Implications of CO<sub>2</sub> pooling on δ<sup>13</sup>C of ecosystem respiration and leaves in Amazonian forest

Abstract

Abstract. 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 collected 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.Correspondence to: A. C. de Araújo (alessandro.araujo@falw.vu.nl) It is likely that lateral drainage of CO 2 enriched in 13 C from upslope areas might have happened when the nights were less stable. Biotic factors such as soil CO 2 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 CO 2 in the low-lying areas of this landscape may confound the interpretation of δ 13 C leaf and δ 13 C Reco .

Seasonality, drivers, and isotopic composition of soil CO<sub>2</sub> fluxes from tropical forests of the Congo Basin

“…The δ 13 C value of various ecosystem components (leaf litter, SOC, soil respired CO 2 , and riverine CO 2 ) were universally enriched in the montane compared to lowland forests (Figure 3). Similar isotopic enrichment with altitude has been shown even within small-scale gradients in Amazonian forests (de Araújo et al, 2008). In the Amazonian study, the relatively enriched values of leaf and ecosystem respiration in the high elevation sites was explained by increased leaf-level photosynthetic capacity https://doi.org/10.5194/bg-2020-133 Preprint.…”

“…(higher leaf nitrogen content and leaf mass per unit area (LMA)), which is decreasing intercellular CO 2 concentrations and reducing leaf discrimination, resulting in increasing 13 C concentrations in the leaves (de Araújo et al, 2008). However, Bauters et al 2017reported decreasing leaf nitrogen content and LMA with higher elevations in tropical forests of the Congo Basin.…”

The role of termite CH<sub>4</sub> emissions on ecosystem scale: a case study in the Amazon rain forest

“…Based on our measurements, no significant difference in the δ 13 C between mound and soil emitted CO 2 was found (-33.7 ‰ (se=2.5) for soil CO 2 , in comparison to -38.1‰ to -34.7‰ for termite mound emitted CO 2 ). In general, the values were more depleted than values found by De Araujo et al (2008), who found a δ 13 C of -30.1 ‰ for valley litter during the dry season (August 2004). To investigate whether our values are representative for other mounds or soils in the valley, and to investigate whether an isotopic difference exists between mound and soil emitted CO 2 , more measurements would be needed.…”

Pooling of CO2within a small valley in a tropical seasonal rain forest

“…de Araújo et al (2008a reported the highest CO 2 concentration in the morning, with a minor peak was at dusk. Previous studies have also reported CO 2 pooling in valleys at nighttime (de Araújo et al 2008b(de Araújo et al , 2010Takagi 2009). …”