Nitrogen (N) is a crucial element in the ocean biogeochemical cycle with complicated transformation processes. A wealth of researches in the past decades have unveiled the importance of one or a few processes, however, a comprehensive study to the almost whole N cycling processes is still rare in estuarine and coastal ecosystems. The eutrophic Pearl River Estuary (PRE) receives dissolved inorganic nitrogen (DIN) from anthropogenic sewage, causing harmful algal blooms and hypoxia seasonally. Therefore, understanding N cycling processes and the influence factors are necessary and can give new insight for controlling and mitigating N pollution in the PRE. Here, we used isotope pairing technique, isotope-tracing technique, and isotope dilution technique combined with quantitative PCR to investigate microbially mediated N cycling processes in surface sediments (0–5 cm) of the Pearl River Estuary (PRE) during summer.
Results/Conclusions
The average potential rates of denitrification, anammox, dissimilatory nitrate reduction to ammonium (DNRA), N2 fixation, gross N mineralization (GNM), and gross ammonium immobilization (GAI) 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 matters, nutrients, and Fe2+/ Fe3+ rather than gene abundances controlling the N transformation rates. The abundances of bacteria 16S rRNA, anammox 16S rRNA, nirS, nrfA, nifH, bacteria-amoA, and archaea-amoA genes were closely associated with organic matters, nutrients, and sediment grain size. Among these pathways, denitrification contributed 41.83–90.13% total nitrate reduction in the PRE, as compared to 0.94–8.58% for DNRA and 8.55–54.56% for anammox. The sediment N-loss flux caused by denitrification and anammox in our study area (1.5×1010 m2) was 6.2×107 mol N d-1, accounting for 32.6% of the riverine DIN fluxes, suggesting that sediment has great significance to the mitigation and controlling of N pollution in this ecosystem. In addition, the total amount of ammonium (NH4+) production through sediment microbial pathways (GNM, N2 fixation, and DNRA) was estimated at approximately 1.2×108 mol N d-1. While the total amount of nitrate (NO3−) consumption (denitrification, anammox, DNRA) was 8.3×107 mol N d-1. This result indicated that sediment also plays a significant role in internal NH4+ source and NO3− sink for the overlying water. Overall, these results highlight the importance of the complicated N cycling in controlling the N budget in the PRE and improve the understanding of both processes and associated controlling mechanisms in the estuarine and coastal ecosystem.