Storm-Scale Temporal Dynamics of Throughfall Dissolved Organic Matter Revealed by High-Resolution Mass Spectrometry

Forest canopies intercept a substantial portion of rainfall, altering not only hydrologic pathways but also the chemical composition of water reaching the forest floor. Throughfall dissolved organic matter (DOM) represents an important input of carbon and nutrients; however, little is known about how its molecular composition changes during individual storm events. Here, we present the first application of high-resolution mass spectrometry (HRMS) to characterize intrastorm dynamics of throughfall DOM. Fifteen sequential samples collected during a precipitation event in a mixed conifer forest showed clear first flush dynamics for dissolved organic carbon, which declined from 20.6 to 5.3 mg-C L⁻¹. Molecular diversity, measured as the number of HRMS features, also decreased from 934 to 439, indicating a decline in compositional complexity after the onset of precipitation. Biosynthetic pathways displayed a strong inverse relationship, with amino acid/peptide intensities decreasing as terpenoid intensities increased (R² = 0.96). In contrast, optical indices and HRMS data diverged in several cases, including temporal patterns in aromaticity, underscoring that widely used optical indices may not be universally applicable outside the systems in which they were developed. Together, these findings demonstrate that the molecular composition of throughfall DOM is highly dynamic within storm events, with pathway-specific responses to rainfall. Such variability suggests that projected changes in storm frequency and intensity under future climate scenarios may substantially alter the composition and ecological role of canopy-derived organic matter. These findings also highlight the need to carefully evaluate methodological choices, including solid-phase extraction approaches that may bias DOM recovery.