Fig. 4 Structural equation model (SEM) illustrating the direct and indirect effects of nitrogen fertilizers (N and NS treatments) on soilphysicochemical properties and alpha diversity (faith_pd index) of bacterial, fungal, and protist communities. Continuous and dashed arrowsrepresent the significant and nonsignificant relationships, respectively. Adjacent number that are labeled in the same direction as the arrowrepresents path coefficients, and the width of the arrow is in proportion to the degree of path coefficients. Green and red arrows indicatepositive and negative relationships, respectively. r2 values indicate the proportion of variance explained by each variable. Significance levels aredenoted with *P < 0.05, **P < 0.01, ***P < 0.001. Standardized total effects (direct plus indirect effects) calculated by the SEM are displayed below theSEM. The low chi-square (χ2), nonsignificant probability level (P > 0.05), high goodness-of-fit index (GFI > 0.90), low Akaike information criteria (AIC), andlow root-mean-square errors of approximation (RMSEA < 0.05) listed below the SEMs indicate that our data matches the hypothetical models