Nitrogen cycling processes in sediments of the Pearl River Estuary: Spatial variations, controlling factors, and environmental implications

Abstract Numerous studies have unveiled the importance of nitrogen (N) transformation processes over the past decades, but comprehensive studies of key N-cycling processes are still rare in estuarine and coastal ecosystems. Here, we used isotope pairing, isotope-tracing, and isotope dilution techniques combined with quantitative polymerase chain reaction to investigate microbial N-cycling processes in surface sediments (0–5 cm) of the Pearl River Estuary. The average rates of denitrification, anammox, DNRA, N2 fixation, N mineralization, and NH4+ immobilization were 1.41 ± 0.89, 0.067 ± 0.033, 0.47 ± 0.28, 0.31 ± 0.30, 1.86 ± 1.09, and 1.30 ± 0.83 μg N g−1 dry d−1, respectively. Sediment grain size, organic matter, nutrients, and Fe2+/Fe3+ rather than gene abundances controlled these rates. The abundances of bacterial 16S rRNA, anammox 16S rRNA, nirS, nrfA, nifH, bacteria-amoA, and archaea-amoA genes were significantly correlated with organic matter, nutrients, and sediment grain size. In general, higher rates and gene abundance was occurred in outer than inner estuary. Among these pathways, denitrification contributed 41.83–90.13% of the total nitrate reduction, as compared to 0.94–8.58% for anammox and 8.55–54.56% for DNRA. The sediment N-loss fluxes caused by denitrification and anammox in our study area (1.5 × 1010 m2) was about 6.2 × 107 mol N d−1, accounting for ~42.1% of the riverine dissolved inorganic N fluxes, suggesting that the sediment of the Pearl River Estuary has great significance to the mitigation and controlling of N pollution in this ecosystem. Additionally, the net NH4+ production via sediment microbial pathways (N mineralization, N2 fixation, DNRA, NH4+ immobilization, and anammox) was estimated at ~ 5.5 × 107 mol N d−1, while the net NO3− consumption (denitrification, anammox, and DNRA) was ~8.3 × 107 mol N d−1. Overall, these results highlight the importance of complicated N-cycling processes in controlling the N budget in the Pearl River Estuary and improve the understanding of both the processes and associated controlling factors in estuarine and coastal ecosystems.