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

Abstract

Rivers are significant emitters of greenhouse gases (GHGs) such as CO 2 , CH 4 , and N 2 O. However, the spatial variability of riverine GHG fluxes and their relationship with dissolved organic matter (DOM) characteristics—such as concentration, composition, sources, and properties—are not well understood, particularly in large river watersheds. This study investigates GHG emissions and the associated regulating factors along the entire course of the Yarlung Tsangpo—a well-known transboundary river. The results indicate that DOM is predominantly composed of tyrosine-like substances (57.1%), followed by protein-like components and humic-like terrestrial materials, suggesting active microbial processes in the river. At the molecular level, the upstream site exhibited lower relative abundances of lignin-like substances (35.6%) but higher concentrations of biolabile components, such as lipid- (21.3%) and protein-like chemicals (19.5%). The DOM is recently produced and primarily derived from biological metabolism, characterized by a wide range of aromaticity and low humification, likely due to the diverse land cover types. The molecular weight of DOM, indicated by slope ratio (S R ), was relatively uniform across the watershed. The Yarlung Tsangpo acts as a net source of GHGs (i.e., CO 2 , CH 4 , and N 2 O). Spatially, the middle reach exhibits the highest flux of CH 4 (range: 317–328 mg m −2 d −1 , mean: 324 mg m −2 d −1 ), but the lowest fluxes of CO 2 (mean flux of 18,189 mg m −2 d −1 ) and N 2 O (average flux of 14.4 mg m −2 d −1 ). The findings of this study provide insights into the dynamics of DOM characteristics and GHG emissions along the entire course of a river traversing diverse biomes.