3.5 Relationship between changes in environmental factors and herb diversity under N addition conditions
Long-term N addition reduced the diversity of understory herbaceous plants. The correlation between the α diversity index of herbs and biochemical factors was analyzed by Pearson correlation. The diversity of understory herbaceous plants was significantly and positively correlated with the diversity of AM fungi (P <0.05; Fig.6), and the colonization rate of AM fungi (R2 =0.66). The diversity of herbs was significantly and positively correlated with total nitrogen content (P <0.05), significantly and negatively correlated with NH4+-N content (P <0.01), and significantly and positively correlated with pH (R2 =0.67). Additionally, herbaceous plant fine root biomass was significantly and positively correlated with diversity (P <0.01).
A structural equation model (SEM) was developed to illustrate the effect of each factor on herb diversity under N addition conditions (Fig. 7), The P-value of χ2 was higher than 0.05 indicated that the model structure was reasonable. The SEM throughput analysis largely (R2 > 90%) explained the effects of biotic as well as abiotic factors on herb diversity. N addition indirectly affected AM fungal diversity and plant diversity though soil pH value, ammonium N, and AP content. Among them, AM fungal diversity was the strongest pathway affecting plant diversity.