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

## TL;DR

This study uses advanced mass spectrometry to analyze the molecular composition of organic acids in Amazonian Podzol soils, revealing how waterlogging and drainage affect carbon storage and soil chemistry.

## Contribution

The study provides the first molecular-level characterization of humic and fulvic acids in Amazonian Podzols using FT-ICR-MS.

## Key findings

- In waterlogged soils, nitrogen, sulfur, or phosphorus-containing compounds decrease with depth, while labile aliphatic molecules increase.
- Well-drained soils show increased NSP compounds and condensed aromatic compounds in deeper horizons due to microbial activity and oxidation.
- The NOSC metric derived from FT-ICR-MS effectively captures molecular changes and can trace podzolization processes.

## Abstract

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.