Effects of straw return methods on the soil organic carbon fractions and pore structure characteristics of Shajiang black soil (Vertisol)

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.