Effects of Straw Returning on Soil Aggregates and Its Organic Carbon and Nitrogen Retention under Different Mechanized Tillage Modes in Typical Hilly Regions of Southwest China

Tillage modes and straw returning influence soil aggregate stability and the distribution of organic carbon (C) and nitrogen (N) in aggregates of different particle sizes. In the typical hilly regions of southwest China, the predominant soil type is purple soil, characterized by heavy texture and high stickiness, with relatively lower soil fertility compared to other soil types. The improper use of fertilizers and field management practices further exacerbates soil compaction. However, abundant straw resources in the region provide an opportunity for comprehensive straw utilization. The effective utilization of straw resources is of significant importance for stabilizing agricultural ecological balance, improving resource utilization efficiency, and alleviating ecological pressure. Previously, most studies have focused on the impact of different mechanized tillage systems on the physical and chemical properties of soil in hilly areas, while research on the preservation of water-stable aggregates’ organic C and N content remains limited. In this study, the soil properties of fields under a winter pea–summer corn rotation for two years were studied with regards to the effects of straw returning on its water-stable aggregate distribution, macroaggregate content (R0.25), mean weight diameter (MWD), geometric mean diameter (GMD), and the organic C and N content in soil aggregates of different particle sizes and at different depths. The effects of five different tillage modes were assessed, namely rotary tillage with straw mixed retention (RTM), conventional tillage with straw burial retention (CTB), no-tillage with straw covered retention (NTC), subsoiling with straw covered retention (STC), and no-tillage without straw retention (NT). Based on the study results, under different tillage modes, straw returning effectively enhanced the soil organic carbon (SOC) and total nitrogen (TN) reserves at the plow layer (0–30 cm), SOC increased by 17.2% to 88%, and TN increased by 8.6% to 85.9%. At the same time, the content of 0.25–2 mm aggregates increased under the straw-return treatments under different tillage patterns. The NT treatment had the lowest R0.25 and MWD and GMD values for soil aggregates at different depths, which were significantly different (p < 0.05) from the other treatment modes. The correlation coefficients between SOC and soil aggregate stability indices ranged from 0.68 to 0.90, with most of them showing highly significant positive correlations (p < 0.01). In conclusion, straw returning under different tillage systems has improved soil aggregate stability and promoted soil structure stability. Specifically, the STC treatment has shown more pronounced effects on soil improvement in the upper soil layer of the hilly regions in southwest China, while the RTM treatment is beneficial for improving the lower soil layer. Therefore, the comprehensive experimental results indicate that the combination of STC and RTM treatments represents the most promising mechanized tillage and straw returning practices for the hilly regions in southwest China.