¹³C/¹²C of atmospheric CO₂ in the Amazon Basin: Forest and river sources

Abstract

Concentrations and the δ 13 C of CO 2 were measured on air in the lower portion of the atmospheric boundary layer in the Amazon basin. Three major trends were observed: (1) a diurnal cycle with an amplitude of 70 to 150 ppm for concentration and 3 to 6‰ for δ 13 C, (2) a vertical gradient of CO 2 and δ 13 C in forest air, and (3) different 13 C composition for CO 2 in air over the river and in the forest. The diurnal changes in CO 2 concentration and δ 13 C of the CO 2 resulted from diurnal cycles in boundary layer mixing and photosynthetic uptake of CO 2 . Photosynthetic uptake of CO 2 increased the δ 13 C to levels up to ∼0.6‰ higher than tropospheric values in forest air samples collected during the afternoon at heights >7 m. An observed 5‰ decrease in δ 13 C of CO 2 in forest air between canopy height and ground level indicates that photosynthetic uptake of biogenic CO 2 is an important factor in determining the δ 13 C of plants growing in the Amazonian rain forest. The observed nighttime increase in CO 2 concentration and δ 13 C decrease differed for air within the forest canopy versus air over the river. The δ 13 C values of the CO 2 sources for air in the forest and over the river were −28‰ and −22‰, respectively. The results indicate that soil and plant respiration were the primary sources of CO 2 input to air in the forest, whereas degassing of CO 2 from the Amazon River is the likely source of CO 2 input to air over the river.