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, 15N 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 μm. 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 μm had the highest 15N2 emission rate of 1.15 mg-N/(m3·d), which was 2.9 times that of 100-200 μm. 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 15N2 production, 15N2O emission rate did not decrease with increasing SPS particle size. For the system containing 8 g L-1 SPS, 15N2O emission rate reached the highest of 1.05 μg-N/(m3·d) in the 50-100 μm SPS system, which was 17.5 times that of 100-200 μm. 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 μm 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.