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

Abstract

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 of 222Rn 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).

Fluxes of CO2, N2O and CH4by222Rn and chamber methods in cold-temperate grassland soil, northern Japan

“…From the results of the 222 Rn method, average diffusion coefficients and relaxation depths of CO 2 and CH 4 were similar to previous values in aerated sandy soils (Dörr and Münnich 1990;Trumbore, 1990;Dueñas et al 1999;Kim and Tanaka 2002). There are yet few reports for a diffusion coefficient and relaxation depth of N 2 O and, as a result, we first report here on these factors for our aerated grassland soil site.…”

“…However, the 222 Rn method does give significant information about trace gas diffusion in the soil throughout the year. Only a small fraction of 222 Rn is produced by recoil from soil particles into the air, depending on the specific surface area of soil particles-and thus depending also on grain size distribution (Trumbore et al 1990;Davidson and Trumbore 1995;Dueñas et al 1999;Kim and Tanaka 2002). 222 Rn flux at the soil surface is affected by 226 Ra contents, as well as soil physical properties such as grain size distribution and resistance to molecular diffusion in the unsaturated soil zone.…”

“…Gas transport in the soil was examined using the radioactive, inert, noble gas 222 Rn (Radon,, which is produced by α-decay of 226 Ra (Radium, half-life: 1602 y), and which has been used as a tracer of unsaturated soil in the method adopted by Born et al (1990), Münnich (1987, 1990), Trumbore et al (1990), Davidson and Trumbore (1995), Dueñas et al (1999), and Kim and Tanaka (2002). The 222 Rn method requires significant additional labor compared to the static chamber method, because of the need to measure both depth profiles of 222 Rn and other trace gases in the soil, and 222 Rn flux at the soil surface.…”

Development of a Compact and Sensitive Electrostatic Radon-222 Measuring System for Use in Atmospheric Observation

“…Similar methodologies were applied to evaluate emissions of CO 2 (Schmidt et al 2003), N 2 O (Schmidt et al 2001), and CFCs (Biraud et al 2000). In another approach, Trumbore et al (1990) estimated the rates of ecosystem gas exchange between the air above and below the soil surface and these between the air inside and above the canopy layer in the Amazon rain forest, using 222 Rn concentration measurements in the soil and the atmosphere. Martens et al (2004) reported on a method for estimating the net ecosystem exchange (NEE) of CO 2 using measurements of atmospheric 222 Rn accumulation in the Brazilian rain forest during night time.…”

Improved mixing height monitoring through a combination of lidar and radon measurements

“…This technique has been used to measure trace gas fluxes in grassland (Obrist et al, 2006), forests (Martens et al, 2004;Trumbore et al, 1990;Ussler et al, 1994), and urban areas (Hammer et al, 2009;Lallo et al, 2009;Yver et al, 2009). More generally, this method can be regarded as a specific example of using mixing depth and concentration measurements to infer surface fluxes (Emeis, 2008;Forster et al, 2012).…”