Abstract: The Xinanjiang Reservoir and the related river reaches were investigated. Sediments were collected along the upstream, reservoir backwater area, central reservoir and downstream the dam. The geochemical phases of phosphorus and nitrogen were determined. The results show that the total phosphorus (TP) and total nitrogen (TN) concentrations in backwater area sediments increase by 37.3% and 34.2% respectively as compared to that in upstream sediments, for the reason of sedimentation and aquaculture input. Due to the impacts of impounding, phosphorus and nitrogen concentrations in downstream sediments decrease by 39.5% and 74.1% respectively as compared to that in reservoir sediments. Among the various forms of phosphorus, the ratio of Ca-bound phosphorus (Ca-P) (13.8%∽31.9%) and Fe-bound phosphorus (Fe-P) (11.5%∽26.2%) in total phosphorus were next only to organic phosphorus (OP) (48%∽69%). The organic phosphorus deposited in the backwater area and central reservoir sediments is gradually decomposed and transformed to Fe-P, leading to the variation in phosphorus forms along the river course. Meanwhile, organic nitrogen (ON) is the main form of TN. Its content is low in the upper reaches sediments, and increases rapidly downward toward the central reservoir, but decreases significantly in the downstream sediments. The percentage of the content of ammoniac nitrogen (NH⁺₄ -N) in TN has less changed along the river channel. The results reveal that the presence of dam has an obvious retention effect on phosphorus and nitrogen, which also changes the stoichiometric ratio of nutrients in sediments.

Key words: phosphorus and nitrogen forms, sediment, Xinanjiang Basin