Cascade reservoirs on nitrogen fractions in upper Mekong sediments against natural river

The construction of cascade hydropower dams in the Lancang River Basin of Southwest China significantly affects sediment nitrogen fractions. To assess these impacts, we contrasted the dammed Lancang River with the undammed Nujiang River. Using the recommended Ruttenberg sequential extraction process, we determined five nitrogen fractions: free nitrogen (F-N), exchangeable nitrogen (Ex-N), carbonate-associated nitrogen (CO₃-N), ironmanganese oxides-bound nitrogen (IM-N), and organic nitrogen (Org-N). There were considerable differences between the properties of sediments across the two rivers and had their effects on nitrogen fraction distribution. The cascade reservoirs possessed higher levels of transferable nitrogen (TranN) and total inorganic nitrogen (TIN) than the natural river section. Nitrogen composition in the natural river was as per Org-N > CO₃-N > IM-N > Ex-N > F-N, but at certain points in reservoirs, it altered to Org-N > IM-N > CO₃-N > Ex-N > F-N. This means that cascade reservoirs favor the release of nitrogen and enhance bioavailable nitrogen. Dynamics of nitrogen fractions depend on environmental factors such as altered water level, reduced flow velocity, silt interception, resuspension of sediment, particle-size distribution, and mineral composition, and axis I explains over 70% of variance.