Characterizing Dissolved Organic Matter Across a Riparian Soil–Water Interface: Preliminary Insights from a Molecular Level Perspective

Riparian
soils are an important source of dissolved organic matter
(DOM) to their connected rivers. The transport of DOM from riparian
soils to rivers exerts a significant impact on both terrestrial and
aquatic organic matter cycles. However, few studies focused on the
underlying changes in DOM composition and potential biogeochemical
processes involved. By combining optical techniques and Fourier transform
ion cyclotron mass spectrometry (FT-ICR MS), here we show the variation
in molecular composition between DOM in the riparian soil (S-DOM),
submerged soil (SS-DOM), and river water (R-DOM) along a typical tributary
of the Yangtze River. Variations in relative inputs of humic- and
aromatic-like sourced DOM (R-DOM > S-DOM > SS-DOM) and protein-like
sourced DOM (SS-DOM > S-DOM > R-DOM) were observed at both optical
and molecular levels, indicating significant alterations in DOM composition
during its transport. In particular, we have identified two preliminary
mechanisms of DOM transport from soils to rivers at the molecular
level by FT-ICR MS: (i) lower molecular weight (MW) (344 ± 9
Da in average) and moderate aromatic (modified aromaticity index;
AImod: 0.28 ± 0.01 in average) DOM compounds are released
to the river without modifications; (ii) moderate MW (370 ± 1
Da in average) and less aromatic (AImod: 0.25 ± 0.01
in average) DOM compounds are produced (likely due to transformation
or degradation of higher MW and aromatic DOM) and released to the
river. Further incubation experiments suggested that DOM compounds
associated with the first mechanism were more refractory (both bio-
and photoresistant) than those of the second one. Therefore, we speculate
that the first mechanism likely relates to the DOM transportation
to the downstream, but the second mechanism mainly contributes to
the in situ CO2 emissions in rivers. Our results (i) highlight
the variation in DOM composition across the soil–river interface;
(ii) confirm the preferential mobilization of specific DOM compounds
from soils to rivers; and (iii) provide an avenue for further investigation
of the mechanisms responsible for the observed changes. In particular,
further studies are encouraged to investigate the spatial and temporal
dynamics of DOM transport along the terrestrial-aquatic-continuum
with different sources or composition of organic matter and under
different hydrological scenarios.