Increased nitrogen (N) deposition due to industrial and agricultural activities poses a significant threat to global biodiversity, disrupting ecosystem functions and services. Above- and belowground communities are closely interdependent and both respond to N enrichment, yet they are frequently studied separately. Whether the biodiversity of these communities responds similarly or synchronously to N inputs remains underexplored. Using a decade-long N addition experiment in a meadow steppe ecosystem, we explored the effects of a gradient of N addition levels (from 0 to 50 g N m−2 year−1) on the diversity of aboveground plants and belowground nematodes at second, sixth, and tenth years after the initiation of the experiment. Our findings revealed asynchronous responses of above- and belowground biodiversity. Plant diversity showed a progressive, time-dependent decline that intensified with both increasing N concentrations and experimental duration. In contrast, nematode diversity exhibited a threshold response: an initial decline at low N levels (<10 g N m−2 year−1) followed by stabilization across higher N concentrations, with no significant temporal intensification of this pattern over the course of the decade-long study. Plant richness declined primarily due to rapid species loss, especially among forbs, with little compensatory gain. In contrast, nematode diversity exhibited a more balanced response, driven by species replacements in which gains offset losses. Bacterivores and omnivores-predators were the most negatively affected nematode groups. This study advances our understanding of ecological responses to nitrogen enrichment by revealing the contrasting long-term dynamics of above- and belowground communities in a meadow steppe ecosystem. While plant diversity deteriorates with increased N input, nematode diversity shows signs of resilience via compensatory turnover, highlighting the potential for belowground biota to buffer ecosystem-level biodiversity loss under chronic N deposition. Our findings underscore the critical need to consider both plant and soil biota simultaneously when assessing the impacts of N deposition on biodiversity.