Transformation processes of total suspended solids and dissolved organic matter in rivers: Influences of different land use sources and degradation processes.

Abstract

The riverine dissolved organic matter (DOM) pool constitutes the largest and most dynamic organic carbon reservoir within inland aquatic systems. Human activities significantly alter the distribution of organic matter (OM) in rivers, thereby affecting the availability of DOM. However, the impact of total suspended solids (TSS) on DOM under anthropogenic influence remains insufficiently elucidated. This study employed Fourier transform ion cyclotron resonance mass spectrometry, DOC characterization, and incubation experiments to investigate how land use and degradation processes influence TSS-DOM transformation in rivers. Our findings revealed that geographical patterns cause significant variations in both DOM composition and TSS content. Anthropogenic impacts led to an increase in autochthonous TSS content and an enhanced relative intensity (RI) of nitrogen (N)- and sulfur (S)-containing compounds in riverine DOM. The presence of TSS increased the bioavailability of DOM from 29.97 % to 33.57 %. However, during both photodegradation and combined degradation processes, the presence of TSS reduced the bioavailability of DOM. The degradation rate constant (k) of DOM decreased as degradation time increased. The k values were significantly correlated with the CHO components in natural rivers and with N- and S-containing components in human-influenced rivers. The degradation rates of DOC under different land uses were 0.05 ± 0.04 d^-1, 0.07 ± 0.06 d^-1, and 0.08 ± 0.06 d^-1 in forested, urban, and cropland-influenced rivers, respectively. The content of aliphatic compounds and the number of CHOS molecules in TSS-containing water were higher than in TSS-free water during the combined process of photochemical and microbial degradation, while the saturation and aromaticity of the compounds were lower. The characteristics of autochthonous DOM were more pronounced under the influence of TSS photorespiration. During drinking water disinfection, these small molecules derived from autochthonous TSS may contribute to an increase in disinfection by-products (DBPs) in drinking water. This study enhanced our understanding of how changes in autochthonous TSS content, driven by geographical heterogeneity and human activities, influence the biogeochemical processes of DOM in water, as well as the underlying molecular mechanisms and implications for water quality safety.