Effects of Rice Straw Ditch-Buried Returns on Soil Organic Carbon Content in Tobacco-Planting Soil and Tobacco Leaf Quality

To address the low carbon sequestration efficiency of the conventional “rice straw return by field soaking” practice in the tobacco–rice rotation region of Fujian Province, a field experiment was conducted with five treatments: conventional rice straw return by field soaking (CK), straw burial in 15 cm deep ditches under ridges (T1), straw burial in 30 cm deep ditches under ridges (T2), straw burial in 30 cm deep ditches between ridges (T3), and combined straw burial in 30 cm deep ditches under and between ridges (T4). The experiment evaluated the effects of different rice straw ditch-burial methods on the distribution of soil organic carbon fractions and soil carbon-acquiring enzyme activities in tobacco-planting soil, and explored their effects on tobacco leaf quality formation. The results showed that ditch-buried rice straw return promoted tobacco plant growth, with T1 exhibiting a more pronounced promoting effect on plant height and leaf expansion. Different ditch-burial methods significantly altered the distribution characteristics of soil organic carbon across different soil layers. The T1 treatment increased the total organic carbon content in the 0~15 cm soil layer by 6.54% compared with CK, favoring the accumulation of particulate organic carbon in the surface soil layer. The T2 treatment significantly increased the total organic carbon, particulate organic carbon, and mineral-associated organic carbon contents in the 15~30 cm soil layer by 18.44%, 39.60%, and 16.50% compared with CK, respectively, and was more conducive to the formation of a stable carbon pool in the deep soil layer. Different ditch-burial methods also significantly affected soil carbon transformation-related enzyme activities in different soil layers. Specifically, the T4 treatment increased the activities of β-glucosidase and peroxidase in the surface soil layer, and the T1 treatment increased the activity of β-glucosidase in the deep soil layer, while the T2, T3, and T4 treatments decrease the activity of polyphenol oxidase in different soil layers. Ditch-buried straw return generally improved the chemical component coordination and sensory quality of tobacco leaves. The T1 and T2 treatments significantly reduced the nicotine content of upper leaves; the T2 treatment decreased the total nitrogen content by 29.6% compared with CK and increased the total sugar content of upper leaves by 15.7%. All ditch-burial treatments significantly reduced the nicotine content of middle leaves, with T2 treatment showing the largest reduction of 27.6%, and the sugar-to-nicotine ratio in middle leaves under T2 treatment was 32.9% higher than that under CK. In addition, the T3 and T4 treatments increased potassium content of middle leaves, and the T1 and T2 treatments had relatively higher comprehensive indices of chemical component coordination. Sensory evaluation indicated that ditch-buried straw return helped improve the aroma quality and overall coordination of cured tobacco leaves without significantly increasing smoke irritation. In summary, rice straw ditch-buried return effectively enhanced the soil carbon sequestration capacity of tobacco-planting soil and the intrinsic quality of tobacco leave. Among the treatments, straw burial in 15 cm deep ditches under ridges (the T1 treatment) showed greater advantages in improving tobacco leaf quality and comprehensive effects, while straw burial in 30 cm deep ditches under ridges (the T2 treatment) was more conducive to the formation of a stable carbon pool in deep soil layer.