Dissolved organic matter and greenhouse gases in the Yarlung Tsangpo

A Riverine Carbon Source

Rivers connect terrestrial, oceanic, and atmospheric carbon pools while processing carbon and nitrogen that can be released as greenhouse gases. Carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes vary widely among rivers because of differences in land cover, hydrology, water quality, and microbial activity. Large watersheds remain difficult to assess because studies often focus on isolated headwater or human-impacted reaches rather than the full flow path.

The Yarlung Tsangpo is a transboundary river on the Tibetan Plateau and the longest river on the plateau, spanning glaciers, pastures, agricultural areas, urban zones, forests, and narrow gorges. Authors Guoliang Li, Jason Feijian Mao, and colleagues from the Chinese Academy of Sciences, Lanzhou Jiaotong University, and the Tibetan Ecology and Environment Monitoring Center investigated dissolved organic matter (DOM) characteristics and greenhouse-gas emissions across the river and its tributaries. They sampled 32 sites in September 2024, measured water quality and CO₂, CH₄, and N₂O fluxes, and conducted a second campaign in May 2025 at four representative sites using ultraviolet–visible spectroscopy, excitation-emission matrix spectroscopy, and Fourier-transform ion cyclotron resonance mass spectrometry.

Biologically Produced Organic Matter

Dissolved organic carbon concentrations ranged from 1.6 to 12.8 mg L⁻¹, with a median of 2.5 mg L⁻¹. Concentrations differed minimally between the main stream and tributaries and also showed no significant differences among upstream, middle-reach, and downstream sections. By contrast, the absorbance-based α₃₀₀ indicator of colored dissolved organic matter was significantly higher in the main stream than in tributaries and generally declined along the direction of flow.

Fluorescence analysis identified three components: a protein-like component, a tyrosine-like substance, and humic-like terrestrial material. The tyrosine-like substance accounted for 57.1% of the fluorescent DOM, followed by the protein-like and humic-like components, indicating active microbial processes and a strong contribution from in situ biological production. At the molecular level, lignin-like substances represented 35.6–53.8% of the DOM, while the upstream site contained fewer lignin-like compounds and higher proportions of biolabile lipid-like and protein-like chemicals than the middle-reach and downstream sites.

Sources and Properties Alongstream

Several indicators supported a predominantly autochthonous and recently produced DOM source. All measured humification index values were below 4, a pattern associated with biological rather than strongly terrestrial DOM, while most slope-ratio values exceeded 1, indicating relatively low molecular weight. Fluorescence index values ranged from 2.3 to 3.0, all above the 1.9 threshold commonly associated with microbial-derived DOM, and the freshness index ranged from 0.72 to 1.67, with a mean of 0.93.

DOM properties varied across the watershed and reflected diverse land cover and environmental conditions. SUVA₂₅₄ values ranged from 0.25 to 17.24 L mg C⁻¹ m⁻¹, averaging 3.29 L mg C⁻¹ m⁻¹; the highest aromaticity occurred at the headwater site and the lowest in downstream waters. Main-stream samples had higher SUVA₂₅₄ values than tributaries, while values declined longitudinally, suggesting a shift from more aromatic and humified material toward less aromatic DOM that may be more strongly affected by microbial transformation.

Greenhouse Gas Fluxes Across Reaches

The Yarlung Tsangpo acted as a net source of CO₂, CH₄, and N₂O across the surveyed watershed. The middle reach had the highest methane flux, ranging from 317 to 328 mg m⁻² d⁻¹, with a mean of 324 mg m⁻² d⁻¹. This reach nevertheless showed the lowest carbon-dioxide flux, with a mean of 18,189 mg m⁻² d⁻¹, and the lowest nitrous-oxide flux, with an average of 14.4 mg m⁻² d⁻¹.

The spatial pattern of emissions was associated with variation in physicochemical conditions and DOM characteristics along the river corridor. The watershed extends from elevations of 550 to 4,992 m, with a median elevation of 3,550 m, and water temperatures ranged from below 10 °C to 22.6 °C. Structural equation modeling was used to assess potential controls on gas fluxes, while gas concentrations were measured by gas chromatography and fluxes were calculated with the thin boundary layer equation and the Ray05 gas-exchange model.

Implications and Future Measurements

The findings indicate that the Yarlung Tsangpo contains DOM that is recently produced and primarily derived from biological metabolism, with low humification and broad variation in aromaticity. The dominance of tyrosine-like and protein-like substances, together with the upstream enrichment of lipid-like and protein-like chemicals, points to spatial differences in microbial processing and organic-matter reactivity. These characteristics are relevant to carbon-cycle assessments because DOM decomposition supplies carbon and electrons that can contribute to CO₂ production, methane formation, and nitrous-oxide generation.

The authors identify seasonal dynamics as an important area for future research to improve understanding of DOM characteristics and greenhouse-gas emissions. They also state that microbial data are needed to clarify the biological mechanisms driving DOM processes and subsequent gas fluxes. In addition, they note that the spectroscopic source indicators used in this study should be complemented by biomarkers and stable isotopes to quantify the contributions of different DOM sources.

Corresponding Author: Jason Feijian Mao

Original Source: https://doi.org/10.1007/s44246-026-00289-x

Contributions: Guoliang Li and Yuli Wang contributed to data curation, investigation, visualization, and writing the original draft. Huifang Zhang contributed to writing, review and editing. Pengchao Xia contributed to data curation, investigation, and validation. Xu Chen and Wenliang Zhou contributed to data curation, investigation, and methodology. Wei Zhao contributed to methodology, resources, validation, supervision, and writing, review and editing. Jason Feijian Mao contributed to conceptualization, data curation, funding acquisition, investigation, methodology, resources, validation, supervision, visualization, and writing, review and editing.