3.5. Structural equation model
SEM analysis indicated that three crop rotation patterns showed the different direct and indirect effects of fertilization on grain yield and NAE (Fig. 7). Under RG rotation, fertilization had a directly positive effect on NAE (0.453) but no significant effect on grain yield. The change of pH showed a negative effect on NAE (−0.337). The change of POXC presented negative effects on grain yield (−0.723) and NAE (−0.776), respectively. Under RR rotation, fertilization showed a directly negative effect on grain yield (−0.343) but no significant effect on NAE. Both the changes of pH and SOC had positive effects on grain yield and NAE under RR rotation. The change of POXC under RR rotation presented lower negative effects on grain yield (−0.502) and NAE (−0.427), respectively compared with that under RG rotation. Fertilization had no significant effect on grain yield and NAE under RW rotation. Only the change of SOC had a positive effect on NAE under RW rotation. In general, fertilization had no direct significant effect on the changes in soil pH, SOC, POXC, and nutrients under the three crop rotation patterns.