The aim of the study was to understand the impact of biochar and straw return on soil aggregates and organic carbon for soil improvement of the newly cultivated purple soil dry slope land in the Three Gorges Reservoir area. In this study, a field test was used to set five treatment pairs with regards to soil aggregate composition and organic carbon distribution:no fertilization(CK), conventional fertilization(NPK), optimized fertilization(GNPK), chemical fertilizer reduction combined with straw(RSD), and chemical fertilizer reduction combined with biochar(BC). The results showed that fertilization can improve the level of soil fertility, especially with the RSD and BC treatments. The soil aggregates of each fertilization treatment were<0.25 mm in size. Compared with the CK, each treatment significantly increased the aggregate content of 0.5-5 mm particles, and the values of MWD, GMD, and R0.25. Further, the treatments significantly reduced the value of D and PAD0.25(P<0.05), and each fertilization treatment significantly increased the soil organic carbon content, of which BC(6.73 g·kg-1) and RSD(5.45 g·kg-1) were significantly better than NPK(5.05 g·kg-1) and GNPK(3.63 g·kg-1). The<0.25 mm aggregates had the highest contribution rate of organic carbon(34.92%-59.49%), while the>5 mm aggregates had the lowest contribution rate of organic carbon(1.55%-6.01%). The BC treatment significantly increased the organic carbon contribution rate of 5-2 mm and 2-1 mm agglomerates(P<0.05), while the contribution rate of NPK, RSD, and GNPK was the most significant for 0.5-0.25 mm(P<0.05). Each fertilization treatment increased the yield of rapeseed and corn, with large inter-annual differences, but the overall difference between treatments was not significant. The stability of soil aggregates and crop yields showed an upward trend with the increase of soil organic carbon. Biochar and straw returning to the field may promote the formation of large and medium aggregates in soil, effectively improve the stability of aggregates, increase organic carbon content, and promote crop yields. It is therefore an effective measure to improve the soil structure of purple soil and improve soil quality.
Keywords: biochar; dry slope; purple soil; soil aggregate; straw.