Abstract: The “Grain for Green” program, a key measure for soil and water conservation and ecological restoration on the Loess Plateau, profoundly affects soil ecological processes. However, existing research mostly focuses on topsoil. How returning farmland to forest regulates bacterial communities in deep soil and the response mechanisms remain unclear. Taking Robinia pseudoacacia plantation (25 a) after returning farmland to forest and cropland with similar fallow periods in a typical arid area of the Loess Plateau as research subjects, with the topsoil layer (0-20 cm) as the reference, we analyzed bacterial diversity, community composition, and their relationship with soil physicochemical factors in deep soil layer (160-200 cm). The results showed that, in the topsoil, the total carbon content of R. pseudoacacia plantation was significantly 7.4% higher than that of cropland, while the available phosphorus content was 80.5% lower. In the deep soil, the total carbon content of R. pseudoacacia plantation was significantly 17.6% higher than that of cropland, and total phosphorus content and soil water content were significantly reduced by 15.6% and 63.5%, respectively. For the inter-layer comparison, total nitrogen content and water content in the deep soil of R. pseudoacacia plantation were significantly lower than those in the topsoil, while soil pH value increased significantly. For cropland, total carbon and total nitrogen contents in the deep soil were significantly lower than those in the topsoil, and pH value also increased significantly. Bacterial α diversity indexes (Chao1 index and phylogenetic diversity index) were significantly higher in topsoil than in deep soil. Moreover, the indices of α diversity in the both soil layers of the R. pseudoacacia plantation were significantly lower than those of cropland. The interaction between land use type and soil layer significantly affected bacterial community composition, together explaining 48.3% of the community variation. The abundance of oligotrophic bacterial groups such as Chloroflexi and Actinomycetes in the deep soil of R. pseudoacacia plantations increased by 19.7% and 65.0% respectively compared to cropland, while the abundance of copiotrophic bacterial groups such as Proteobacteria decreased by 26.9%. The response of bacterial community to environmental factors exhibited significant vertical differentiation. The topsoil community was regulated by nitrogen, phosphorus, and water availability, while the deep soil community was mainly driven by soil carbon storage and water content. Compared with croplands, R. pseudoacacia plantations had higher bacterial network complexity and stability, with significant differences between topsoil and deep soil. In summary, returning farmland to forest altered soil physical and chemical properties, and reshaped the composition and vertical distribution of soil bacterial communities, promoting a shift towards oligotrophic bacteria in deep soils, thereby affecting nutrient cycling, carbon sequestration, and water balance in deep soils.

Key words: Grain for Green, the Loess Plateau, bacterial community structure, deep soil, dryland