## 3 Results and Discussion
### SOC and Nutrient Contents  
Both straw return modes and soil layer depths had significant effects on SOC and nutrient contents. And the interaction effect between straw return modes and soil depths was also significant for the contents of SOC and nutrients (Table 2). With the increase of soil depth, SOC and nutrient contents gradually and significantly decreased in the CK and SMU treatments. In contrast, SOC and total N contents in the 20-30 cm depth were significantly larger, and available N content in the 10-20 cm depth was significantly larger than those in the other depths in the SDP treatment. However, \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Source of Variation** & **d.f.** & **SOC** & \multicolumn{2}{c}{**Total N**} & \multicolumn{2}{c}{**Total P**} & \multicolumn{2}{c}{**Available N**} & \multicolumn{2}{c}{**Available P**} \\ \hline  & & \(F\) & \(p\) & \(F\) & \(p\) & \(F\) & \(p\) & \(F\) & \(p\) & \(F\) & \(p\) \\ \hline Return modes (A) & 2 & 1675.1 & \textless{}0.01 & 396.3 & \textless{}0.01 & 63.2 & \textless{}0.01 & 1664.5 & \textless{}0.01 & 1020.1 & \textless{}0.01 \\ Soil depths (B) & 3 & 3613.8 & \textless{}0.01 & 1260.0 & \textless{}0.01 & 853.8 & \textless{}0.01 & 3437.1 & \textless{}0.01 & 11,206.1 & \textless{}0.01 \\ A \(\times\) B & 6 & 232.0 & \textless{}0.01 & 83.2 & \textless{}0.01 & 21.1 & \textless{}0.01 & 201.0 & \textless{}0.01 & 395.0 & \textless{}0.01 \\ Error & 24 & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of two-way ANOVA for the effects of maize straw return modes and soil layer depths on SOC and nutrient contents. Figure 2: Maize grain yield (**a**), hundred grain weight (**b**), ear number (**c**), and grain number per ear (**d**) under different maize straw return modes. Vertical bars represent the standard deviation of means (\(n=3\)). The bars having different lowercase and uppercase letters denote significant differences among straw return treatments (\(p<0.05\)). CK—no maize straw returning; SMU—maize straw multibing; SDP—maize straw deep ploughing. total P and available P contents gradually and significantly decreased with the increase of soil depths in the SDP treatment (Figure 3). It has been previously reported that tillage systems have a profound influence on crop root growth. The favorable hydrothermal conditions in mulching-based no-tillage systems could increase root vitality and expansion and, thus, enhance the utilization of soil moisture and available nutrients for crop growth. Moreover, mulching-based no-tillage could significantly increase root length, surface area, and tip number in the 0-40 cm soil layer [18]. In a study by [PERSON] et al. (2014), they reported a positive correlation between root proliferation with soil nutrient absorption capacity and crop yield in mulching-based no-tillage systems [19]. Relative to conventional and minimum tillage, deep tillage was more beneficial for root growth [20]. The downward growth of roots may need to absorb more P nutrients, which would result in the decrease of P nutrients in deeper soil depths. In comparison with the CK treatment, SOC and nutrient contents in the 0-40 cm soil depths were significantly greater in both the SMU and SDP treatments. The exception was for the SMU treatment, where total N content was significantly lower and total P content did not significantly differ in the 30-40 cm soil layer as compared to the CK treatment (Figure 3). Similar results were also reported in some previous studies [21; 22; 23; 24; 25]. In contrast, some other studies found that straw mulching did not significantly influence SOC [26; 27; 28; 29]  
Figure 3: Soil organic carbon (**a**), total nitrogen (**b**), total phosphorus (**c**), available nitrogen (**d**), and available phosphorus (**e**) under different maize straw return modes. Vertical bars represent the standard deviation of means (n = 3).