Distinct Non-conservative Behavior of Dissolved Organic Matter after Mixing Solimões/Negro and Amazon/Tapajós River Waters

Abstract

Positive and negative electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry and 1H NMR revealed major compositional and structural changes of dissolved organic matter (DOM) after mixing two sets of river waters in Amazon confluences: the Solimões and Negro Rivers (S + N) and the Amazon and Tapajós Rivers (A + T). We also studied the effects of water mixing ratios and incubation time on the composition and structure of DOM molecules. NMR spectra demonstrated large-scale structural transformations in the case of S + N mixing, with gain of pure and functionalized aliphatic units and loss of all other structures after 1d incubation. A + T mixing resulted in comparatively minor structural alterations, with a major gain of small aliphatic biomolecular binding motifs. Remarkably, structural alterations from mixing to 1d incubation were in essence reversed from 1d to 5d incubation for both S + N and A + T mixing experiments. Heterotrophic bacterial production (HBP) in endmembers S, N, and S + N mixtures remained near 0.03 μgC L–1 h–1, whereas HBP in A, T, and A + T were about five times higher. High rates of dark carbon fixation took place at S + N mixing in particular. In-depth biogeochemical characterization revealed major distinctions between DOM biogeochemical changes and temporal evolution at these key confluence sites within the Amazon basin.

Chemodiversity of Dissolved Soil Organic Matter from Amazon Rainforest as Influenced by Deforestation

“…Conversely, there is an increase in the formation of aromatic compounds and/or a decrease in the decomposition of aromatic compounds. Our findings are consistent with previous studies that have described how land use reduces the accumulation of amino sugars and the decomposition of recalcitrant compounds [ 9 , 10 , 32 ]. The lower microbial carbon biomass and microbial respiration at deforested sites reduce oxygen supply and carbon substrate diffusion in the soil solution [ 1 , 35 ].…”

“…In the context of the Amazon basin, the chemodiversity of DOM compounds is crucial for promoting soil organic matter decomposition and soil organisms’ activity [ 32 ]. Some DOM compounds can also serve as indicators of soil degradation [ 33 , 34 ].…”