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Denitrification in river sediments is the hotspot of nitrogen removal and nosZ I gene is essential for reducing nitrous oxide (N 2 O) emissions. However, few studies tried to link nosZ I communities with variations of denitrification rates in sediments along the high-elevation rivers. Here, we investigated the spatial variation of potential denitrification rates of sediments along a section (hereafter YJ) of the middle reaches of the Yarlung Zangbo River in the Qinghai-Tibetan Plateau. We also used the real-time quantitative PCR (qPCR) and high-throughput sequencing techniques to evaluate the abundance and composition of nosZ I-containing microbial groups. The influences of physicochemical factors and denitrifier communities on potential denitrification rates were further revealed through structural equation modeling. The obtained results indicated that potential denitrification rates and N 2 O/(N 2 O + N 2) ratio in the sediments along YJ section were greatly different. Moreover, the alpha diversity and composition of nosZ I-containing microbial community in river sediments differed remarkably, mainly driven by the ammonia nitrogen (NH 4 +-N), organic matter (OM) and pH in sediments. The relative abundances of Zoogloeaceae , Oxalobacteraceae , Rhodospirillaceae and Bradyrhizobiaceae significantly differed among five groups (P < 0.05). Structural equation modeling further suggested that nitrogen nutrients directly influenced the potential denitrification rates, while total phosphorus (TP) showed indirect effects on potential denitrification rates through modulating denitrifier abundances and nosZ I community. The abundance and composition of nosZ I community were powerful predictors in regulating denitrification rates and N 2 O/(N 2 O + N 2) ratio. Our findings highlight that the nosZ I-containing microbial groups play a non-negligible role in nitrogen removal and N 2 O mitigation in high-elevation river sediments. • Denitrification rates and nosZ I community spatially varied greatly. • Physicochemical factors and community explained 89.9% of denitrification variation. • Physicochemical properties indirectly control denitrification by altering community. • nosZ I community was a powerful predictor for denitrification and N 2 O emission. [ABSTRACT FROM AUTHOR]

The evaluation of trace elements (TEs) in sediments of dam lakes is crucial for maintaining ecosystem health. Therefore, it is very important to determine their concentrations, pollution status, sources, controlling factors and ecological risks in these ecosystems. Here, for the first time, we analyzed 14 TEs (Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd, Sb, Hg and Pb) and sediment properties (organic matter (OM) and pH) in sediments from four dam lakes (Erfelek, Derbent, Suat Uğurlu and Saraydüzü) in the Black Sea Region of Turkey, which are used for different purposes and located on the different rivers or streams with different pollution levels. The results indicated that Al, V, Mn, Fe, Cu, Zn, As, Pb and Sb concentrations were lower in the Erfelek Dam Lake which has been used for drinking water supply, indicating that it is more protected than other dam lakes. However, Al, V, Mn, Fe, Co, Cu and Sb concentrations were higher in the Suat Uğurlu Dam Lake on the Yeşilırmak River which is moderately polluted. According to sediment contamination indices, there was low contamination in the sediments of all dam lakes. Similarly, the assessment of ecological risk posed by TEs indicated low eclogical risk in the dam lakes. Sediment OM and pH were found to be important control factors affecting the distribution of TEs in the dam lakes sediments. Correlation and factor analyses suggested that all TEs in the sediments of dam lakes mainly originated from geogenic sources. • The contens of 14 TEs in the sediments of four dam lakes were compared. • Ecological risk indices and SQGs were used to assess ecological risks of TEs. • Effects of OM and pH on the accumulation of TEs in sediments were investigated. • Factor and correlation analyses were conducted for source identification of TEs. [ABSTRACT FROM AUTHOR]