activity concentration in soil air, following final labelling, increased for a period of about 24 hours, then declined for a week, after which it was relatively stable during the final month until plant maturity. The total soil respiration under the growing maize crop varied between 3 and 5 g C·m^-2·d^-1. After the third labelling, when little soil CO₂ was expected from the heterotrophic activity because of exhaustion of the most available carbon from the previous year's crop residues, root respiration was maximum at 3.6 g·m^-2·d^-1. (author). 8 refs, 5 figs