The carbon isotope of a leaf (δ 13 C leaf ) is generally more negative in riparian zones than in areas with low soil moisture content or rainfall input. In Central Amazonia, the small-scale topography is composed of plateaus and valleys, with plateaus generally having a lower soil moisture status than the valley edges in the dry season. Yet in the dry season, the nocturnal accumulation of CO 2 is higher in the valleys than on the plateaus. Samples of sunlit leaves and atmospheric air were

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... ted along a topographical gradient in the dry season to test whether the δ 13 C leaf of sunlit leaves and the carbon isotope ratio of ecosystem respired CO 2 (δ 13 C Reco ) may be more negative in the valley than those on the plateau. The δ 13 C leaf was significantly more negative in the valley than on the plateau. Factors considered to be driving the observed variability in δ 13 C leaf were: leaf nitrogen concentration, leaf mass per unit area (LMA), soil moisture availability, more negative carbon isotope ratio of atmospheric CO 2 (δ 13 C a ) in the valleys during daytime hours, and leaf discrimination ( leaf ). The observed pattern of δ 13 C leaf might suggest that water-use efficiency (WUE) is higher on the plateaus than in the valleys. However, there was no full supporting evidence for this because it remains unclear how much of the difference in δ 13 C leaf was driven by physiology or δ 13 C a . The δ 13 C Reco was more negative in the valleys than on the plateaus on some nights, whereas in others it was not.