Vertical patterns of soil water uptake by plants in a primary forest and an abandoned pasture in the eastern Amazon: an isotopic approach

This study evaluated the water uptake patterns of a primary forest and of the savanna-like vegetation of an abandoned pasture in an eastern Amazon site. We used natural stable isotope abundance in the soil profile, as well as plots irrigated with deuterated water to determine time and depth of soil water uptake by plants in different functional groups. Natural isotopic abundance was not suitable for identification of depth of water uptake by plants, but experiments using labeled water were. We found that the label percolation rate in the soil profile of the forest was lower than that observed in the pasture. Fourteen months after application, the label peak was located at 1.8 m depth in the forest and at 3 m depth in the pasture. Isotopic analysis of sap water from trees and lianas in the forest during the dry season showed that trees acquired labeled water at a deeper level in the soil profile compared to that acquired by lianas. Depth of water uptake by lianas seems to vary on a seasonal basis. In the pasture the 'colonizer' shrub (Solanum crinitum Lamb.) took up labeled water only from the surface layer of the soil profile (similar to 20%), whereas the most abundant coexisting grass (Panicum maximum Jacq.) acquired it from the top meter. None of the pasture plants were able to acquire labeled water after one rainy season when the label pulse was deep in the soil (> 1 m deep). These results have implications for studies of forest water cycle in which the soil volume used as source of water for plant transpiration is still unknown, and for an understanding of plant succession in the forest regeneration process of abandoned pastures in the eastern Amazon.