| Summary: | Ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) play an important role in nitrogen (N) cycling of the estuarine sediments. However, the shifts of potential ammonia oxidation rates (PARs) of AOA and AOB along different salinity gradients are not well understood. Here, salinity-treated (0, 5, 15, 25 and 35 ‰) incubation experiments of sediment from the Pearl River Estuary were performed to explore the influence of salinity on ammonia oxidation process, including the PARs and gene abundances of AOA and AOB. Significant shifts of PARs and amoA gene abundances of ammonia oxidation process along salinity gradient were observed during the incubation (0–60 d). After a long-term incubation, both PARtotal (4.61 ± 0.93 μg N g−1 d−1) and PARaob (3.05 ± 1.04 μg N g−1 d−1) reached their maximum values at 25 ‰ salinity, and PARaoa (1.96 ± 0.10 μg N g−1 d−1) reached its maximum values at 35 ‰ salinity, which showed that PARs had higher values under high salinity conditions. AOB amoA gene was more abundant than AOA under moderate and high salinity, and only a significant linear correlation was observed between PARaob and AOB abundances. In addition, the ammonia oxidation activities showed a wide range of salinity tolerance, which is worthy of further study on community structure and transcript richness. Our study illustrates that salinity plays a key role in regulating the ammonia oxidation process in estuarine sediments, and has guiding significance for N fixation and N loss associated with saltwater incursion.
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