Nitrogen addition drives changes in arbuscular mycorrhizal fungal richness through changes in plant species richness in revegetated alpine grassland

Nitrogen (N) addition not only promotes the restoration of degraded grasslands, but also threatens ecosystem functioning through the loss of species richness. Thus, a deep understanding of the effect of N addition on the richness of key organisms in restored grasslands is critical to sustainably restoring degraded grasslands. We conducted a 4-year N addition experiment to investigate the response of both plant and arbuscular mycorrhizal (AM) fungal richness to the combined addition of ammonium (Am) and nitrate (Ni) in a revegetated grassland rehabilitated (with a focus on restoration) on the Qinghai–Tibet Plateau. Both nitrogen forms were added at three levels: 0, 10, and 20 g N m⁻² year⁻¹. By itself, Ni addition of 20 g N m⁻² year⁻¹ (Ni20) reduced both plant and AM fungal richness, while Am addition of 20 g N m⁻² year⁻¹ (Am20) had no significant effect on them. However, when Ni and Am were combined, only Ni20 plus Am20 among all combinations reduced both plant and AM fungal richness. Both soil nitrate-N and plant species richness jointly drove changes in AM fungal richness, but plant species richness was the main factor affecting AM fungal richness under N addition. Our results suggest that minimizing the loss of AM fungi caused by plant species loss resulting from N addition is a key means to sustainably restore degraded grasslands.