<p>Global nitrogen (N) enrichment promotes soil organic carbon (SOC) accumulation but often causes biodiversity loss in plants and soil microbes, creating a central challenge for achieving co-benefits of carbon accumulation and biodiversity conservation. The extent to which biodiversity contributes to SOC accumulation, and how these trade-offs can be mitigated under N enrichment remains poorly understood. Here, we conduct a global meta-analysis of N enrichment experiments encompassing 2141 observations from 275 studies to evaluate the contributions of plant, bacterial, and fungal diversity to SOC accumulation and to determine associated ecological thresholds. Our findings indicate that biodiversity, especially in plant communities, is the most important factor for SOC accumulation. We identify three distinct N thresholds characterized by different ecological responses: biodiversity loss accelerates at 50 kg N ha⁻¹ yr⁻¹, soil degradation persists at 200 kg N ha⁻¹ yr⁻¹, and SOC accumulation declines beyond this point. Given that the promoting effects of bacterial, fungal, and plant diversity on SOC diminish or even reverse at 50, 67, and 74 kg N ha⁻¹ yr⁻¹, respectively, we suggest limiting fertilization to around 50 kg N ha⁻¹ yr⁻¹, as the trade-off between biodiversity loss and SOC accumulation under N enrichment is potentially minimized.</p>