The impact of endogenous organic detritus on differentiation of nitrate reduction pathway in sediments

In order to explore the impact of endogenous organic detritus on differentiation of nitrate reduction pathway and mechanism, our study hypothesizes that the source of organic matter leads to differences in its chemical structure, thus affecting the nitrate reduction pathway. An indoor incubation experiment was conducted by adding different organic detritus from nitrogen-fixing and non-nitrogen-fixing cyanobacteria, green algae, and submerged macrophyte (sediment mixed thoroughly with different algal detritus and topped with 60 cm of water). The chemical components of different organic detritus degradation were mainly composed of aliphatic and aromatic compounds from cyanobacterial detritus as well as from green algae and macrophyte detritus, respectively, but the proportion was entirely different. Although the abundance of functional genes involved in the nitrogen cycle is similar in all groups, the microbial community structures are vastly different. The dominant microbial community structure and nitrate reduction rate as well as their negative relationship all indicated the discrepancy between ecological function and dominant microbial community structure. This suggested that the minority microbial community plays a dominant role in the nitrate reduction process. However, there is a high consistency between nitrate reduction rates and nitrogen nutrient levels. In addition, the dissimilatory nitrate reduction to ammonium (DNRA) predominated (10 ~ 35 μmol/kg/h), followed by denitrification (0.2 ~ 1.4 μmol/kg/h) in the nitrate reduction process. Therefore, the degradation of endogenous organic detritus promoted the nitrogen retention process mainly carried out by a minority microbial community, contributing to maintain the original eutrophic state in water bodies.