Department of Earth System Science

Measurements of the emission of ²²²Rn from Amazon forest soils, and profiles of ²²²Rn in air were used to study the ventilation of the soil atmosphere and the nocturnal forest canopy. The emission of ²²²Rn from the yellow clay soils dominant in the study area averaged 0.38±0.07 atom cm⁻² s⁻¹. Nearby sand soils had similar fluxes, averaging 0.30 ± 0.07 atom cm⁻² s⁻¹. The effective diffusivity in the clay soil (0.008±0.004 cm² s⁻¹), was lower than that for the sand soil (0.033±0.030 cm² s⁻¹). Profiles of ²²²Rn and CO₂ in air showed steepest concentration gradients in the layer between 0 and 3 m above the soil surface. Aerodynamic resistances calculated for this layer from ²²²Rn and CO₂ varied from 1.6 to 18 s cm⁻¹, with greater resistance during the afternoon than at night. Time averaged profiles of²²²Rn in the forest canopy measured during the evening and night were combined with the soil flux measurements to compute the resistance of the subcanopy to exchange with overlying air. The integrated nocturnal rate of gas exchange between the canopy layer (0 to 41 m) and overlying atmosphere based on ²²²Rn averaged 0.33±0.15 cm s⁻¹. An independent estimate of gas exchange, based on 13 nights of CO₂ profiles, averaged 0.21±0.40 cm s⁻¹. These exchange rates correspond to flushing times for the 41 m canopy layer of 3.4 and 5.5 hours, respectively. Comparison of ²²²Rn and CO₂ profiles show that the nocturnal production of CO₂ by above-ground vegetation was about 20% of the soil emission source, consistent with data from eddy-correlation experiments (Fan et al., this issue).