3.2.2 Herb diversity, fine root biomass, and AM fungal colonization rate
The Shannon index and Simpson index of herb plants decreased gradually with increasing gradient of N addition concentration, and the MN and HN treatments were significantly lower than CK (Table 3); The fine root biomass and AM fungal colonization rate decreased significantly with the increase of N addition concentration gradient, and the biomass decreased by 78.6% in the HN treatment, the AM fungal colonization rate decreased significantly in the MN and HN treatments, and decreased by 21.95% in the HN treatment. These indicated that herb diversity and AM fungal colonization rate of fine roots were significantly reduced when the applied N concentration exceeded 40 kg N ha-1year-1 in this experiment. The fine root biomass of herb plants was significantly reduced when the applied N concentration exceeded 20 kg N ha-1 year-1.