Removal Capacities and Environmental Constrains of Denitrification and Anammox Processes in Eutrophic Riverine Sediments

Abstract

Excessive nitrogen (N) loading has had severe consequences in coastal zones around the world. Denitrification and anammox are major microbial pathways for removing N in aquatic environments before it is exported to the coast. To assess two processes in eutrophic riverine systems, the denitrification and anammox and their bacterial participants were investigated in sediments of the Xiaoqing (XQ) River and Jiaolai (JL) River in Northeast China. By combining the evidence from N15 isotope tracing experiment and functional gene-based analysis, it was found that denitrification and anammox are ubiquitous along the investigated riverine sediments. The denitrification varied from 39.38 to 1433.01 nmol N2 m−2 h−1. Moreover, the anammox rates were in the range of 15.91 to 1209.97 nmol N2 m−2 h−1. Quantitative PCR results revealed that the nirK and nirS genes were in the order of 104–106 copies g−1 and 103–105 copies g−1, respectively, in both river sediments, while the hzsA was in the order of 106–105 copies g−1 in XQ at approximately two orders of magnitude compared with JL. The phylogenetic analysis of functional genes revealed the high diversity of the denitrifier and low diversity of anammox bacteria. Variance partitioning analyses verified that the grain particle characteristics were the major factor group determined the N removal efficiency. The denitrification and anammox processes were estimated to have removed 16.1% of the inorganic nitrogen inputs before being exported to Laizhou Bay, which highlights that a more extensive understanding of the regularity of the N removal processes is important in the technical remediation of eutrophication problems.