Tibet’s Yarlung Tsangpo Revealed as Strong Greenhouse Gas Source Along Its Course

A team of researchers surveyed the Yarlung Tsangpo along its entire course, from glacier-fed headwaters above 4,900 meters elevation down to downstream valleys at roughly 550 meters, and found that the river acts as a net source of all three greenhouse gases, with fluxes that varied along the flow path even though dissolved organic carbon concentrations differed minimally among the upstream, midstream, and downstream sections.

In September 2024, the team visited 32 sites along the main river and its tributaries, collecting surface water samples in duplicate and measuring dissolved gases using the headspace equilibration technique, in which river water is mixed with ambient air in a syringe and the equilibrated gas phase is analyzed by gas chromatography.

According to the study's introduction, dissolved organic matter is a central factor regulating riverine greenhouse gas emissions: microbial decomposition of DOM through in situ respiration and metabolism is a significant source of riverine carbon dioxide, and it provides electrons for denitrification, thereby facilitating the production of nitrous oxide. The introduction also notes that methane emissions have been positively correlated with DOM concentration in previous studies. The introduction states that findings on how DOM composition shapes gas fluxes remain inconsistent: some work suggests humic-like substances enhance methane production, other studies report they suppress it, and aromatic compounds have been shown to correlate negatively with denitrification potential, subsequently affecting the production capacity of nitrous oxide. The Yarlung Tsangpo, whose watershed spans sandy pastures, four major Tibetan cities, glacial terrain containing over 10,000 glaciers—nearly 30 percent of all glaciers on the Tibetan Plateau—and forested gorges, was chosen by the authors as a case study to assess the spatial variability and driving forces of riverine greenhouse gas fluxes across diverse land cover and land use.

The fluorescence analysis identified the composition of the river's dissolved organic matter. Using excitation-emission matrix spectroscopy with parallel factor analysis, the researchers identified three fluorescent components in the DOM: a protein-like component, a tyrosine-like substance, and a humic-like terrestrial material. The tyrosine-like substance dominated at 57.1 percent of the relative abundance, followed by the protein-like component, with humic-like terrestrial material trailing behind. That pattern matters because tyrosine-like and protein-like compounds are typically associated with labile material derived from phytoplankton exudates and microbial metabolism, while low humic content signals weak influence from soils surrounding the river. The authors state that, collectively, these data suggest active microbial processes in the studied river system, particularly as the river drains several rapidly degrading glaciers, which introduce small organic compounds into the fluvial system that are biologically diverse and can support microbial metabolism downstream.

The molecular-level analysis sharpened this picture further. FT-ICR MS showed that lignin-like substances, generally considered markers of terrestrial plant material, were the most abundant compound class overall, comprising between 35.6 and 53.8 percent of identified molecules. But the upstream site stood apart: it contained fewer lignin-like substances—35.6 percent—along with elevated proportions of biolabile components, including lipid-like chemicals at 21.3 percent and protein-like chemicals at 19.5 percent. Elemental compositions dominated by CHO formulas, followed by CHON and CHOS, were consistent across the glacier-fed stream, middle reach, and downstream sites, while the upstream sample showed relatively lower CHON abundance and higher CHONS and CHOS content.

A suite of optical indices indicated that the river's DOM was produced relatively recently and is predominantly of biological origin. The humification index stayed below 4 at every site; the authors state that a lower HIX value suggests autochthonous DOM of biological origin, indicating a primary source from plankton and microorganisms. The fluorescence index ranged narrowly from 2.3 to 3.0, entirely above the 1.9 threshold that indicates microbial derivation—a value notably higher than those reported for the Epulu, Yangtze, and Ganges rivers. The freshness index, which tracks recently produced DOM, averaged 0.93, exceeding mean values for the Brahmaputra, Connecticut, and Yangtze rivers and for 60 streams across the contiguous United States. The slope ratio, an indicator of DOM molecular weight, exceeded 1 at most sites and differed only slightly across the watershed, indicating a relatively uniform molecular weight distribution.

Not everything about the river’s organic carbon was uniform, however. The specific ultraviolet absorbance at 254 nanometers, a proxy for aromaticity, spanned an unusually wide range from 0.25 to 17.24 liters per milligram carbon per meter, far broader than the ranges reported for other major world rivers. The team attributes this spread to the diverse land cover draining into the river and variation in rainfall. The highest aromaticity, indicated by the SUVA254 value, was recorded at the headwater site, where nearly one-third of the area is underlain by permafrost; the authors state that permafrost-derived water contributes significantly to terrestrial-origin DOM and that natural wetlands in the area may also contribute, while the lowest values occurred downstream, which the authors attribute to increased microbial activity. Thermodynamic indicators told a complementary story: the highest mean double-bond equivalent value was observed at the middle reach and downstream sites, while the lowest was recorded at the upstream site, and the highest mean Gibbs free energy value, which corresponded to the lowest activity, was also observed at the upstream site; the authors therefore conclude that DOM molecules are relatively more inert at the upstream site compared to the other three sites.

The greenhouse gas measurements revealed the river to be a strong net emitter. Carbon dioxide flux ranged from 15,227 to 22,688 milligrams per square meter per day, averaging 18,189—exceeding the maximum previously reported for Tibetan Plateau rivers and aligning with flux ranges from the Congo, Mekong, and Amazon. Methane fluxes ranged narrowly from 317 to 328 milligrams per square meter per day, and nitrous oxide fluxes from 12 to 17, averaging 14.4 milligrams per square meter per day, higher than values under 1.79 previously reported for large Tibetan Plateau rivers. Spatially, the pattern was counterintuitive: the middle reach, which receives most of the watershed’s tributaries and is influenced by four major Tibetan cities including Lhasa and Shigatse, showed the highest methane flux, with a mean of 324 milligrams per square meter per day, but the lowest fluxes of carbon dioxide and nitrous oxide. The authors note that this diverges from reported trends of higher carbon dioxide emissions in urbanized river sections compared to non-urban reaches, and they attribute the increased carbon dioxide flux in the downstream area to higher water temperatures and biolabile permafrost/glacier-derived organic matter enhancing microbial respiration in the upper stream area.

Structural equation modeling identified the physical and chemical conditions of the water—temperature, pH, and total nitrogen—as the primary direct drivers of all three gas fluxes, with these parameters also acting indirectly by regulating the properties, composition, and content of dissolved organic matter. Because temperature correlates strongly with altitude across the river’s vast elevation range of roughly 550 to 5,000 meters, the authors suggest a potential altitudinal gradient in greenhouse gas release, though they caution that microbial analysis, which was beyond the scope of this study, is needed to confirm the biological mechanisms. The team also notes an important limitation: molecular characterization was limited to four representative sites without paired greenhouse gas measurements, so future work should prioritize paired sampling, seasonal monitoring, and complementary source-tracing methods such as biomarkers and stable isotopes.

The authors state that the findings can advance understanding of greenhouse gas emissions and the associated regulating factors across a river course that traverses diverse biomes. Rivers collectively emit enormous and uncertain quantities of greenhouse gases, with carbon dioxide fluxes globally ranging from a few hundred to over 50,000 milligrams per square meter per day, and most existing studies examine only isolated sections such as headwaters or polluted urban reaches. By documenting dissolved organic matter characteristics and greenhouse gas emissions along the entire course of a river traversing diverse biomes, the study provides insights into the dynamics of DOM characteristics and GHG emissions in a large river watershed. The authors conclude that the Yarlung Tsangpo is a strong source of greenhouse gases, primarily influenced by physicochemical parameters, and they encourage seasonal dynamics studies and microbial data for an in-depth understanding of the biological mechanisms driving DOM processes and subsequent greenhouse gas fluxes.

Subject of Research: Greenhouse gas emissions and dissolved organic matter characteristics along the Yarlung Tsangpo River on the Tibetan Plateau

Article Title: Characteristics of dissolved organic matter and greenhouse gas emissions in the Yarlung Tsangpo, a large transboundary river

Article References: Li, G., Wang, Y., Zhang, H., Xia, P., Chen, X., Zhou, W., Zhao, W., & Mao, J. F. (2026). Characteristics of dissolved organic matter and greenhouse gas emissions in the Yarlung Tsangpo, a large transboundary river. Carbon Research, 5(1), Article 62. https://doi.org/10.1007/s44246-026-00289-x

Image Credits: AI Generated

DOI: 10.1007/s44246-026-00289-x

Keywords: Yarlung Tsangpo, Tibetan Plateau, greenhouse gases, dissolved organic matter, carbon dioxide, methane, nitrous oxide, river emissions, FT-ICR MS, fluorescence spectroscopy, microbial metabolism, carbon cycle