Non-symbiotic N₂ fixation represents one of the main avenues of nitrogen (N) input into ecosystems, and thus plays a critical role in the N cycling in alpine wetlands. However, it remains unclear how environmental changes may influence N fixers and their N-fixing ability. We conducted a mesocosm experiment in the Zoige wetland to examine the responses of soil non-symbiotic N₂ fixation in alpine wetlands to rising water table and N addition. The mesocosm consisted of 27 plots filled with in situ peat cores sampled from the Zoige wetland. We established nine treatment combinations, i.e., three levels of water table crossed by three levels of N addition, with three replicates for each combination. We measured the rate of non-symbiotic N₂ fixation, soil carbon and N contents of the top soil, aboveground biomass and plant N contents for each plot. The rate of non-symbiotic N₂ fixation ranged from 0.003 to 7.35 μg N g^-1 d^-1, being higher in flooded than well-drained soils. High soil water content promoted N₂ fixation, especially under nitrogen addition. The possible explanation for stronger water effects under N addition may be that soil organic carbon, which is required by non-symbiotic N fixers, can be rapidly released by phytoplankton after N addition in flooded plots. This is evidenced by higher soil organic carbon under nitrogen addition. We concluded that water table is the main factor affecting non-symbiotic N₂ fixation of the top soil in Zoige Wetland, and that N addition enhances the extent to which high soil water content can promote non-symbiotic N₂ fixation.