Geochemical Characteristics and Influence to Overlying Water of Nitrogen in the Sediments from Cascade Reservoirs of the Lancang River

In order to reveal the effect of nitrogen forms in sediments on nitrogen cycling in overlying water of reservoirs, the distribution characteristics of total nitrogen and bioavailable nitrogen form was analyzed by chemical sequential extraction method in ten representive sediment samples collected in three cascade canyon reservoirs along Lancang River, China. The factors that influence the release of nitrogen form and its potential risk to the overlying water were discussed. The result indicated that there were abrupt changes in the contents of nitrogen speciations between the upstream and downstream of the cascade dams, and the single reservoir also shown obvious spatial gradient characteristics. The concentrations of total nitrogen (TN) ranged from 829.8 mg/kg to 2013.2 mg/kg and the relative concentrations of different nitrogen form ranked according to the following sequence: the non-transferable N (NT-N, 39.19%~62.36%) > organic matter-sulfide fraction N (OSF-N, 23.79%~44.56%) > ion exchangeable N (IEF-N, 6.57%~12.18%) > carbonate fraction N (CF-N, 2.24%~4.22%) > iron-manganese oxides fraction N (IMOF-N, 1.53%~3.87%). IEF-N, CF-N, IMOF-N and OSF-N were the transferable nitrogen forms. The highest content of transferable nitrogen in the sediments from the front of dam of reservoirs (Xiaowan, Manwan, Dachaoshan) was consistent with the reservoir age and pollution degree. The nitrogen concentration in the overlying water was closely related to the distribution of IEF-N from the sediments. OSF-N was an important source of nitrogen in the overlying water. Moreover, NH₄⁺-N was the main component of IEF-N, CF-N and IMOF-N. However, the concentrations of NH₄⁺-N were relatively higher in the sediments from the front of dams where water bodies flow slowly, indicating that the coupling relationship between eutrophication risk and the cycle of nitrogen in the sediments.