This study aimed to explore the characteristics of changes in the bacterial and fungal communities and functions in the rhizosphere soil and their influences on sugarcane growth during the tillering stage under different potassium application levels, so as to provide a theoretical basis and production guidance for potassium application during the tillering stage of sugarcane.

Using the new variety YZ1696 as the experimental material, a three-year continuous field trial was conducted during the sugarcane tillering stage, applying five different potassium fertilizer levels (0, 75, 150, 225, and 300 kg·hm^-2) to investigate the effects on the growth rate of sugarcane, the structure and function of the rhizosphere soil bacterial and fungal community, and the sucrose yield of sugarcane.

(1) Within two months after potassium application during the tillering stage, the absolute growth rate of sugarcane initially increased and then decreased as the potassium application level increased. After three consecutive years of targeted application of different potassium fertilizer treatments, the growth performance of new plantings, one-year-old ratoons, and two-year-old ratoons was consistent, showing that both the sugarcane yield and sucrose content first increased and then decreased with increasing potassium fertilization gradient, and the growth rate, yield, and sucrose content of sugarcane were the optimal under a potassium application of 150 kg·hm^-2. (2) Different potassium application treatments had significantly influenced the abundance of bacteria and fungi in the rhizosphere soil and the composition of the microbial community and diversity. The differences in bacterial abundance mainly concentrated in Proteobacteria, Acidobacteriota, Actinobacteriota, Nitrospirota, and Firmicutes, while the differences in fungal abundance mainly concentrated in Basidiomycota, Ascomycota, and Chytridiomycota. The structure and diversity of the rhizosphere bacterial and fungal communities exhibited a trend of decreasing first and then increasing with the increase in potassium application level, and a fertilization inflection point was at a potassium application of 150 kg·hm^-2. (3) Correlation analysis between the abundance of the main microbial communities and the sucrose yield of sugarcane under different potassium application treatments indicated that the abundances of bacterial Actinobacteriota and fungal Proteobacteria, Basidiomycota, Ascomycota, and Chytridiomycota were significantly correlated with the sucrose yield of sugarcane. These microorganisms played positive roles in nitrogen fixation, organic matter decomposition, nutrient cycling, and inhibition of pathogenic bacteria in the soil.

Appropriate potassium application during the tillering stage could improve the structure and function of the rhizosphere soil microbial community, thereby promoting sugarcane growth. However, different potassium application levels had varying effects on sugarcane yield, sucrose content, the functional microbial communities of bacteria and fungi, among which the potassium application level of 150 kg·hm^-2 had the most significant effect on optimizing the structure of the soil microbial community and enhancing the sucrose yield of sugarcane.

The research focused on the production potential of sugarcane main varieties under the different planting modes in mountainous and hilly regions, in order to explore the relationship between the varieties and mechanical adaptation, screen the adaptable varieties for mechanical production, and clarify the main characteristics of the varieties, thus to provide the theory for promoting the completely mechanical production of sugarcane in mountainous and hilly regions.

Two different ecological types of sugarcane production regions in mountainous regions of Yunnan were selected to study the production potential of four main planting varieties, including LC05-136, YZ05-49, YZ05-51, and YZ08-1609, under two different planting modes (the completely mechanical or manual production).

The yield and sucrose content of sugarcane varied between the two factors of varieties and planting modes, and the yield was significantly affected. Under the mechanical production, the yield of YZ08-1609 was significantly higher than that with the treatment of artificial production (P<0.01), while YZ05-51 showed the opposite trend (P<0.01), under the mechanical production, the sucrose content of YZ08-1609 is slightly higher than that with the artificial production (P>0.05), while these of LC05-136 and YZ05-49 were slightly decreased (P>0.05). The planting density and the emergence rate of sugarcane varied with both varieties and production modes. The planting density under the machinery production was generally higher than that under the artificial production, while the emergence rate was generally lower than that with the treatment of the artificial production. The emergence rate of YZ05-51 under the machinery production significantly decreased (P<0.05), while the emergence rate of YZ08-1609 remained relatively stable (P>0.05). The main agronomic traits of sugarcanes varied with both varieties and the production modes. The millable stalks of YZ08-1609 under the machinery production were significantly higher than that under the artificial production (P<0.05); the industrial characteristics were mainly influenced by the variety. YZ08-1609 owned higher growth potential under the machinery production, with significantly higher yield and millable stalks compared with these under the artificial production, and slightly higher sucrose content; YZ05-51 owned higher growth potential under the artificial production with the performance of higher yield under the artificial production.

Different sugarcane varieties owned the different adaptabilities to the machinery production, like some sugarcane varieties were suitable for the machinery production and some were suitable for the manual production. During pushing forward the progress of the application of mechanical production in sugarcane planting, the adaptability of varieties should be fully considered. Overall, sugarcane varieties suitable for the machinery production should own higher sugarcane yield and more millable stalks, and the sucrose content should be less affected or even slightly higher than that under the manual production.