Effects of Different Aeration Strategies and Ammonia-Nitrogen Loads on Nitrification Performance and Microbial Community Succession of Mangrove Constructed Wetlands for Saline Wastewater Treatment

In highly salinized environments, nitrification is the phase that limits the rate of nitrogen transformation and removal. Therefore, this study concentrated on the impacts of different aeration strategies and NH4+–N surface loading on the nitrification performance of mangrove constructed wetlands and explored the potential microbial response mechanism. The results showed that both the continuous aeration constructed wetlands (CA-CW) and intermittent aeration constructed wetlands (IA-CW) achieved a nitrification efficiency of more than 98% under an NH4+–N surface loading of 1.25-4.7 g/(m2·d). Periodic changes in dissolved oxygen concentration and low ammonia-nitrogen loading cause mangrove roots to secrete allelochemicals, such as dibutyl phthalate, phenol, 4-(1,1-dimethylpropyl)-, which in turn encourages the growth of nitrification and denitrification groups such as Candidatus Nitrocosmicus, Nitrolancea, Truepera, Pontibacter, Halomonas and Sulfurovum in the root layer of IA-CW1 and strengthens the transformation and removal of nitrogen. Intermittent aeration increased the secretion of 13-Docosenamide, 5-Tetradecene, Phenol, 2,4-bis(1,1-dimethylethyl)-, and strengthened Nitrospira, Nitrosomonas, and Comammox Nitrospira under conditions of high ammonia-nitrogen loading (HAL), which may account for the effective ammonia removal in IA-CW2. According to PICRUSt2, HAL and continuous aeration increase the relative abundance of functional nitrification genes (amoCAB and hao) and decrease the expression of denitrification-related genes (nirK, NarGHI/NarAB, norBC, and nosZ). Considering the quantitative PCR results, the NH4+–N surface loading rate was the primary factor driving the succession of the ammonia-oxidizing archaea or ammonia-oxidizing bacteria groups. The enrichment of complete ammonia-oxidizing bacteria promoted by the periodic change in the dissolved oxygen concentration was an important contributor to the efficient nitrification of saline sewage under HAL. Overall, this study revealed that the dominant AOMs in mangrove CWs could be significantly altered by regulating the aeration modes and pollution loads to adjust the rhizosphere organic matter in situ, thereby resulting in more efficient nitrification.