2.1 Study area and sediment sampling
­The Yarlung Tsangpo River is the largest river located on the southern edge of the Tibetan Plateau, with a length of approximate 2,057 km and catchment area of 2.4 × 105 km2 (Yao et al., 2010). It originates from the Jemayangdrung Glacier on the northern slope of the Himalayas and flows from west to east across the southern region of the Tibetan Plateau. The land use types along the Yarlung Tsangpo River changes from bare land, to grassland, to water and wetland, to infrastructure construction and cropland, and ultimately changes to forest as elevation decreases (Li et al., 2013). The main land use type in the Yarlung Tsangpo River basin is grassland, which accounts for about 60% of the total area of this basin. Grassland is mainly distributed in the middle and upper reaches of the Yarlung Tsangpo River basin, forest land is mainly distributed in the lower reaches of this basin, and urban land and cropland are mainly distributed in the lowland valley area of the Yarlung Tsangpo River basin (Fig. 1). The main channel of the Yarlung Tsangpo River is joined by several major tributaries, including the Lhasa River, the Nyang River, the Nyangqu River, and the Parlung Tsangpo River.
All surface sediment samples were collected in May 2019. The sampling campaign was conducted over the course of two weeks during which no extreme weather (e.g., heavy rainfall or regional debris flow) occurred. A total of 81 sampling sites were chosen. Briefly, 41 sites were sampled along the main channel from Zhongba County to Mainling County (~ 1211 km), 40 sites were selected from three major tributaries: Lhasa River (11 sites), Niyang River (16), and Parlung Tsangpo River (13) (Fig. 1 and Table S1).
At each sampling site, three riparian-zone surface sediments (0-10 cm depth) were randomly collected using a sterilized plastic spoon and mixed thoroughly to form a composite sample. Subsequently, composite samples were stored in sterilized Nasco Whirl-Pak sample bags, sealed, and transported to the laboratory in iceboxes. In the laboratory, subsamples were stored at 4°C for sediment physicochemical characterization and -80°C for genomic DNA extraction.