Time variations of the CO2 concentration and the randon 222 daughter activity in the atmosphere are observed 16 m above the ground in the outskirts of Heidelberg (Germany), situated in the rather densely populated and intensively cultivated upper Rhine valley. These parallel recorded data series are used to estimate the CO2 respiration rate from the soil and from plant leaves. Atmospheric concentration variations of both gases appear to be merely due to variations in atmospheric mixing. From direct measurements of concentration variations of both gases appear to be merely due to variations in atmospheric mixing. From direct measurements of concentration profiles in the soil air we deduce the 222Rn exhalation rate, which is nearly constant with time and about 4000 dpm m-2 h-1. This flux is consistent with the one derived from observed pileup of the 222Rn activity of an air mass during passage over the continent. The constant 222Rn exhalation rate quoted serves as a reference for atmospheric dilution of the soilborne CO2. The comparison of the two concentrations leads to a total CO2 respiration flux to the atmosphere of 21 mmol CO2 m-2 h-1 for a short time period in August 1980. Direct measurements derived from CO2 concentration profiles in the soil yield an average regional CO2 flux from the soil alone of 12 mmol m-2 h-1 with a maximum of 20 mmol m-2 h-1 in June and July and a minimum of 3 mmol m-2 h-1 or less in winter. The different 222Rn exhalation rates from the continent and ocean (where it is smaller by 2 orders of magnitude) are further used to identify the continental or the maritime origin of air masses.