To investigate the appropriate nitrogen addition concentration for synergistically restoring productivity and maintaining biodiversity in degraded alpine steppe， this study conducted a randomized block experiment in the northern Xizang alpine steppe （elevation 4634 m）. Six nitrogen addition gradients （0，5，10，15，20， and 25 g·m^-2·a^-1 N） were set under natural conditions and with phosphorus application. Mixed-effect models were used to analyze the effects of nitrogen addition on plant aboveground biomass， community structure， and diversity. The results showed that short-term nitrogen addition significantly increased plant aboveground biomass， with the promoting effect strengthening as nitrogen concentration increased. Simultaneously， nitrogen addition significantly reduced plant diversity， with the inhibitory effect intensifying with higher nitrogen concentrations， accompanied by an increase in the importance value of grasses and a decrease in the importance values of sedges and forbs. Phosphorus application enhanced the positive effect of nitrogen addition on aboveground biomass but did not significantly exacerbate the inhibition of plant diversity. This study reveals that moderate nitrogen addition （10~15 g·m^-2·yr^-1N） with phosphorus application may be an effective restoration measure for degraded alpine steppe， providing a scientific basis for the sustainable management of alpine steppe ecosystem.

Nitrogen addition  /  Phosphorus limitation  /  Alpine steppe  /  Plant aboveground biomass  /  Plant species diversity  /  Community structure