Sediment nitrate dissimilatory reduction processes in the river-lake ecotone of Poyang Lake, China: Mechanisms and environmental implications

Purpose: The river-lake ecotone in lake ecosystems can strongly influence sediment nitrate dissimilatory reduction processes. However, the mechanisms underlying these processes in river-lake ecotone ecosystems are still poorly understood. This study aims to investigate the rates of sediment nitrate dissimilatory reduction processes in Poyang Lake and clarify the mechanisms and environmental implications of these processes. Materials and methods: Sediment samples from five river-lake ecotones and lake ecosystems were collected from Poyang Lake, China. Sediment nitrate reduction and N₂O production rates were measured using ¹⁵N isotope tracing experiments. The abundance of denitrifiers, anammox 16S rRNA bacteria, and nrfA genes was quantified using the polymerase chain reaction method. Correlation analysis, redundancy analysis, and stepwise linear regression were used to evaluate the mechanisms of sediment nitrate reduction processes. Results and discussion: Sediments in the river-lake ecotone showed significantly higher denitrification (DEN), N₂O production, dissimilatory nitrate reduction to ammonium (DNRA), and anammox rates compared to lake ecosystems. DEN was the dominant process contributing to nitrate reduction, accounting for 73.36% and 74.13% in the river-lake ecotone and lake ecosystem, respectively. DEN, N₂O, and DNRA rates were significantly positively correlated with TOC, sulfide, and Fe²⁺ contents, which control the abundance of denitrifying and nrfA genes, ultimately increasing these rates. Conclusion: The annual input of reactive N to the lake from the watershed is almost equivalent to the annual N removal. However, higher DNRA and N₂O production rates indicate that approximately 23.59% of the annual N input to the lake may be transformed to NH₄⁺ or N₂O. N retention and N₂O production in the river-lake ecotone significantly exceed those in the lake ecosystem, highlighting the ecotone as a hotspot for eutrophication risk and N₂O emissions.