Contribution of vegetation to the water cycle of the Amazon basin: An isotopic study of plant transpiration and its water source

Previous models which have been developed to explain the water cycle in the Amazon have been unable to locate or quantify sources of direct evaporation (E) in the evapotranspiration process of the basin (E + T). The proportion of water vapor generated by plant transpiration relative to the total forest vapor returned to atmosphere (T/(E + T)) remains unknown. An isotopic technique was developed to record this proportion at two sites in the Amazon basin with contrasting hydrological characteristics. The results show that plant transpiration accounts for the major source of water vapor to evapotranspiration flux. It was also observed that this proportion increased slightly during the dry season at a site in the eastern Amazon, whereas no effect was apparent at a site in the western Amazon where the dry season is less intense.A search for the source of water for plant transpiration was also conducted at an eastern Amazon site using a label pulse chase experiment. The results indicate different water uptake patterns between forest plants and abandoned pasture vegetation. Vines in the forest, as well as shrubs and grasses in the pasture, acquired labeled water from the surface very quickly three days after the soil was irrigated with deuterated water. Fourteen months after label application forest trees evidenced higher labeled water uptake when the label was deeper than 1 m depth in the soil. The roots of pasture grass acquired water deeper in the soil than the roots of shrubs, but neither could access the labeled water when it was deeper than 1 m. Water infiltration rates were consistent with the plant water uptake patterns, with higher values recorded for the pasture than for the forest.This study identifies plant transpiration as a major source of water vapor during the dry season and describes the important function of the eastern Amazon forests as a moisture reservoir (deep soil water) for the dry season