Preliminary study on nitrogen and phosphorus releases from creek sediments in Shanghai suburbs

A laboratory study was conducted to observe the release of nitrogen and phosphorus from the sediments under both anaerobic and aerobic conditions. The samples used were five creek sediments and a fish-pond sediment (as a comparison) obtained from suburban Shanghai. High loads of nitrogen and phosphorus were found in the creek sediments. Total nitrogen of the sediments ranged from 1.17 to 5.95 g kg^-1; total phosphorus from 608.63 to 2033.95 mg kg^-1. Making up more than 90% of the total nitrogen, organic nitrogen was the dominant nitrogen fraction in the sediments; whereas inorganic phosphorus was the dominant phosphorus fraction, which made up more than 85 percent of the total phosphorus. Ca-bound phosphorus fraction dominated inorganic phosphorus, which occupied more than 50% of the total. A large amount of ammonium was released from the sediments under both aerobic and anaerobic conditions, and the anaerobic releases were slightly greater than the aerobic. In addition, ammonium contents in the aerobic waters decreased sharply after reaching the peaks because of strong nitrification, compared with the relatively maintained ammonium peaks in the anaerobic waters. Anaerobic phosphate releases were much greater than the aerobic and the released ortho-phosphate was mainly from Fe-bound phosphorus. Ammonium and ortho-phosphate releases from the sample of the dredged creek were the lowest, showing that creek dredging could effectively remove contaminants from the surface of sediments and weaken the release potentials of nitrogen and phosphorus. Ammonium and ortho-phosphate releases from the fish-pond sediment were greater than those from the creek sediments though its total nitrogen and phosphorus were not the highest, which was probably due to the larger amount of biologically degraded organic matter in the fish-pond sediment.