Lanthanum-(La(III)) concentration in water resources is increasing exponentially in recent years following its widespread use as farm inputs via crop fertilizer production. However, studies on the effects of La(III) on nitrogen removal, conversion and mechanism are still unclear. Consequently, this study investigated the effects of La(III) on partial-nitritation-(PN) process performance and bacteria activities via sludge morphology, extracellular polymeric substance (EPS) composition, organic functional group changes, and microbial community structure. The study revealed hormesis condition within the PN batch assays upon La(III) dosage, thus, dose-response phenomenon exhibited sludge stimulation at low La(III) dosage and inhibition at high-dose. Scanning electron microscope-energy spectroscopy (SEM-EDX), confocal laser scanning microscope (CLSM), and Fourier transform infrared spectroscopy (FTIR) confirmed efficient removal of La(III) (96.76%-99.93%) was achieved with polysaccharide and polysaccharide-protein complex through biosorption and biosynthesis. 16S rRNA gene sequencing demonstrated that La(III) significantly promoted the abundance of Nitrosomonas and identified Aquamicrobium as the ammonia oxidizing bacteria with high tolerance to La(III). Furthermore, phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) further revealed metabolic detoxification as pathways where enzymatic reactions neutralized and solubilized toxins. This study unraveled a strategic approach and insight into how Nitrosomonas and other functional genera regulate and control their activities against La(III) toxicity and subsequently enhance the PN process.