Soil bacterial communities are central to nutrient cycling and fertility, yet the long-term ecological consequences of converting forest to orchard systems remain poorly understood. This study investigated the impacts of forest-to-orchard land-use conversion and prolonged orchard cultivation (9 and 16 years) on soil nutrient dynamics and bacterial community structure in a subtropical red soil hilly region of southern China. Soil physicochemical properties, bacterial community composition, co-occurrence networks, and predicted metabolic pathways were analyzed to assess microbial responses. Land-use conversion significantly increased soil nutrient availability, especially available phosphorus, which reached 298.86 mg·kg -1 in the 16-year orchard. Orchard establishment also shifted bacterial community composition, with Proteobacteria becoming more abundant and Acidobacteria declining. Co-occurrence network analysis revealed initially more complex microbial interactions in orchard soils, including the emergence of Verrucomicrobiota taxa absent from forest soils, but network complexity declined after 16 years of cultivation. Soil organic matter and available phosphorus were key drivers of changes in community structure. Predicted functional profiles indicated a clear metabolic shift from nutrient-conserving pathways (e.g., organic nitrogen degradation prevalent in forest soils) to enhanced biosynthesis and fermentation pathways in orchard soils. This shift reflects a transition in microbial strategy from resource-conserving to fast-cycling under prolonged cultivation. Overall, these findings highlight the strong influence of land-use change and soil nutrient status on microbial community assembly and function, and underscore the need for nutrient-sensitive management to sustain soil health and ecosystem services in orchard systems.

The vertical distribution of soil microorganisms in soil indicates the restoration degree of degraded soil ecosystems. We took the untreated bare land and vegetation restoration sample plot in the red soil erosion area of southern China as the object of study; comparatively analysed the soil bacterial community changes in the 0 to 10, 10 to 20, 20 to 30 and 30 to 40 cm soil layers; and explored the environmental factors driving the change in the soil bacterial community. The poor nutrient conditions created by soil erosion increased the competitiveness of autotrophs and made Chloroflexi the dominant phylum of bacteria. Soil erosion led to the gradual similarity of soil bacterial communities in the 0 to 10, 10 to 20 and 20 to 30 cm soil layers. However, only the relative abundance of Actinobacteria changed in different soil layers in the erosion area, mainly due to the inconsistent distribution of soil organic carbon caused by erosion affecting the change in the Actinobacteria relative abundance in the soil layer. After vegetation restoration, the soil properties of the eroded land were obviously improved, and the dominant bacterial phylum changed from autotrophic bacteria ( Chloroflexi) to heterotrophic bacteria ( Actinobacteria). The change in community structure existed only in the 0 to 30 cm soil layer in the restoration area, while the community structure changed to mainly Proteobacteria in the 30 to 40 cm soil layer. The change in the respective proportions of Chloroflexi, Proteobacteria and Actinobacteria was the main reason for the difference in soil bacterial community structure among soil layers. The change in soil aggregates caused by vegetation restoration was the main environmental factor driving the variation in soil bacterial community structure, and the formation of aggregates was closely related to soil organic carbon. The vertical distribution of Actinobacteria in different soil layers can indicate the degree of soil ecosystem restoration in the red soil erosion area of southern China, and the relationship between Actinobacteria and soil organic carbon was significant.