Coupled Nitrification-Denitrification Caused by Suspended Sediment (SPS) in Rivers: Importance of SPS Size and Composition

Suspended sediment (SPS) is ubiquitous in rivers, and SPS with different particle sizes and compositions may affect coupled nitrification-denitrification (CND) occurring on SPS significantly. However, there is no related research report. In this work, N-15 isotope tracer technique was adopted to explore the CND in systems containing SPS (8 g L-1 and 1 g (L-)1) collected from the Yellow River with various particle sizes, including <2, 2-20, 20-50, 50-100, and 100-200 mu m. The results showed that the CND occurred on SPS and the CND rate was negatively related to particle size; both nitrification and denitrification rate constants increased with decreasing SPS particle size. For instance, SPS (8 g L-1) with a particle size below 2 mu m had the highest N-15(2) emission rate of 1.15 mg-N/(m(3).d), which was 2.9 times that of 100-200 rho m. This is because SPS with a smaller particle size had a larger specific surface area and a higher organic carbon content, which is beneficial for bacteria growth. Both the nitrifying and denitrifying bacteria population were positively correlated with CND rate (p < 0.05). Different from the N-15(2) production, (N2O)-N-15 emission rate did not decrease with increasing SPS particle size. For the system containing 8 g SPS, 15N(2)O emission rate reached the highest of 1.05 mu g-N/(m(3).d) in the 50-100 mu m SPS system, which was 17.5 times that of 100-200 mu m. Similar results could be found from the system with 1 g L-1 SPS. This is due to the fact that the oxygen concentration at the SPS-water interface increased with SPS particle size, and the oxygen conditions might be most suitable for the production of N2O in the 50-100 mu m system. This study suggests that SPS size and composition play an important role in nitrogen cycle of river systems, especially for the production of N2O.