## 3 Results and Discussion
### SOC and Nutrient Contents
The findings of SEM demonstrated that straw return modes had a direct and positive effect, but soil depths had a negative effect on SOC and nutrient contents. The total N and  
Figure 4: [PERSON]’s correlation coefficients between soil organic carbon and nutrient contents and maize yield under different maize straw return modes. Significance levels are denoted with \({}^{+}p<0.05\) and \({}^{**}p<0.01\). SOC—soil organic carbon; TN—total nitrogen; TP—total phosphorus; AN—available nitrogen; AP—available phosphorus; HGW—hundred grain weight; EN—ear number; GNPE—grain number per ear; GY—grain yield. Figure 5: Principle component analysis (PCA) plot of soil organic carbon and nutrient contents (**a**) and redundancy analysis (RDA) plot of relationships between soil organic carbon and nutrient contents with maize yield (**b**) under different maize straw return modes. CK—no maize straw returning; SMU—maize straw mulching; SDP—maize straw deep ploughing; SOC—soil organic carbon; TN—total nitrogen; TP—total phosphorus; AN—available nitrogen; AP—available phosphorus; HGW—hundred grain weight; EN—ear number; GNPE—grain number per ear; GY—grain yield. available N exhibited direct and positive effects on HGW and GNPE; available P exhibited a direct and positive effect on EN, and SOC, total P, and available P exhibited direct and negative effects on GNPE. Moreover, HGW, GNPE, and EN had a direct and positive effect on GY. The larger positive standardized path coefficients were observed between HGW with total N (13.3) and available N (6.58) (Figure 6). Our results suggested that straw return modes indirectly affected maize yield by directly and preferentially affecting total N and available N contents, which was consistent with our hypothesis. Therefore, an optimizing N management strategy is important to achieve high maize yield with straw return. In conclusion, the results from the five-year field experiment showed that straw return modes had contradicting influences on the distributions of SOC and nutrients within soil depths. Both SMU and SDP significantly increased SOC, total N, available N, total P, and available P contents in all soil layers compared with CK. SMU was more beneficial for increasing SOC and nutrient contents within the surface soil, while SDP was more beneficial within the subsurface soil. Total N, available N, and SOC were major factors controlling maize yield. The straw return modes indirectly affected maize yield by directly and preferentially affecting total N and available N contents. An optimizing N management strategy is important to achieve high maize yield with straw return. We recommend the combination of rotation tillage and straw return for enhancing carbon sequestration,  
Figure 6: Structural equation model relating soil organic carbon and nutrient contents to maize grain yield as affected by maize straw return modes and soil layer depths. R2 indicates the proportion of variance explained by each variable. Numbers on arrows are standardized path coefficients; arrow thickness represents the magnitude of the standardized path coefficient. Red arrows indicate positive effects, blue arrows indicate negative effects, and dashed arrows indicate non-significant paths that were removed in the final model. Significance levels are denoted with * \(p\) < 0.05, **_p_ < 0.01, and **_p_ < 0.001. \(\chi^{2}\)/df—non-significant chi—square test; _P_—probability; GFI—goodness-of-fit index; RMSEA—root mean square error of approximation; SRM—straw return mode; SOC—soil organic carbon; TN—total nitrogen; TP—total phosphorus; AN—available nitrogen; AP—available phosphorus; HGW—hundred grain weight; EN—ear number; GNPE—grain number per ear; GY—grain yield. improving nutrient availability, and long-term sustainable maize production in rain-fed areas of Northeast China in future agricultural practices. However, a long-term study is essential to confirm the findings from the present study. This experiment was conceived and designed by [PERSON], [PERSON] and [PERSON].