Beyond Hydrology: Exploring the Factors Influencing the Seasonal Variation of the Molecular Composition of Riverine Dissolved Organic Matter

The export of dissolved organic matter (DOM) from rivers is essential for linking terrestrial and marine carbon reservoirs in the global carbon cycle. However, there is limited knowledge regarding how the molecular composition of riverine DOM changes under different hydrological conditions, especially during extreme rainfall events. Moreover, the factors beyond hydrology that impact DOM composition have not been well defined. To address these gaps, samples were collected from a human-impacted medium-sized subtropical monsoonal river across various hydrological conditions throughout a complete hydrological cycle. Utilizing high-resolution mass spectrometry, it was discovered that the solid-phase extractable DOM (SPE-DOM) during the high-flow (1 < runoff (Q): annual mean runoff (Q_m) < 3) and extreme-rain (Q:Q_m > 3) periods exhibited a higher number of molecular formulae, lower H/C, higher O/C, and a higher proportion of carboxylic-rich alicyclic molecules compared to the low-flow period (LFP) (Q:Q_m < 1). These alterations were attributed to input from more diverse sources, particularly a greater input from soil organic matter with higher oxidation degrees. Additionally, the P-containing formulae were more enriched during the extreme-rain period, likely from agricultural lands and sediment release. Conversely, the fraction of S-containing formulae was significantly higher during the LFP, possibly due to the amplified influence of anthropogenic input. Furthermore, the DOM aromaticity did not fluctuate with runoff but was significantly associated with temperature. In summary, the study indicated that the composition of DOM varied significantly under different hydrological conditions, with temperature and anthropogenic activities identified as crucial factors influencing riverine DOM export.