Atmosphere‐biosphere exchange of CO2 and O3 in the central Amazon Forest

Measurements of vertical fluxes for CO 2 and O 3 were made at a level 10 m above the canopy of the Amazon forest during the wet season, using eddy correlation techniques. Vertical profiles of CO 2 and O 3 were recorded continuously from above the canopy to the soil surface, and forest floor respiration was measured using soil enclosures. Nocturnal respiration of CO 2 by the forest ecosystem averaged 2.57 kgC/ha/h, with about 85% from the forest floor. During the daytime, CO 2 was taken up at a mean rate of 4.4 kgC/ha/h. Net ecosystem uptake of carbon dioxide increased with solar flux by 0.015 (kgC/ha/h)/(W m −2 ), corresponding to fixation of 0.0076 moles CO 2 per mole photons (about 0.017 moles CO 2 per mole of absorbed photons at photosynthetically active wavelengths). The relationship between net ecosystem exchange and solar flux was virtually the same in the Amazon forest as in forests in Canada (Desjardins et al., 1982, 1985) and Tennessee (Baldocchi et al., 1987 a,b ). The relatively high efficiency for utilization of light (about 30% of the theoretical maximum) and the strong dependence of net CO 2 uptake on solar flux suggest that light may significantly regulate net ecosystem exchange and carbon storage in the tropical forest. Changes in the distribution of cloud cover, associated for example with climatic shifts, might induce globally significant changes in carbon storage. Rates for uptake of O 3 averaged 2.3×10 11 molecules cm −2 s −1 in the daytime (10 hours, 700–1700 hours), dropping by roughly a factor of 10 during the 14 hours from dusk to dawn. The mean O 3 deposition velocity at 40 m was 0.26 cm s −1 in the night and 1.8 cm s −1 in the day. Diurnal variation of O 3 deposition was regulated both by stratification of the atmospheric boundary layer and by stomatal response to light and water deficit. The total flux of O 3 to the forest was limited largely by supply from the free troposphere above. Deposition of O 3 to the forest canopy appears to be a regionally, and perhaps globally, important sink for tropospheric O 3 .