Increasing Nitrogen (N) deposition is widely considered to be an environmental problem that leads to biodiversity loss and reduced ecosystem resilience. However, N fertilization is also an important management measure that enhances community primary production and coverage, thereby promoting the restoration of degraded grasslands. Currently, empirical evaluations of these contrasting impacts are scarce. In this study, we evaluated the dual effects of N enrichment on biodiversity and ecosystem functioning at different organizational levels (i.e., plant species, functional groups, and community) by adding N at 0, 25, 50, 100, and 200 kg N hm^-2 a^-1 to two types of alpine grasslands in the northern Tibetan Plateau:alpine meadow and alpine meadow-steppe. We also estimated the N saturation thresholds of the different alpine grassland types. Nitrogen addition had no effect on species diversity in the alpine meadow type. On the other hand, plant species number and diversity decreased gradually with increasing N addition rate in the alpine meadow-steppe. The aboveground biomass increased gradually with increasing N addition rate in the first two years of fertilization in the alpine meadow. With extended fertilization time (after 2015), the aboveground biomass first increased and then decreased in the alpine meadow-steppe. Additionally, in the alpine meadow-steppe, the aboveground biomass first showed an increasing trend followed by a decreasing trend under increasing N addition rates. At the start of fertilization (before 2015), grass aboveground biomass in the alpine meadow increased with increasing N addition rate, and the grass and legumes aboveground biomass increased with the extension of N addition time (after 2016). Sedge aboveground biomass changed from a gradual increase at the beginning of N addition (2014) to an initial increase followed by a decrease (2015), and finally to a gradual decreasing trend (2017) in the alpine meadow, indicating that N addition is not conducive to sedge growth. Nitrogen addition only significantly increased the aboveground biomass of other forbs in 2016. The alpine meadow-steppe showed different biomass patterns. In the first three years of N addition (before 2015), the grass aboveground biomass initially increased and then decreased with increasing N addition rate. Under an extended N addition period (after 2016), grass biomass gradually increased. The aboveground biomass of sedges and forbs initially increased and then decreased. The N saturation thresholds of the alpine meadow and the alpine meadow-steppe, as estimated by the responses of grasses, were 109.5 and 125.8 kg N hm^-2 a^-1, respectively. The sensitivity of the alpine meadow was significantly higher than that of the alpine meadow-steppe. This shows that increased N deposition in the future will have different effects on different types of alpine grasslands. These differences in N saturation thresholds should also be taken into account when fertilization is carried out aiming to recover different types of alpine grasslands.