The Amazon River mobilizes organic carbon across one of the world’s largest terrestrial carbon reservoirs. Quantifying the sources of particulate organic carbon (POC) to this flux is typically challenging in large systems like the Amazon River due to hydrodynamic sorting of sediments. Here, we analyze the composition of POC collected from multiple total suspended sediment (TSS) profiles in the mainstem at Óbidos, and surface samples from the Madeira, Solimões and Tapajós Rivers. As hypothesized, TSS and POC concentrations in the mainstem increased with depth and fit well to Rouse models for sediment sorting by grain size. Coupling these profiles to Acoustic Doppler Current Profiler discharge data, we estimate a large decrease in POC flux (from 540 to 370 kilograms per second) between the rising and falling stages of Amazon River mainstem. The C/N ratio, stable and radiocarbon signatures of bulk POC are less variable within the cross-section at Óbidos, and suggest that riverine POC in the Amazon River is predominantly soil-derived. However, smaller shifts in these compositional metrics with depth, including leaf wax n-alkanes and fatty acids, are consistent with the perspective that deeper and larger particles carry fresher, less degraded organic matter sources (i.e., vegetation debris) through the mainstem. Overall, our cross-sectional surveys at Óbidos highlight the importance of depth-specific sampling for estimating riverine export fluxes. At the same time, they imply that this approach to sampling is perhaps less essential with respect to characterizing the composition of POC sources exported by the river.

Carbon allocation to non-structural carbohydrates (NSC) is essential for plant metabolism playing an important role in tree responses to drought. It is still unclear if and how interspecific hydraulic trait variation modulates NSC concentration dynamics in different plant organs, particularly in tropical tree species. We investigated whether drought-resistance strategies (inferred from hydraulic traits) explain seasonal and interannual NSC dynamics in leaves, branches, trunks, and roots in seasonal eastern Amazon tree species in Brazil. We measured NSC concentration in eight abundant species during three years, including the end of the wet and dry seasons of the typical regular years (2013-2014) and the extreme drought induced by El Niño–Southern Oscillation in 2015 (ENSO). Organs have an important contribution to explain the starch (ST), soluble sugar (SS), and NSC variance among trees. We showed seasonal and year-to-year homeostasis in ST and SS concentrations in a majority of organs during 2013 and 2014, but SS increased in all organs during the extreme ENSO drought, while the ST concentration did not. The increase in SS concentration was more evident in woody organs from species with intermediate and tolerant drought strategies. The drought-tolerant species maintain higher root starch concentrations and mobilize more SS during extreme drought.