Plantation type regulates molecular characteristics and chlorine reactivity of soil-derived DOM: Enhanced DBPs...

Plantation forests are rapidly expanding worldwide and are increasingly recognized as critical regulators of downstream drinking water quality. Diverse plantation types (e.g., legumes and non-legumes) differ fundamentally in their plant-soil-microbe interactions, yet how these differences influence the properties of dissolved organic matter (DOM) and its propensity to form disinfection byproducts (DBPs) during chlorination remains largely unknown. In this study, we comprehensively investigated the regulatory role of plantation type in shaping the molecular characteristics and chlorine reactivity of soil-derived DOM. Our results revealed that DOM from legume A. mangium soils contained elevated total dissolved nitrogen (TDN), nitrate nitrogen (NO₃⁻-N) and dissolved organic nitrogen (DON) than non-legume Eucalyptus soils. Spectroscopic analysis further demonstrated that DOM from legume soils was enriched in terrestrial humic-like components C1 but a lower proportion of protein-like substance C2 relative to non-legume soils. At the molecular level, DOM composition in non-legume soils was dominated by fresh, labile, saturated and reduced substances, whereas DOM in legume soils contained more structurally complex, aromatic, unsaturated and oxidized compounds. These properties and composition feature markedly enhanced the chlorine reactivity of legume-derived DOM, leading to a higher DBPs formation potential. For instance, the average total organic halogen (TOX) after chlorination of DOM was significantly higher in legume soils (0.21 mg/L) than in non-legume soils (0.14 mg/L). Overall, these findings underscored that plantation type was a key ecological driver of the properties and chlorine reactivity of soil-derived DOM, with legume plantations posing a greater risk of DBPs formation in downstream waters. These insights necessitate the need to incorporate forest type into watershed management and drinking water safety strategies.