Effect of particle size and composition of suspended sediment on denitrification in river water

Abstract

Rivers with high suspended sediment (SPS) concentration are common worldwide, and previous studies reported the occurrence of denitrification on SPS. In this work, effect of particle size and composition of SPS on denitrification in river water was studied in laboratory. The 15N isotope tracer technique was used to investigate the denitrification in water containing 8gL−1 SPS with different particle sizes, including <20μm, 20–50μm, 50–100μm, and 100–200μm. The results showed that the denitrification rate was negatively related to particle size, and the SPS with particle size below 20μm had the highest 15N2 emission rate of 0.27mg-N/m3·d, which was twice that of 100–200μm. The denitrifying bacteria population in the system decreased with the increase of particle size, which was positively correlated with denitrification rate (p<0.05). There was a positive correlation between organic carbon content of SPS and denitrifying bacteria population (p<0.01), indicating that organic carbon is a key factor influencing denitrifying bacteria. Different from the 15N2 production, 15N2O emission rate reached the highest of 1.02μg-N/m3·d in the system containing SPS of 20–50μm, which was 14.8 times that of 100–200μm. This was due to the difference in denitrifying bacteria species in different systems due to different oxic/anoxic conditions around SPS. This study suggests that not only the SPS concentration but also the SPS size and composition should be considered in studying the nitrogen cycle in river systems, especially for the production of N2O.