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

Abstract

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±0.99 μmol m −2 s −1 in the wet season, 6.39±0.59 in the dry season) was significantly higher than raw uncorrected, eddy covariance‐derived CO 2 NEE (5.96±0.51 wet season, 5.57±0.53 dry season), but agrees with corrected eddy covariance results (8.65±1.07 wet season, 6.56±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.