Broad molecular classification based on stoichiometric ratio relationships has been used extensively to characterize the chemical diversity of aquatic dissolved organic matter (DOM). However, variability in the molecular composition within this classification has remained elusive, thus limiting the interpretation of DOM dynamics, especially with respect to transport versus transformation patterns in response to hydrologic or landscape changes. Here, leveraging high-frequency spatiotemporal sampling during rainfall events at a Critical Zone Observatory project site in Clear Creek, Iowa, we apply a metabolomics-based analysis validated with fragmentation using tandem mass spectrometry to uncover patterns in the molecular features of the DOM composition that were not resolved by classification based on stoichiometric ratios in the chemical formulae. From upstream to downstream sites, beyond the increased aromaticity implied by changes in the stoichiometric ratios, we identified an increased abundance of flavonoids and other phenylpropanoids, two important subgroups of aromatic compounds. The stoichiometric analysis also proposed a localized decline in the abundance of lipid-like compounds, which we attributed specifically to medium-chain and short-chain fatty acids; other lipids such as long-chain fatty acids and sterol lipids remained unchanged. We further determined in-stream molecular transitions and specific compound degradation by capturing changes in the molecular masses of terpenoids, phenylpropanoids, fatty acids, and amino acids. In sum, the metabolomics analysis of the chemical formulae resolved molecular variability imprinted on the stoichiometric DOM composition to implicate key molecular subgroups underlying carbon transport and cycling dynamics in the stream.

基于化学计量比关系的广泛分子分类已广泛用于表征水生溶解有机物 (DOM) 的化学多样性。然而，这一分类中分子组成的变异性仍然难以捉摸，因此限制了对 DOM 动力学的解释，特别是在响应水文或景观变化的运输与转化模式方面。在这里，利用爱荷华州克利尔克里克关键区观测站项目现场降雨事件期间的高频时空采样，我们应用了基于代谢组学的分析，并使用串联质谱法进行了碎片化验证，以揭示 DOM 组成的分子特征模式没有通过基于化学式中的化学计量比的分类来解决。从上游到下游，除了化学计量比变化所暗示的芳香性增加外，我们还发现黄酮类化合物和其他苯丙烷类化合物的丰度增加，这是芳香族化合物的两个重要亚组。化学计量分析还提出了类脂化合物丰度的局部下降，我们特别将其归因于中链和短链脂肪酸；其他脂质如长链脂肪酸和甾醇脂质保持不变。我们通过捕获萜类、苯丙烷、脂肪酸和氨基酸的分子量变化，进一步确定了流内分子转变和特定化合物降解。总共，