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.