【Objective】 Biochar and diatomite have gained considerable attention as soil amendments; however, their effects and underlying mechanisms on nitrification processes as well as ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in coastal saline soils remain poorly understood. 【Method】 A 42-day indoor incubation experiment was conducted using typical coastal saline soil from the Yellow River Delta. Five treatments were applied: control without fertilizer (CK), urea (U), urea with dicyandiamide (DCD), urea with diatomite (DE), and urea with biochar (BC). The effects of these treatments on soil nitrification were evaluated, and the abundance and community structure of AOA and AOB were analyzed using quantitative PCR and high-throughput sequencing. 【Result】 The results demonstrated that DCD significantly inhibited nitrification in the short term, with an inhibition rate of 73.1%, however, its inhibitory effect diminished over time. Biochar also showed short-term nitrification inhibition but subsequently promoted AOA abundance. In contrast, diatomite did not significantly inhibit nitrification but notably enhanced AOB abundance. Correlation analysis revealed that the amoA gene abundances of AOA and AOB exhibited significantly opposite regulatory effects on the nitrification process. AOA showed significant positive correlations with ammonium nitrogen (NH??-N) content and nitrification inhibition rate (NIR), but a significant negative correlation with nitrate nitrogen (NO??-N) content. Conversely, AOB demonstrated significant negative correlations with NH??-N content and NIR, but highly significant positive correlations with NO??-N content and net nitrification rate (NNR). Notably, the correlation between AOB and NNR was significantly stronger than that of AOA. Cluster analysis indicated that the community structures of AOA and AOB in the BC and DCD treatments were more similar, which may be related to the fact that both regulate the soil nitrogen transformation process and microbial activity. 【Conclusion】 This study elucidates the differential regulatory mechanisms of biochar and diatomite on ammonia-oxidizing microbial communities in coastal saline soils, offering a novel theoretical foundation for the amelioration of saline soils.