###### Abstract  
Returning crop residues to the field after harvesting is a proven effective strategy for improving soil fertility, carbon sequestration, and crop productivity. However, the relationships between crop residue return modes, SOC and nutrient contents, and crop yields are still unclear. In this study, a field trial was conducted to investigate the effects of different maize straw return modes, i.e., straw mulching (SMU), straw deep ploughing (SDP), and control without straw return (CK), on soil organic carbon (SOC) and nutrient contents in soil layers of 0-40 cm in a Mollisol. The relationships between straw return modes, SOC and nutrient contents, and maize yield were evaluated. Compared with CK, SMU and SDP significantly increased SOC, total nitrogen (N), available N, total phosphorus (P), and available P contents in all soil layers. Relative to SMU, SOC, total N, available N, total P, and available P contents were significantly lower in soil layers of 0-10 cm, but they were significantly higher in soil layers of 20-40 cm in SDP. Redundancy analysis indicated that total N, available N, and SOC were major factors controlling maize yield. Structural equation modeling further showed that straw return modes indirectly affected maize yield by directly and preferentially affecting total N and available N contents. The results indicated that SMU and SDP were beneficial for increasing SOC and nutrient contents at the surface and subsurface soils, respectively. Optimizing a nitrogen management strategy is important to achieve high maize yield with straw return. crop residue; soil property; causal relationship; soil depth; maize monoculture 2023132148  
rotation than with wheat-maize rotation with the condition of straw returning [5]. The return modes of crop residues are one of the critical factors that impact soil fertility and crop productivity. Based on a meta-analysis from 420 publications in China, [PERSON] et al. (2021) found that straw mulching was more beneficial to soil nutrient accumulations in areas with average annual rainfall of <400 mm and in the soils with neutral and alkaline pH values, loam texture, a straw returning amount of >9000 kg per hectare, and a nitrogen fertilizer application rate of <100 kg per hectare; by contrast, straw burying was more favorable to soil nutrient accumulations in areas with average annual rainfall of >800 mm and in soils with acidic pH values and clay texture [6]. Therefore, understanding the impacts of different return modes of crop residues on soil quality and crop productivity are necessary for the rational utilization of crop residues in agricultural ecosystems. Mollisols, which are recognized as inherently fertile and productive soils around the world, are mainly distributed in the northeast region of China [7]. The Mollisol region of Northeast China is also one of the major maize (_Zea mays_ L.) production areas in China and is known as the Golden Maize Belt of the world [8]. The total maize sown area in Northeast China has more than 6 \(\times\) 10\({}^{6}\) ha, which accounts for about 30% of the nation's total maize production [7]. However, the Mollisols in Northeast China have experienced serious degradation, including SOC and nutrient depletion, soil structure deterioration, and mollic epipedon thickness reduction, due to natural (e.g., erosion and climate change) and human disturbances (e.g., unreasonable tillage and fertilization regimes) during the last several decades [9; 10; 11; 12]. With the improvement in the degree of agricultural mechanization, direct returning maize straw through mulching or deep ploughing to the field have been widely applied for combatting soil degradation in this region. Previous investigations have mainly concentrated on the impacts of either the mulching [13] or deep ploughing of maize straw [14] on SOC and nutrient contents in the Mollisols of Northeast China. For example, [PERSON] et al. (2021) studied the impact of straw mulching on SOC content [13]. [PERSON] et al.