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; AI(mod): 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 (AI(mod) : 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.