Acceleration of Denitrification in Turbid Rivers Due to Denitrification Occurring on Suspended Sediment in Oxic Waters

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 (N-15) 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.