4.3 Effect of N addition on understory herb plants
Cases of nitrogen deposition-induced soil acidification leading to reduced plant diversity have been reported (Kimmel et al., 2020).In this study, we found that the diversity of understory herb plants in the forest also decreased with decreasing soil pH value. In addition, excess N can also cause nutrient imbalance. Large-scale input of exogenous N can break the original stable stoichiometry, thus causing a nutrient imbalance in the soil and plants, which can also lead to the loss of some species of plants. N deposition indirectly affected plant species diversity by altering the composition of soil microbial communities (Bobbink et al., 2010). In the present study, it was also found that the varies of understory plant diversity was linearly and positively correlated with AM fungal diversity and AM fungal colonization rate (Fig.6), this also verified that the function of AM fungi in maintaining the stability of ecosystems and preserving biodiversity should not be underestimated. In addition, plant fine root biomass was also significantly reduced by long-term N addition, and a small amount of N input increased plant root biomass (Frew, 2022), In contrast, excessive N addition decreased the fine root biomass with increasing time of N application. The SEM clearly shows that AM fungal diversity is the strongest influence pathway affecting herb diversity under N addition conditions (Fig. 7), which is further evidence of the ability of AM fungi in maintaining understory herb diversity.