Molecular Characterization of Humic and Fulvic Acids of Waterlogged and Well-Drained Amazonian Podzols

The Amazon rainforest is the largest tropical rainforest in the world. Amazonian Podzol soils, characteristic of this region, are known to store substantial amounts of organic carbon in both their surface and deep horizons. Despite decades of research, the molecular-level composition of these soils remains uncharacterized. This study addresses this knowledge gap by employing ultrahigh resolution mass spectrometry, namely, Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS), to determine the molecular composition of humic acid (HA) and fulvic acid (FA) fractions from two Amazonian Podzol profiles of varying levels of groundwater exposure (waterlogged vs well-drained). In the waterlogged soil compounds containing nitrogen, sulfur, or phosphorus (NSP) decreased with increasing depth while labile carboxyl-containing aliphatic molecules (CCAM) increased. CCAM were likely preserved through complexation with metals or from kinetically stalled degradation processes. In the well-drained soil compounds containing NSP increased with increasing depth likely due to elevated microbial productivity in the deeper horizons. Oxidation reactions in the well-drained soil profile also led to the production of condensed aromatic compounds (ConAC), which were responsible for the significant carbon sequestration observed in the deeper horizons. The molecular fingerprints of the samples of this study could be successfully parametrized by the nominal oxidation state of carbon (NOSC) derived from FT-ICR-MS suggesting this metric for tracing the podzolization process in future studies of podzol soils. The findings of this study demonstrate the utility of molecular fingerprinting in soil science and emphasize the critical role of hydrology on the molecular composition and carbon dynamics of Amazonian Podzol soils.