Background and aims <p>Plant belowground biomass (BGB) is critical for ecosystem functions but remains understudied compared to aboveground biomass (AGB). Nitrogen (N) enrichment is known to impact BGB, yet the underlying mechanisms are poorly understood. Methods.</p> <p>We conducted a multi-level N enrichment experiment that manipulates forms and quantity of N in a temperate grassland by adding three chemical forms of N (Ca(NO₃)₂, NH₄NO₃, and (NH₄)₂SO₄) with six levels (0, 4, 8, 16, 24, 32&#xa0;g N m⁻²&#xa0;year⁻¹) in 2021–2023. Plant biomass, light asymmetry, soil inorganic N, pH, and exchangeable metal ions were measured to reveal the mechanisms of BGB responses to inorganic N enrichment.</p> Results <p>Plant BGB initially increased at low N rates but declined with further enrichment, forming an unimodal response. The threshold of plant BGB occurred at 16.0, 21.1, and 24.4&#xa0;g N m⁻²&#xa0;year⁻¹ for (NH₄)₂SO₄, NH₄NO₃, and Ca(NO₃)₂, respectively. Below the threshold, increases in soil available N and plant AGB promoted BGB accumulation. Beyond the threshold, oxidized N (NO₃⁻) mainly suppressed BGB by enhancing soil available N and intensifying light competition, whereas reduced N (NH₄⁺) exerted stronger negative effects by additionally inducing soil acidification, ammonium toxicity, and metal toxicity.</p> Conclusion <p>Our results demonstrate that the threshold of BGB response to N enrichment is dually regulated by biomass allocation – which could be induced by both oxidized and reduced N, as well as cation toxicity – which is more related to reduced N. These findings highlight that the dual mechanisms determing the BGB response to N enrinshment are N-form-dependent.</p>