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.河岸带土壤是连通河流中溶解态有机质（dissolved organic matter, DOM）的重要来源。DOM从河岸带土壤向河流的输运过程，对陆地与水生生态系统的有机质循环均具有显著影响。然而，目前鲜有研究聚焦于该过程中DOM组成的内在变化及其潜在的生物地球化学机制。本研究结合光学光谱技术与傅里叶变换离子回旋共振质谱（Fourier transform ion cyclotron mass spectrometry, FT-ICR MS），针对长江典型支流沿线的河岸带土壤DOM（S-DOM）、淹水土壤DOM（SS-DOM）与河水DOM（R-DOM）的分子组成差异展开分析。在光学与分子层面均观测到：类腐殖质与类芳香族DOM的相对输入量呈现R-DOM > S-DOM > SS-DOM的规律，而类蛋白DOM的相对输入量则为SS-DOM > S-DOM > R-DOM，表明DOM在输运过程中其组成发生了显著改变。尤为重要的是，本研究通过FT-ICR MS在分子层面揭示了DOM从土壤向河流输运的两类初步机制：（1）低分子量（MW，平均344±9 Da）、中等芳香性（改良芳香度指数AImod，平均0.28±0.01）的DOM组分未经修饰直接释放进入河流；（2）中等分子量（平均370±1 Da）、低芳香性（AImod，平均0.25±0.01）的DOM组分经生成（推测源于高分子量芳香族DOM的转化或降解）后释放进入河流。后续培养实验结果表明，与第一类机制相关的DOM组分相较于第二类，具有更强的难降解性（兼具抗生物降解与抗光降解特性）。据此我们推测，第一类机制大概率对应DOM向下游的输运过程，而第二类机制则主要驱动河流原位CO₂排放。本研究结果：（1）明确了土壤-河流界面间DOM组成的差异；（2）证实了特定DOM组分从土壤向河流的优先迁移现象；（3）为解析观测到的组成变化机制提供了研究方向。尤为值得关注的是，未来研究可进一步探究在不同有机质来源/组成以及不同水文情景下，陆地-水连续体沿线DOM输运的时空动态规律。