Effect of High-density Planting of Flue-cured Tobacco on the Physicochemical Properties and Bacterial Community Structure of Tobacco Planting Soil

To evaluate the potential impact of high-density planting as a yield-enhancing measure on soil health, the flue-cured tobacco variety Yunyan 99 was used as the material in this study, and two planting modes of conventional density and high density were established in the tobacco-growing area of Hanzhong, Shaanxi Province. Soil samples were collected from two soil layers of topsoil (0~10 cm) and subsoil (10~20 cm), and their physicochemical properties were analyzed. Illumina MiSeq high-throughput sequencing technology was used to study the structure of soil bacterial communities, and redundancy analysis (RDA) was used to explore the correlation between soil physicochemical factors and bacterial communities. The results showed that: ①High-density planting significantly increased soil bulk density and moisture content, with higher values in the subsoil layer compared to the topsoil layer. ②High-density planting led to significant soil acidification, and the contents of organic matter, total nitrogen, available nitrogen, total phosphorus, available phosphorus, total potassium, and available potassium all decreased, indicating that soil nutrient consumption increased. ③High-density planting reduced the richness (number of species), species diversity and evenness of bacterial communities in different soil layers, and altered the vertical distribution pattern of bacterial communities with more in the topsoil and less in the subsoil. ④High-density planting significantly reduced the relative abundance of Actinobacteria participated in organic matter metabolism and Proteobacteria involved in promoting nutrient cycling, and increased the relative abundance of Acidobacteria and Chloroflexi associated with barren soil environment. ⑤High-density planting significantly increased the relative abundance of Norank_ f__Norank_o__Vicinamibacterales that might promote tobacco disease susceptibility, and the relative abundance of RB41 and Norank_ f__Vicinamibacteraceae that might promote soil acidification and reduce soil organic carbon in the topsoil, while reduced the relative abundance of Sphingomonas that could promote plant growth and inhibit plant pathogens. ⑥RDA analysis showed that soil moisture content was significantly positively correlated with bacterial α diversity, and total nitrogen, pH, and available potassium were significantly positively correlated with bacterial community evenness. In summary, high-density planting of flue-cured tobacco not only alters soil physicochemical properties and microenvironment, causes soil acidification and accelerates nutrient depletion, but also drives the succession of soil bacterial communities towards to adapt to barren habitats, and may increase the risk of soil-borne diseases. The research results can provide a theoretical basis for scientifically evaluating the sustainability of this model and formulating corresponding soil management and microecological regulation strategies.