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

Abstract. The carbon isotope of a leaf (δ¹³C_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 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 δ¹³C_leaf of sunlit leaves and the carbon isotope ratio of ecosystem respired CO₂ (δ¹³C_Reco) may be more negative in the valley than those on the plateau.

The δ¹³C_leaf was significantly more negative in the valley than on the plateau. Factors considered to be driving the observed variability in δ¹³C_leaf were: leaf nitrogen concentration, leaf mass per unit area (LMA), soil moisture availability, more negative carbon isotope ratio of atmospheric CO₂ (δ13C_a) in the valleys during daytime hours, and leaf discrimination (Δ_leaf). The observed pattern of δ¹³C_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 δ¹³C_leaf was driven by physiology or &delta¹³C_a. The δ¹³C_Reco 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 (R_soil) 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 δ¹³C_leaf and δ¹³C_Reco.