Radon fluxes in tropical forest ecosystems of Brazilian Amazonia: night‐time CO₂ net ecosystem exchange derived from radon and eddy covariance methods

Abstract

Radon-222 (Rn-222) is used as a transport tracer of forest canopy-atmosphere CO 2 exchange in an old-growth, tropical rain forest site near km 67 of the Tapajós National Forest, Pará, Brazil. Initial results, from month-long periods at the end of the wet season (June-July) and the end of the dry season (November-December) in 2001, demonstrate the potential of new Rn measurement instruments and methods to quantify mass transport processes between forest canopies and the atmosphere. Gas exchange rates yield mean canopy air residence times ranging from minutes during turbulent daytime hours to greater than 12 h during calm nights. Rn is an effective tracer for net ecosystem exchange of CO 2 (CO 2 NEE) during calm, night-time hours when eddy covariance-based NEE measurements are less certain because of low atmospheric turbulence. Rn-derived night-time CO 2 NEE (9.00 AE 0.99 lmol m À2 s À1 in the wet season, 6.39 AE 0.59 in the dry season) was significantly higher than raw uncorrected, eddy covariance-derived CO 2 NEE (5.96 AE 0.51 wet season, 5.57 AE 0.53 dry season), but agrees with corrected eddy covariance results (8.65 AE 1.07 wet season, 6.56 AE 0.73 dry season) derived by filtering out lower NEE values obtained during calm periods using independent meteorological criteria. The Rn CO 2 results suggest that uncorrected eddy covariance values underestimate night-time CO 2 loss at this site. If generalizable to other sites, these observations indicate that previous reports of strong net CO 2 uptake in Amazonian terra firme forest may be overestimated.

Tropical forests in a changing environment

“…The method models air movement and CO 2 concentrations between the vegetation and the atmosphere. The model assumptions fail at the low wind speeds commonly observed at night in tropical forests, which alone could fully explain the reports of substantial net carbon uptake by tropical forests [70]. The second possibility is that early eddy flux studies might have been conducted in maturing forests, where net CO 2 uptake is to be expected.…”

“…Limits on net carbon uptake by old-growth tropical forests Recent reports of net carbon uptake by old growth tropical forests sometimes exceed the size of the missing global carbon sink [69]. Estimates range from w0.5 Mg C ha K1 y K1 for repeated censuses of forest inventory plots [33] to 1-6 Mg C ha K1 y K1 for eddy flux measurements of net ecosystem CO 2 exchange [70,71]. Three types of evidence now suggest that these values are too high.…”

Observations of HNO<sub>3</sub>, ΣAN, ΣPN and NO<sub>2</sub> fluxes: evidence for rapid HO<sub>x</sub> chemistry within a pine forest canopy

“…Analysis of the NO to NO 2 ratio is simplified if we can assume steadystate which requires residence times longer than 100 s. The residence time can be calculated by Eq. 9 (Martens et al, 2004),…”

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).…”