Evaluating the Molecular Compound Heterogeneity of Dissolved Organic Matter (DOM) during Coupled Release-Diffusion Kinetic Processes in Soil.

Soil dissolved organic matter (DOM) leaching critically regulates subsoil carbon stocks, yet how molecular heterogeneity governs the coupled release-diffusion kinetics remains elusive. Here, we employed a novel column system with in situ porewater samplers and Fourier transform ion cyclotron resonance mass spectrometry to resolve DOM molecular behaviors at unprecedented resolution. By establishing the compound-specific interfacial release preference index and apparent diffusive mass-transfer coefficient, coupled with interpretable machine learning algorithms, we revealed strong molecular-level selectivity in both processes: the O/C ratio and oxygen content predominantly controlled interphase release, while the aromaticity index and double bond equivalent governed intrapore diffusion. These findings challenge the simplification of bulk dissolved organic carbon in current soil carbon turnover models and provide estimated molecular parameters essential for mechanistically predicting subsoil carbon stability under changing climate conditions.