A river network can be viewed as an open ecosystem interlinked longitudinally, laterally, and vertically by hydrologic processes, and it also plays a very important role in denitrification under the global nitrogen cycle context. In fact, denitrification is a process that diverse microorganisms utilize nitrogen oxides as electron acceptor to produce energy, therefore microorganisms are important participants and regulators of denitrification processes. However, our understanding of the correlations between denitrification activity and denitrifying microbial functional gene abundances is limit. We conducted a research in the Jinshui River with a length of 87 km, located in the south aspect of the Qinling Mts (Shaanxi, China), and we have investigated the primary environmental factors and denitrification rate using isotope pairing technique in sediments along the longitudinal gradient. Additionally, we quantified microbial community composition by employing denaturing gradient gel electrophoresis (DGGE) and abundance of corresponding genes involved in denitrification processes using real-time quantitative PCR (RT-PCR) techniques. Afterwards, we analyzed the mechanism of denitrification responsive to the environmental factors and microbial processes in the river system.
We assessed denitrification rates at 16 sampling sites in the Jinshui River in January, April, July, and October 2016. The result indicated that there were wide spatial variations in denitrification rate with an arrange of 1.03~81.32ng/(g*h) in the river bed in the riverine ecosystem. The highest denitrification rate occurred in summer and in sampling sites with large forest cover in its adjacent area. We analyzed the denitrifying microorganism by high-throughput sequencing that could visually display community species’ composition diversity and phylogenetic relationship, and discovered that most of denitrifying microbes belonged to the class of β Proteobacteria and a small part belonged to γ Proteobacteria. Generally, there were fewer species with the highest abundance in July and more species with the lowest abundance in January. The total mRNA was extracted from the samples, and the reverse transcription PCR was performed with specific primers. Then, qualitative and quantitative analysis of mRNA by denaturing gel gradient electrophoresis detected five groups of functional gene transcripts. The research results could provide new knowledge of denitrification processes and its associated microbial processes in river ecosystem along the longitudinal gradient.