Novel insight of dynamic characteristics of typical terrestrial plant litter as potential organic source to waterbodies

Plant litter emerges as a notable organic source in moderately/lightly polluted waterbodies, which has been the main pollution type in the world. However, previous researches overlooked the long-term organic dynamic characteristics and mechanistic release-degradation process of plant litter. This study performed an immersion experiment (224d) of fresh (wet/dry) and rotten leaf litter, investigating organic evolution via multiscale approaches: dissolved organic carbon (DOC), fluorescence dissolved organic matter (FDOM), and DOM molecular analysis. Results revealed that dry litter discharged the maximum net organic matter, peaking on the 4th day (76.48 mg DOC/g volatile solid (VS)), and max fluorescence organic matter peaking on the 8th day (protein-like) and 24th day(humic-like). Then a multiphase-release-degradation model effectively described the underlying release and degradation kinetics of DOC and FDOM for fresh litter (R² of 0.725-0.967), identifying 2-3 main organic components and 1-3 main fluorescence components based on distinct kinetics. Maximum release was higher from dried litter (112.7 mg/g VS) than wet litter (97.12 mg/g VS), which released slower and less. Crucially, a three-stage humification process for litter in water was discovered as an important organic revolution process, evidenced by a red-shift (20-50 nm excitation, 30 nm emission) in humic-like fluorescence peaks. This progressive humification was further demonstrated by an increase in larger molecules from 5.16 % to 26.35 % and a rise of ubiquitous humic substances from 75 % to 93 % across stages. This study elucidated the litter-driven organic might not be ignored with high organic discharge level, and early salvage is a constructive method to control the influence. The study was of great importance for understanding the organic dynamic as well as the development of the pollution control.