## 4 Discussion
This could be attributed to RT treatment improving soil aeration, increasing surface oxygen levels, thereby enhancing the activity of aerobic bacteria and boosting the metabolic activity of aerobic microorganisms in the topsoil [54]. Deep tillage (PT) disturbed the structure of deeper soil layers and reduced soil compactness [55]. The highest bacterial Chao 1 index in the PTS treatment at the 20-40 cm depth was explained from the perspective of physical disruption. However, the Simpson index remained unaffected by straw return. These findings align with [PERSON] et al. [56], who reported inconsistent patterns in bacteria Simpson indices across different studies. For fungal communities, diversity and richness varied depending on the soil environment and farmland management practices [57; 58]. Straw return also increased the Chao 1 index of soil fungi across treatments. The RTS treatments showed the highest fungal diversity in soil at 0-20 cm depth, likely due to straw-induced modifications in SOC content, soil bulk density, and moisture conditions, which collectively influenced fungal community development [55]. Notably, the PTS treatment displayed the highest fungal diversity indices (Chao 1 and Shannon) in deeper soil layers; potential reason include the following: (1) deep tillage practice was found to alter soil compaction and aeration [57], improving the microbial environment and thus increasing microbial diversity; (2) straw was plowed into the deep soil layer, modifying the soil substrate and serving as an energy source for microbial activity [59; 60]. [PERSON] et al. [61] demonstrated that _Proteobacteria_ in soil primarily participate in organic matter transformation and soil structure formation, being closely associated with carbon utilization. In this study, straw incorporation increased the content of labile organic carbon, which consequently led to an increase in the relative abundance of _Proteobacteria_. Moreover, deep tilling of straw also further enhanced the abundance of _Proteobacteria_ in deeper soil [60; 62]. Compared to CK, straw return also increased the relative abundance of _Bacteroidetes_. As a beneficial bacteria phylum involved in carbon and nitrogen cycles, _Bacteroidetes_ increased as the carbon content from straw in the soil increased, which explained the rise in their relative abundance. Certain microorganisms within _Bacteroidetes_ possess the capability to utilize carbon sources, and straw return specifically promoted the enrichment of these microbial groups [63]. Notably, _Bacteroidetes_ can secrete _Carbohydrate_-active enzymes in soil that facilitate the decomposition of polysaccharides into plant-available forms [64]. In this study, _Ascomycota_ and _Basidiumycota_ were the dominant soil fungal phyla across different soil layers and treatments, with _Ascomycota_ showing the highest relative abundance. _Ascomycota_ are well adapted to soil environments and represent a dominant phylum in soils, consistent with previous studies [1]. Exogenous input of organic matter has been shown to effectively increase the relative abundance of _Ascomycota_ in soil [65]. The results of this study demonstrate that straw return effectively increased soil labile organic carbon content, thereby promoting the growth and multiplication of microorganisms of _Ascomycota_. Studies have shown that _Ascomycetes_ respond faster to changes in soil labile organic carbon, while _Basidiumycota_ can decompose to produce specific enzymes to degrade decomposition-resistant carbon compounds in soil [66]. In addition, NTS increased the relative abundance of _Ascomycetes_ and _Zygomycotain_ in the subsurface soil layer (0-20 cm), which may be attributed to improved soil physicochemical conditions under straw mulching. [PERSON] et al. similarly demonstrated that straw return could promote soil moisture storage, increase soil surface temperature, and improve the topsoil environment, consequently increasing the relative abundance of _Ascomycetes_ and _Zygomycota_[67].