AbstractIncreased atmospheric nitrogen (N) deposition affects biodiversity in terrestrial ecosystems. However, we do not know whether the effects of N on above‐ground plant β‐diversity are coupled with changes occurring in the soil seed bank. We conducted a long‐term N‐addition experiment in a typical steppe and found that above‐ground β‐diversity increased and then decreased with increasing N addition, whereas below‐ground β‐diversity decreased linearly. This suggests decoupled dynamics of plant communities and their soil seed bank under N enrichment. Species substitution determined above‐ and below‐ground β‐diversity change via an increasing role of deterministic processes with N addition. These effects were mostly driven by differential responses of the above‐ground vegetation and the soil seed bank β‐diversities to N‐induced changes in environmental heterogeneity, increased soil inorganic N concentrations and soil acidification. Our findings highlight the importance of considering above‐ and below‐ground processes simultaneously for effectively conserving grassland ecosystems under N enrichment.