## 3 Results and Discussion
### SOC and Nutrient Contents
The bars different lowercase and uppercase letters denote significant differences among straw return treatments and among soil depths, respectively (\(p\) < 0.05). CK—no maize straw returning; SMU— maize straw mulching; SDP— maize straw deep ploughing. and total N [30] contents in the 20-40 cm soil depth. These conflicting outcomes may be attributed to the influences of many factors, including climate conditions, soil properties, and straw returning years. For example, [PERSON] et al. (2021) showed that straw mulding had no significant influence on SOC content in the 20-40 cm soil depth in an Argiudoll of Northeast China after a three-year field experiment [28]. However, [PERSON] et al. (2020) observed that straw mulding markedly increased SOC content in the 20-40 cm soil depth in a Haplic Cambisol of South Africa after a five-year field experiment [24]. Thus, further studies were needed to ascertain the main factors that affected SOC content under the condition of SMU. The contents of SOC and total N in the 0-20 cm soil depth were 1.30-11.9% and 2.16-3.54% larger in the SMU than in the SDP treatment, while SOC and total N contents in the 20-40 cm soil depth were 11.4--9.2% and 8.12-10.5% lower in the former than in the latter treatment, respectively. However, no significant difference was found for SOC content between the SMU and the SDP treatments in the 10-20 cm soil depth. Furthermore, available N, total P, and available P contents were significantly greater in the SMU than in the SDP treatment in the 0-10 cm soil depth, while they were significantly greater in the SDP than in the SMU treatments in the other soil depths (Figure 3). The results proved our hypothesis that the responses of SOC and nutrient contents to straw return modes depended on soil layer depths. In the SMU treatment, straw was mulched on the soil surface, and the decomposition of straw accordingly led to more SOC and nutrients concentrated on the surface soil. However, straw was plugued to the 20-30 cm soil depth in the SDP treatment, which resulted in higher SOC and nutrient contents in the subsurface soil. Furthermore, deep ploughing can break plow pan layers, which subsequently facilitates the growth of crop roots in the subsurface soil and raises the inputs of carbon and nutrients from maize roots [31]. It was previously reported that straw mulding reduced soil erosion [32] but caused shallow and stringent plough layer thickness [21]; on the contrary, straw deep ploughing enlarged plough layer thickness [33] while could increase the risk of soil erosion [34]. Based on our present and previous studies, straw mulding and deep ploughing have their own advantages and disadvantages. Thus, we recommend the combination of rotation tillage (i.e., alternation of no-tillage and deep ploughing) and straw return in future agricultural practices. Relationships between Maize Straw Return Modes, Soil Depths, SOC and Nutrient Contents, and Maize Yield  
The Pearson correlation results showed that SOC and nutrient contents were positively correlated with each other, and there were also positive correlations between SOC and nutrient contents with GNPE. However, a negative correlation was detected between SOC and nutrient contents with EN (Figure 4). According to the PCA analysis, the first two axes contributed 98.1% and 1.95% of the SOC and nutrient contents, respectively. The SOC and nutrient contents were clearly distinguished among the three straw return treatments (especially between the CK and SMU treatments versus the SDP treatment) in the ranking diagram (Figure 5a), suggesting that the SDP mode had a more remarkable influence on SOC and nutrient contents. The results of the RDA analysis indicated that SOC and nutrient contents explained 87.5% of the variation in maize yield. RDA1 and RDA2 interpreted 86.8% and 0.66% of maize yield, respectively. The total N, available N, and SOC were significant factors in shaping maize yield, which explained 49.8%, 31.8%, and 5.70% of the total observed variance in maize yield, respectively (Figure 5b).