Implications of CO2 pooling on δ13C of ecosystem respiration and leaves in Amazonian forest, Biogeoscience

The carbon isotope of a leaf (d13Cleaf) 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 CO2 is higher in the valleys than on the plateaus. Samples of sunlit leaves and atmospheric air were collected along a topographical gradient in the dry season to test whether the d13Cleaf of sunlit leaves and the carbon isotope ratio of ecosystem respired CO2 (d13CReco) may be more negative in the valley than those on the plateau. The d13Cleaf was significantly more negative in the valley than on the plateau. Factors considered to be driving the observed variability in d13Cleaf were: leaf nitrogen concentration, leaf mass per unit area (LMA), soil moisture availability, more negative carbon isotope ratio of atmospheric CO2 (d13Ca) in the valleys during daytime hours, and leaf discrimination (¿leaf). The observed pattern of d13Cleaf 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 d13Cleaf was driven by physiology or &delta13Ca. The d13CReco was more negative in the valleys than on the plateaus on some nights, whereas in others it was not. It is likely that lateral drainage of CO2 enriched in 13C from upslope areas might have happened when the nights were less stable. Biotic factors such as soil CO2 efflux (Rsoil) and the responses of plants to environmental variables such as vapor pressure deficit (D) may also play a role. The preferential pooling of CO2 in the low-lying areas of this landscape may confound the interpretation of d13Cleaf and d13CReco