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The chemistry of major dissolved ions (Ca2+, Mg2+, Na+ + K+, HCO3−, SO42−, Cl−) and dissolved silica (SiO2) in the river water of the Huanghe (Yellow River), China, was studied from historical records at 100 stations in the drainage basin for the period 1958–2000 (not all the parameters were continuously monitored during the entire period). This river system (750,000 km2) presents an exceptional temporal and spatial water chemistry variability compared to other major rivers. The total dissolved solid (TDS) concentration of the Huanghe varied over 2 to 3 orders of magnitude throughout the basin, with a median TDS concentration of 452 mg/L, which is about 4 times the world spatial median value (WSM). In particular, the concentrations of Na+ + K+, SO42−, and Cl− were 10–20 times higher than in other major world rivers. Similar to the Changjiang (Yangtze River) and many other Himalayan rivers, the TDS at a given station is seasonally variable and inversely related to river runoff with a variation factor less than 2.0, despite a water dilution of fourfold to fivefold in the summer flood season. In addition to chemical weathering of sedimentary rocks, evaporation and fractional crystallization were found to be the major natural process controlling the major element chemistry of the Huanghe, owing to the abundance of loess and clastic rocks under arid and semiarid climates. A persistent increasing trend from year to year has been observed in the concentrations of TDS and all the salts except for HCO3− at all main‐channel stations except for the uppermost Lanzhou. The rate of increase in the TDS concentration was the highest in the middle reaches (10.52 mg/L yr−1 at Toudaoguai) and remained to be unusually high in the lower reaches (5.5 mg/L yr−1 at Stations Luokou and Lijin). The increasing trend coincided with a significant decrease in water discharge at most of the main‐channel stations in the past 40 years, and is attributed to increasing regulations of reservoirs and intensive water withdrawal for irrigation and diversions, as well as many other anthropogenic processes. Despite the increasing trend in TDS concentrations, the seaward TDS flux to the sea by the Huanghe has decreased dramatically from more than 20 × 109 kg/yr in the 1960s to less than 10 × 109 kg/yr at present, due to the sharp decrease in water discharge (sometimes completely dried up) in the lower reaches of the river. The significant decreases in sea‐ward fluxes of water and dissolved salts are expected to have profound physical, ecological, and socio‐economic impacts on the lower reaches of the river, the coastal areas and the Bohai Sea.

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Significance Stream/river carbon dioxide (CO 2 ) emission has significant spatial and seasonal variations critical for understanding its macroecosystem controls and plumbing of the terrestrial carbon budget. We relied on direct fluvial CO 2 partial pressure measurements and seasonally varying gas transfer velocity and river network surface area estimates to resolve reach-level seasonal variations of the flux at the global scale. The percentage of terrestrial primary production (GPP) shunted into rivers that ultimately contributes to CO 2 evasion increases with discharge across regions, due to a stronger response in fluvial CO 2 evasion to discharge than GPP. This highlights the importance of hydrology, in particular water throughput, in terrestrial–fluvial carbon transfers and the need to account for this effect in plumbing the terrestrial carbon budget.

High suspended sediment (SPS) concentration exists in many rivers of the world. In the present study, the effects of SPS concentration on denitrification were investigated in airtight chambers with sediment samples collected from the Yellow River which is the largest turbid river in the world. Results from the nitrogen stable ((15)N) isotopic tracer experiments showed that denitrification could occur on SPS in oxic waters and the denitrification rate increased with SPS concentration; this was probably caused by the presence of low-oxygen microsites in SPS. For the water systems with both bed-sediment and SPS, the denitrification kinetics fit well to Logistic model, and the denitrification rate constant increased linearly with SPS concentration (p < 0.01). The denitrification caused by the presence of SPS accounted for 22%, 38%, 53%, and 67% of the total denitrification in systems with 2.5, 8, 15, and 20 g L(-1) SPS, respectively. The activity of denitrifying bacteria in SPS was approximately twice that in bed-sediment, and the denitrifying bacteria population showed an increasing trend with SPS concentration in both SPS and bed-sediment, leading to the increase of denitrification rate with SPS concentration. Furthermore, the denitrification in bed-sediment was accelerated by increased diffusion of nitrate from overlying water to bed-sediment under agitation conditions, which accompanied with the presence of SPS. When with 8 g L(-1) SPS, approximately 66% of the increased denitrification compared to that without SPS was attributed to denitrification on SPS and 34% to agitation conditions. This is the first report of the occurrence of denitrification on SPS in oxic waters. The results suggest that SPS plays an important role in denitrification in turbid rivers; its effect on nitrogen cycle should be considered in future study.