## 4 Discussion  
Effects of Integrated Straw Return and Tillage Practices on Soil Organic Carbon and Its Labile Fractions, and Microbial Communities  
SOC serves as a critical component of the soil carbon pool, playing a pivotal role in soil nutrient cycling. Extensive studies have demonstrated that corn straw return increases organic matter input, significantly enhancing both total SOC and labile organic carbon fractions [41]. The findings of this study align with these observations, showing elevated SOC content across both soil layers under straw incorporation treatments. The NTS, RTS, and PTS treatments all significantly increased the organic carbon content in the surface soil (0-20 cm). Existing studies have shown that combining straw return with NT (1) reduced the soil disturbance frequency and effectively maintained soil stability [42]; (2) reduced soil organic matter (SOM) mineralization and enhanced SOC sequestration in the topsoil [21]. Under the NTS treatments, the straw left on the soil surface is more susceptible to changes in external environmental conditions, resulting in C loss during straw decomposition process [43]. In the 0-20 cm layer, the RTS treatment exhibited particularly pronounced SOC accumulation. This enhancement may be attributed to improved straw-soil contact through rotary tillage, which optimizes soil aeration and  
Figure 6: Pearson\({}^{\prime}\)s correlation analysis of crop yield relative to SOC labile carbon and bacterial and fungal diversity community in different integrated straw return and tillage practices. Abbreviations: SOC (soil organic carbon), ROC (readily oxidizable organic carbon), DOC (dissolved organic carbon), MBC (microbial biomass carbon). water infiltration while stimulating microbial activity to accelerate straw decomposition. In contrast, the RTS treatment releases more C into the soil for SOC formation than straw covering [44; 45]. These mechanisms collectively favor SOC sequestration under RTS treatment [46]. The PTS treatment facilitated SOC accumulation in deeper soil profiles (0-40 cm) through direct input of exogenous carbon to subsoil layers and enhanced root growth and rhizodeposition induced by deep tillage [47]. Furthermore, the formation of carbon-rich macroaggregates, along with changes in soil porosity and enzyme activity, contributed to the enhanced storage of soil organic carbon under conservation tillage and straw return [48]. As sensitive indicators of soil quality, labile SOC fractions (DOC and ROC) respond rapidly to agricultural management practices like straw return [49]. DOC represents a readily available microbial carbon source, while ROC comprises oxidizable and mineralizable organic components [10]. The findings of this study suggest two mechanisms for the increase in organic carbon content of labile SOC fractions under different straw return treatments: (1) direct input of decomposable straw-derived carbon elevated DOC and ROC pools; (2) straw decomposition enhanced microbial activity, driving the transformation of organic carbon [50]. The RTS and PTS treatments had the highest DOC content in the 0-20 cm and 20-40 cm soil layers, respectively. This might be because in RTS, the straw was fully mixed with the soil, which promoted the decomposition rate of straw and facilitated the rapid release of dissolved substances [51]; the PTS treatment promoted the migration of active organic carbon in the soil. During the agricultural production process, soil bacteria can be affected by farming practices [52]. The Chao1 and Shannon indices can be used to evaluate the diversity and richness of microbial communities [53]. The straw returning treatments (NTS, RTS, and PTS) increased the Chao1 and Shannon indices of soil bacteria to varying degrees. This suggested that the substantial nutrients present in straw promoted the growth and reproduction of microorganisms, thereby enhancing bacterial diversity in farmland soils. The RTS treatment significantly increased the bacterial Chao 1 and Shannon indices of soil at 0-20 cm depth.