Abstract: 【Objective】To ascertain the effect of straw returning quantity on the soil physical characteristics and establish a recycling model for planting by-products. At the same time, to provide a scientific basis for the utilization of straw resources in the central hilly area of Sichuan basin.【Method】Herein, we examined the effects of different amount of straw returned to the field (0% straw returned (RMW₀), 30% straw returned (RMW₃₀), 50% straw returned (RMW₅₀) and 100% straw returned (RMW₁₀₀)) on the physical characteristics at the cultivated soil layer of the winter wheat-summer maize rotation system, based on long-term field trials (2006-present) using a combination of in situ monitoring and computed tomography microscanning (CT).【Result】(1) Straw returns to the field could significantly improve soil permeability, water holding capacity and hydraulic conductivity, and the improve effect increases significantly with the amount of straw returned to the field . Compared to the RMW₀ treatment, soil bulk density in RMW₃₀, RMW₅₀ and RMW₁₀₀ treatments reduced significantly by 15.2%, 11.7% and 17.9%, respectively. Whereas soil porosity of these treatments were significantly increased by 18.4%, 13.7% and 21.3%, respectively. In addition, the Saturated hydraulic conductivity of RMW₁₀₀ treatment was as high as 1.62 mm min^-1, and the soil hydraulic conductivity was superior to other treatments. (2) Straw returning promoted the development of existing pores into larger ones and significantly improved pore uniformity and connectivity. The RMW₁₀₀ and RMW₅₀ treatments improved the macropore composition of the soil better than that of the RMW₃₀ and non-return treatments. The average pore diameter of the RMW₁₀₀ treatment tends to be larger and inter-pore connectivity is optimal. The homogeneity of the pores in the RMW₅₀ treatment was significantly improved and the pore size distribution was more appropriate than that of other treatments. (3) Compared to the RMW₀ treatment, the number of >2 mm agglomerates increased significantly and the number of 0.25-2 mm agglomerates decreased significantly after straw returned to the field, which was beneficial to the formation of large soil water-stable agglomerates and promoted the transformation of medium to large agglomerates. Both RMW₅₀ and RMW₁₀₀ treatments improved significantly better than that of RMW30 treatment. (4) Principle components analysis showed soil bulk density, water-stable aggregate with diameter larger than 0.25 mm and large pore were the main indicators of the physical characteristics of cultivated soils in calcareous purple soils. The first and second principal components explained 57.8% and 23.62% of the physical properties of the soil. The physical characteristics of RMW₅₀ and RMW₁₀₀ treatments were close to each other, and showed significant divergence from the RMW₀ and RMW₃₀ treatments on the PC1 and PC2 axes. 【Conclusion】On the basis of no significant difference of crop yield in the central hilly area of Sichuan basin, there were differences in the effects of different straw returning quantities on the physical properties of cultivated soil layer, with no significant differences between 50% and 100% straw returning effects, but significantly better than that of 30% and 0% of straw incorporation. The specific straw application rate should be selected according to the local conditions.

Key words: straw returning, purple soil, winter wheat-summer maize rotation system, soil aggregate, soil pore characteristics