Enhanced deposition of atmospheric nitrogen (N) resulting from anthropogenic activities has negative impacts on plant diversity in ecosystems. Several mechanisms have been proposed to explain the species loss. Ion toxicity due to N deposition-induced soil acidification has been suggested to be responsible for species loss in acidic grasslands, while few studies have evaluated the role of soil-mediated homeostasis of ions in species loss under elevated N deposition in grasslands with neutral or alkaline soils. To determine whether soil-mediated processes are involved in changes in biodiversity induced by N deposition, the effects of 9-year N addition on soil properties, aboveground biomass (AGB) and species richness were investigated in an Inner Mongolia steppe. Low to moderate N addition rate (2, 4, 8 g N m⁻² yr⁻¹) significantly enhanced AGB of graminoids, while high N addition rate (≥ 16 g N m⁻² yr⁻¹) reduced AGB of forbs, leading to an overall increase in AGB of the community under low to moderate N addition rates. Forb richness was significantly reduced by N addition at rates greater than 8 g N m⁻² yr⁻¹, while no effect of N addition on graminoid richness was observed, resulting in decline in total species richness. N addition reduced soil pH, depleted base cations (Ca²⁺, Mg²⁺ and K⁺) and mobilized Mn²⁺, Fe³⁺, Cu²⁺ and Al³⁺ ions in soils. Soil inorganic-N concentration was negatively correlated with forb richness and biomass, explaining 23.59% variation of forb biomass. The concentrations of base cations (Ca²⁺and Mg²⁺) and metal ions (Mn²⁺, Cu²⁺ and, Fe³⁺) showed positively and negatively linear correlation with forb richness, respectively. Changes in the metal ion concentrations accounted for 42.77% variation of forb richness, while reduction of base cations was not associated with the reduction in forb richness. These results reveal that patterns of plant biodiversity in the temperate steppe of Inner Mongolia are primarily driven by increases in metal ion availability, particularly enhanced release of soil Mn²⁺.