Figure 4.

Structural equation modelling (SEM) showing the potential causal relationships among the AOA, AOB, comammox clade-A, and comammox clade-B abundances, agricultural management mode (including fertilization, straw, and earthworm), soil physicochemical factors (including pH, SOC, TN, NH₄⁺-N, and NO₃⁻-N), and potential nitrification rate (PNR). The arrow’s width indicates the intensity of the causal influence. The positive and negative correlations between the indicators are shown by the red and blue arrows, respectively. The path coefficients are represented by the numbers in the grey boxes above the arrows. Significant (p < 0.05) and insignificant (p > 0.05) routes are shown by solid and dotted lines, respectively.