Shajiang black soil (Vertisol) is known to have low soil organic carbon (SOC) and high bulk density, but it limits crop productivity. Straw return can be expected to serve as an important practice of agricultural management to improve the soil structure and SOC content. Three tillage practices were widely implemented: no-tillage (NTS), rotary tillage (RTS), and deep ploughing (DPS). The objective of this study was to explore the optimal straw return for high soil quality and crop production in Shajiang black soil. The six-year field experiment was conducted to quantify the long-term effects of different straw return methods on the SOC and its fractions (particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)), as well as the pore structure characteristics. Soil samples were collected from depths of 0-10, >10-20, and >20-40 cm. X-ray computed tomography (CT) was adopted to visualize and quantify the soil pore structure. The results indicated that there was no significant difference (P>0.05) in the SOC and its fractions, total porosity, global connectivity, hydraulic radius, and macroporosity (>50 μm) among all treatments in the 0-10 cm soil layer. Compared with the RTS, the NTS decreased the SOC content in the >10-20 and >20-40 cm soil layers by 14.1% and 23.7%, respectively (P<0.05). While the NTS treatment raised significantly the proportion of MAOC to SOC in the >20-40 cm soil layer, compared with the RTS (P<0.05), There was no significant increase in the MAOC content in the 0-40 cm soil layer. The NTS treatment also significantly decreased the POC content and the proportion of POC to SOC in the >20-40 cm by 55.9% and 42.6%, respectively (P<0.05). However, the NTS had no significant effect on the pore structure parameters in the >10-40 cm soil layer (P>0.05) compared with the RTS. By contrast, the DPS increased the SOC content by 12.7% and 44.1% in the >10-20 and >20-40 cm depths, compared with the RTS, respectively (P<0.05). Furthermore, the content of POC, MAOC, and proportion of POC to SOC in the >20-40 cm soil layer increased by 116%, 42.4% and 49.6%, respectively (P<0.05). The DPS also significantly increased the porosity of >200 μm, the porosity of the largest interconnected pore network, hydraulic radius, and global connectivity in the >10-20 and >20-40 cm layers (P<0.05). Besides, the DPS treatment significantly increased the fractal dimension in the 0-10 and >10-20 cm soil layers (P<0.05). The soil macroporosity under DPS treatment was dominated by >500 μm pores, where the porosity accounted for 39.0%, 41.5%, and 34.2% of the macroporosity in the 0-10,>10-20, and >20-40 cm soil depths, respectively. Compared with the RTS, the DPS significantly increased the porosity of >200-300, >300-400, >400-500 and >500 μm in the >10-40 cm soil layer (P<0.05). Additionally, the POC content showed a linear positive correlation with the soil macroporosity (>50 μm) (R²=0.643, P<0.01), global connectivity (R²=0.488, P<0.05), porosity of the largest interconnected pore network (R²=0.564, P<0.05), and hydraulic radius (R²=0.749, P<0.01). The POC and SOC contents were positively correlated with >200-300, >300-400, >400-500, and >500 μm porosity (P<0.05), indicating that the pore structure of soil was a significant influence on the straw decomposition, as well as the POC and SOC accumulation. In conclusion, deep ploughing with straw return can be expected to improve the SOC accumulation and macroporosity in the >10-40 cm layer. The finding can provide a strong reference for the sustainable strategy on soil quality and straw management in Shajiang black soil.

To investigate the effects of different fertilization regimes on organic carbon content and the contribution characteristics of carbon from different sources in Shajiang black soil, a long-term fertilization experiment was conducted over nine consecutive years. Treatments of five fertilization strategies were established: no fertilization (CK), chemical fertilization alone (NPK), chemical fertilization with straw return (NPK+S), commercial organic fertilizer with straw return (COF+S), and combined application of commercial organic fertilizer and nitrogen fertilization with straw return (N+COF+S). The study analyzed the distribution of soil organic carbon (SOC), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) in the 0-10 cm and 10-20 cm soil layers, as well as the contributions of carbon from different sources. The results showed that in the 0-10 cm layer, compared with CK, the SOC content increased by 25.2%, 100.0%, and 72.1% (P＜0.05) under NPK+S, COF+S, and N+COF+S treatments, respectively. In the 10-20 cm layer, only the COF+S treatment significantly increased SOC content by 38.3% (P＜0.05). In both soil layers, the COF+S treatment increased the mass percentage of POC by 53.6% and 30.4% (P＜0.05), respectively. All fertilization treatments enhanced the contribution of POC to SOC (P＜0.05) while reducing the contribution of MAOC (P＜0.05). In the 0-10 cm layer, compared with CK, the four fertilization treatments increased the C content of POC by 115.9%, 155.6%, 266.2%, and 197.4% (P＜0.05), respectively. For the MAOC fraction, NPK+S, COF+S, and N+COF+S treatments increased the C content by 14.9%, 56.2%, and 39.0% (P＜0.05), respectively. In the 10~20 cm layer, NPK+S, COF+S, and N+COF+S treatments significantly increased the C content of POC by 148.3%, 275.8%, and 194.5% (P＜0.05), respectively, but only the COF+S treatment increased the C content of MAOC by 27.4% (P＜0.05). Across all treatments in the 0~20 cm soil layer, fungal-derived carbon consistently represented the primary source contributing most significantly to SOC. In the 0-10 cm layer, compared with CK, all four treatments reduced the contribution of bacterial-derived carbon to SOC by 5.1%, 4.3%, 24.3%, and 18.3% (P＜0.05). Compared to CK, all four fertilization treatments increased the contribution of plant-derived to SOC in the 0~20 cm layer, with COF+S showing the highest increase 30.2% (P＜0.05) in the 0-10 cm layer and 10.2% (P＜0.05) in the 10-20 cm layer. In both soil layers, SOC fractions and the contributions of plant-derived and microbial-derived carbon were significantly positively correlated with SOC (P＜0.05). Correlation analysis revealed that across both soil layers and all treatments, SOC fractions exhibited statistically significant positive correlations with plant-derived carbon, and significant negative correlations with fungal- and bacterial-derived carbon (P＜0.05). The study demonstrated the combined application of commercial organic fertilizer with straw return enhanced the contribution of plant-derived carbon to SOC in both soil layers of Shajiang black soil. This enhancement reached statistical significance (P＜0.05) when the commercial organic fertilizer was applied at the adequate rate of kg/(hm²·a). Therefore, this combined practice effectively enhanced the carbon pool level in Shajiang black soil. This was achieved by enriching the soil with organic materials, which suppressed bacterial transformation of exogenous carbon, thereby preserving more plant residues in the form of POC within the soil.