Measurements of soil and canopy exchange rates in the Amazon rain forest using 222 Rn

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)