Effects of Soil Salinity on Nitrification and Ammonia-Oxidizing Microorganisms in Coastal Reclaimed Farmland Soil

Nitrification, which is controlled by ammonia-oxidizing archaea (AOA) and bacteria (AOB), is the key step in the nitrogen cycle. Coastal reclaimed farmland is in the conversion stage from coastal mudflat to cultivated farmland and usually suffers from soil salinization. The effects of soil salinity on nitrification and ammonia-oxidizing microorganisms in this soil are rarely reported. Soils with different salinities were collected to conduct an incubation experiment. qPCR and Illumina MiSeq sequencing were used to examine the effects of soil salinity on the abundance, diversity, and community structure of AOA and AOB. Results showed that nitrification was inhibited by increased salinity, while AOA and AOB were both inhibited by soil salinity even if the number of amoA gene copies of AOB was greater than those of AOA. With respect to community diversity, AOA and AOB communities showed a significant shift along the salinity gradient, with AOB diversity indices decreasing significantly with soil salinity, and AOA diversity indices increasing with soil salinity and then decreasing. The community structure of AOB was significantly altered by soil salinity, and the Nitrosomonas cluster increased with soil salinity. However, the effect of soil salinity on the AOA community structure was not significant. Phylogenetic analysis showed that the Nitrosophaera cluster and the Nitrosospira cluster were the predominant clusters in AOA and AOB, respectively. The conclusion could be drawn from the above results that soil salinity inhibited nitrification by suppressing the activities of ammonia-oxidizing microorganisms and influencing their community structures. Meanwhile, ammonia-oxidizing bacteria adapt to soil salinity by means of a strategy in which salt-tolerant species replace salt-intolerant species.