Molecular-level insight into the role of soil-derived dissolved organic matter composition in regulating photochemical reactivity

Soil-derived dissolved organic matter (DOM) links soil and water carbon pools and is an important source of photochemically produced reactive intermediates (PPRIs) in aquatic environments. Despite its importance, the variations in photochemical reactivity of soil-derived DOM molecules in producing PPRIs across broad geographical regions, and the factors driving these variations, remain unclear. Herein, we resolved the apparent quantum yields (Φ(PPRIs)) of hydroxyl radicals (•OH), singlet oxygen (¹O₂), and excited triplet-state DOM (³DOM*) for irradiated DOM from 22 representative soil reference materials in China, and linked them to soil pH, mineral weathering degree, and DOM characteristics. Generally, the average Φ(PPRIs) values of the soil-derived DOM followed the order of Φ(³DOM*) (1.67× 10^-2) > Φ(¹O₂) (1.47× 10^-2) > Φ(•OH) (7.31× 10^-5). The DOM from less weathered soils showed higher Φ(•OH) and Φ(³DOM*) and comparable Φ(¹O₂) than that from more weathered soils. The differences were mainly regulated by the abundance of humic-, lignin-, tannin-, and aromatic-like compounds, as indicated by the correlation and random forest model analyses. Partial least squares and multiple linear regression analyses identified DOM molecular weight, nominal oxidation state of carbon, and soil chemical index of alteration as effective predictors of •OH yields. Soil chemical index of alteration emerged as a prioritized predictor of ³DOM* yields, while the electron-donating capacity and humic-like compound content of the soil-derived DOM were effective predictors of ¹O₂ yields. This study advances our understanding of how mineral weathering processes regulate the photochemical reactivity of soil-derived DOM in the aquatic environment across wide geographical regions.