Carbon Isotope Fractionation of Dissolved Organic Matter Due to ^•OH-Based Oxidation

^•OH-based oxidation plays a crucial role in dissolved organic matter (DOM) transformation and carbon flux, whereas quantifying the contribution of this pathway remains challenging. Here we combined the concentration with the carbon isotope analysis of DOM and its generated CO₂ to quantify the contribution of ^•OH-based oxidation. Results showed that the ¹³C enrichment factors (ε values) were -8.1‰ to -8.9‰ for benzene ring oxidation in aromatic compounds, -4.2‰ to -28.9‰ for lower-molecular-weight organic acids, and -13.0‰ for DOM from sediment. The fractionation of sediment DOM reflects the average ε value of humic substances and organic acids. These ε values were more negative than those of the photochemical and microbial processes, enabling the identification of DOM transformation mechanisms. Using an end-member mix model, we found that the proportion of ^•OH-based mineralization in total CO₂ emission ranged from 20.9% to 39.8% for 100 g/L sediment oxidation by 5-20 mM H₂O₂ under pH-neutral condition within 2 h and was only 2% for oxidation by air under the same conditions. We also found that inorganic carbon degassing contributed greatly to CO₂ emission during sediment oxidation. This study presents a new isotope-based tool to quantitatively assess the contribution of ^•OH-based oxidation to the emission of CO₂ from DOM.