Plantation forest stand age affects the characteristics and the chlorine reactivity of soil-derived dissolved organic matter.

Plantation forest areas are rapidly expanding worldwide. Forests at different stand ages exhibit distinct patterns in litterfall input, soil microbial diversity, and enzyme activity, all of which potentially affect the properties of dissolved organic matter (DOM). DOM is an important precursor of disinfection byproducts (DBPs). Its quantity and chemical composition are key factors governing DBP formation potential and toxicity during water treatment. Therefore, changes in DOM characteristics driven by forest stand age can play a critical role in shaping drinking water quality. However, how plantation forest stand age affects DOM and its derived DBP remains poorly understood. Herein, this study comprehensively investigated the variations in DOM characteristics as well as the formation and toxicity of DBPs across three different stand ages (20, 30, and 34 years). Results revealed notable age-related differences in both the characteristics and composition of DOM. Specifically, a distinct shift in the abundances of DOM fluorescent components was observed. For example, the percentages of two humic-like components, C1 and C3, increased from 21 % and 19 % in mid-aged stage to 55 % and 24 % in mature stage, respectively. In contrast, the protein-like component C2 descended sharply from 60 % in the mid-aged stage to 57 % in the premature stage, and significantly down to 21 % in the mature stage. Molecular-level analysis further confirmed a transformation from fresh and biolabile substances to more complex, chemically stable and recalcitrant compounds. More importantly, the observed variations in DOM characteristics and composition driven by increasing stand age enhanced its reactivity with chlorine, leading to a higher DBP formation potential and increased microtoxicity. For instance, DOM from mature stands showed higher reactivity with chlorine, as indicated by an increase in total organic halogen (TOX) from 0.11 mg/L in middle-aged forest to 0.20 mg/L in mature forest. Similarly, microtoxicity levels rose from 0.27 to 0.35 mg Zn²⁺ equivalents as the forest age progression. Correlation analysis revealed that the primary precursors of DBPs and associated microtoxicity were not the fresh, labile DOM fractions of microbial origin, but rather the highly aromatic, unsaturated, and humified fractions of DOM. Overall, these findings highlighted the critical role of plantation forest stand age in shaping DOM properties and reactivity with chlorine. These insights underscored the importance of incorporating forest stand age into watershed management and the need for age-informed water quality management strategies in forested watersheds.