Effects of tillage methods on soil physical properties and spatial distribution of flue-cured tobacco (Nicotiana tabacum) roots in mountainous tobacco fields

To explore the effects of different tillage methods on the yield and output value of flue-cured tobacco (Nicotiana tabacum) in mountainous tobacco fields and to reveal how deep tillage and subsoiling in mountainous tobacco fields increases yield and efficiency, a field experiment was conducted, and soil physical characteristics, soil water content, spatial distribution of the tobacco root system, and the growth of tobacco plants were investigated. A flue-cured tobacco variety, 'K326', was planted and subjected to tillage methods of 20 cm of rotary tillage (control, RT20), 30 cm of deep tillage (DT30), and 30 cm (ST30) and 40 cm (ST40) of subsoiling tillage. The results showed that DT30, ST30, and ST40 significantly increased the yield and output of the flue-cured tobacco when compared with RT20. The yield was increased by 12.2%, 12.3% and 16.0%, meanwhile the output was increased by 10.5%, 13.8%, and 21.8% under DT30, ST30, and ST40 treatments, respectively. Moreover, deep tillage and subsoiling tillage significantly improved the physical structure of soil subsurface layer in the range of 20-40 cm. The soil bulk density was decreased by DT30, ST30, and ST40 treatments; the soil bulk density values under DT30, ST30, and ST40 treatments were 6.1%, 5.3%, and 8.0% lower than that of RT20 treatment, respectively. However, the treatments increased the capillary porosity of the soil; capillary porosity values under those three treatments were 11.3%, 13.1%, and 21.6% higher than that under RT20 treatment, respectively. Additionally, the soil water content of the 20-40-cm soil layer was also increased by 4.9%, 2.3%, and 5.7% under DT30, ST30, and ST40 treatments compared with RT20 treatment, respectively, when measured at the rosette stage. At the budding stage, it was still increased by 4.5%, 3.8%, and 5.6% under DT30, ST30, and ST40 treatments over RT20 treatment, respectively. Deep tillage and subsoiling tillage treatments increased the absolute fresh weight and promoted vertical growth of tobacco roots as well as alleviated the crowding of the upper roots. The DT30, ST30, and ST40 treatments increased the root depth index by 5.32%, 8.26%, and 16.20% compared with RT20 treatment, respectively. The difference in the fresh weight of the tobacco roots was the most significant among different treatments of soil subsurface (depth of 20-40 cm). Among them, the fresh weight of tobacco roots in the 20-30-cm soil layer with deep ploughing and deep loosening measures was 162.2%-469.0% higher than that with the control, and in the 30-40-cm soil layer it was 56.5%-292.9% higher. It also turned out that subsoiling and deep tillage improved soil physical properties such as soil bulk density, porosity, and moisture in mountainous tobacco fields, which enhanced the soil environment for tobacco planting and benefited tobacco root growth. These treatments also allowed for optimization of the horizontal and vertical distribution of the tobacco root system. Furthermore, these tillage treatments significantly increased the dry matter of flue-cured tobacco, which resulted in increased yield and output value of tobacco.