Denitrification prevails over anammox in the Yellow River wetland, Sanmenxia, China

PurposeThe aim of this work was to determine the contributions of denitrification and anammox to nitrogen loss in the sediments of the Yellow River wetland and to explore the abundance of functional genes involved in these two processes.MethodsThree samples were collected from areas with different vegetation covers: a P. australis-covered site (LW), a T. orientalis-covered site (XP), and a site without vegetation coverage (GT). The potential anammox and denitrification rates were estimated using nitrogen isotope tracer technology by quantifying N-29(2) and N-30(2) produced in a strictly anaerobic incubation of sediment with labeled (NO3-)-N-15 as the reactive substrate. According to the production rates of N-29(2) and N-30(2), denitrification and anammox rates were simultaneously measured. Metagenomic sequencing was used to analyze the abundances of functional microbes and functional genes.ResultsThe total nitrogen removal rates (denitrification + anammox) in the vegetation-covered sites were three to six times that in the GT (GT: 0.28 mg N kg(-1) d(-1); LW: 1.73 mg N kg(-1) d(-1); XP: 0.94 mg N kg(-1) d(-1)). The denitrification rates at the LW, XP, and GT were 1.50 mg N kg(-1) d(-1), 0.80 mg N kg(-1) d(-1), and 0.07 mg N kg(-1) d(-1), respectively. The anammox rates at the GT, LW, and XP were 0.21 mg N kg(-1) d(-1), 0.23 mg N kg(-1) d(-1), and 0.14 mg N kg(-1) d(-1), respectively. The denitrification rates were six times higher than the anammox rates in the vegetation-covered sites and thus represented the dominant nitrogen removal pathway, accounting for 85% (LW) and 86% (XP) of nitrogen loss. However, anammox was the dominant process of N-2 production at the GT. Metagenomic analysis showed that the GT did not meet the lower limit of gene abundance detection. The anammox gene was not detected, and the abundance of the denitrification gene was high, which was consistent with the denitrification rate. In addition, the continuous accumulation of (NH4+)-N-15 and the abundance of functional genes indicated the existence of the DNRA process in the Yellow River wetland.ConclusionDenitrification is the main nitrogen removal pathway in Yellow River wetland sediment, and vegetation coverage accelerates the nitrogen removal rate. Moreover, the DNRA process is of great significance for the efficient utilization of nitrogen in the Yellow River wetland.

Automated summary

The purpose of this study was to determine the contributions of denitrification and anammox to nitrogen loss in the sediments of Yellow River wetland, as well as explore the abundance of functional genes involved in these processes. Samples were collected from areas with different vegetation covers, and nitrogen isotope tracer technology and metagenomic sequencing were used for analysis. The results showed that denitrification was the main nitrogen removal pathway, accounting for 85% to 86% of nitrogen loss, while anammox was dominant in N2 production in the areas without vegetation coverage. The study also revealed the significance of the DNRA process in the efficient utilization of nitrogen in Yellow River wetland.