| Because farmyard manure containing heavy metals was often used in tobacco fields,the production of flue-cured tobacco faces the risk of heavy metals exceeding the standard and heavy metal pollution in the tobacco soil.This study tried to explore the experiment by preparing soil culture experiments with different concentration gradients of Cd(0,2.5,5,7.5 and 10 mg kg-1 pig manure farm manure),setting up a simulated field flue-cured tobacco continuous cropping and tobacco-rice rotation,and explored effects of cadmium-containing（Cd）farmyard manure on flue-cured tobacco production,flue-cured tobacco Cd absorption and transportation,and Cd residue in soil under two planting systems.The morphological changes,migration and accumulation dynamics of Cd in red soils and the ecological risk of soil Cd pollution when applied with cadmium-containing farmyard manure under two planting systems were evaluated.The main findings were as follows:（1）The agronomic indicators and the biomass of each organ in the third season of flue-cured tobacco under the tobacco-rice-tobacco rotation were significantly higher than those under the flue-cured tobacco continuous cropping.For the second season of continuous cropping of flue-cured tobacco,there was no significant difference in dry weight of flue-cured tobacco between different concentration gradient Cd treatments.In the second season of tobacco-rice rotation,compared with the control（0 Cd）,the root and whole plant dry weight of the rice treated with 2.5,5,7.5,and 10 mg kg-1 Cd were significantly reduced.Moreover,the maximum net photosynthetic rate of flue-cured tobacco in the second season of continuous cropping of all Cd concentration treatments was higher than that of the second season rice of tobacco-rice rotation.The results show that the cadmium tolerance of flue-cured tobacco is significantly higher than that of rice.（2）For the flue-cured tobacco grown in the third season of flue-cured tobacco continuous cropping and tobacco-rice rotation,the Cd content in the tobacco leaves under the treatments of 7.5 and 10 mg/kg Cd was significantly higher than that of the control（0Cd）.Among all Cd treatments,the continuous cropping flue-cured tobacco was the international flue-cured tobacco safety limit 2-3 times the value,1-1.5 times under tobacco-rice rotation.Compared with continuous cropping of flue-cured tobacco,tobacco-rice rotation significantly reduced the Cd content of the flue-cured tobacco in various organs and the whole plant.For the second season of continuous cropping flue-cured tobacco,the Cd accumulation in the lower leaves>Cd accumulation in the middle leaves>Cd accumulation in the middle leaves>Cd accumulation in the upper leaves treated with exogenous addition of Cd,and the accumulation of tobacco leaves in vigorous growing period of flue-cured tobacco was above 74 ug.In the second season of tobacco-rice rotation,root Cd accumulation>stem and leaf Cd accumulation>grain Cd accumulation under different concentration gradient Cd treatments.Most of the Cd absorbed by rice accumulates in the roots,and the grain Cd accumulation is above 25μg,but Cd content in rice grains treated with different Cd concentration gradient is generally lower than the national limit value for heavy metals in rice(0.2 mg kg-1).（3）TF root-stem and TF stem-leafin the flue-cured tobacco in the third season of tobacco-rice rotation with different concentration gradients Cd treatment were lower than those in the third season of flue-cured tobacco continuous cropping.For the second season of continuous cropping flue-cured tobacco,there was no significant difference in TF root-stembetween different Cd concentration treatments.The Cd transport coefficient between each part showed a trend of TF root-stem>TF middle leaf-upper leaf>TF lower leaf-middle leaf,Most of the absorbed Cd is accumulated in the lower leaves of flue-cured tobacco.Tobacco-rice rotation for the second season rice,TF stems,leaves-grains treated with exogenous addition of Cd were higher than TF roots-stems and leaves,and cadmium-containing farmyard manure treatment increased the Cd transport from stems and leaves to grains.The Cd transport coefficient between the various organs of flue-cured tobacco is generally higher than the Cd transport coefficient between the corresponding organs of rice.（4）The results of soil stratification analysis showed that under continuous cropping flue-cured tobacco,the Cd brought by farmyard manure gradually migrated to the lower layer of the soil.After three seasons of planting,the low Cd concentration(2.5,5 mg kg-1Cd)treatments accumulated in 0-10 cm soil layer,the high Cd concentration(7.5,10 mg kg-1 Cd)treatments accumulated in the 10-30 cm soil layer,while low Cd concentration treatment under the tobacco-rice rotation accumulated in the 0-20 cm soil layer.The high concentration of Cd mostly accumulated in the 20-30cm soil layer.Under the tobacco-rice rotation,the Cd brought by the farm manure migrates to the bottom layer faster than the flue-cured tobacco continuous cropping,which may be related to the rice hydroponics.The residual amount of Cd in rhizosphere soil after continuous cropping flue-cured tobacco was greater than that of tobacco-rice rotation.（5）Whether it is continuous cropping flue-cured tobacco or tobacco-rice rotation,after three cropping,the Cd content of rhizosphere soil in pots still exceeded the Cd threshold of heavy metals in farmland soils in my country.However,the potentially ecological risk of soil Cd decreased after tobacco-rice rotation,which was lower than the risk of continuous cropping flue-cured tobacco.For the three seasons of flue-cured tobacco in continuous cropping,the soil Cd risk assessment code under different concentration gradient Cd treatments was reduced by 56.13%,51.28%,36.90%,18.97%,and 36.92%.The low Cd concentration treatment eventually turned into a medium risk,and the high Cd concentration treatment finally changed into high risk.After tobacco-rice rotation（the entire three cropping）,the soil Cd risk assessment code of different concentration gradient Cd treatments changed from extreme risk or high risk to high risk.In summary,in tobacco fields where Cd farmyard manure was applied,the implementation of tobacco-rice rotation planting system helped increase the productivity of flue-cured tobacco,improved the safety and quality of tobacco leaves caused by cadmium pollution,and reduced the ecological risk of soil Cd pollution.The results of this study provided theoretical and practical references for tobacco farmers to rationally apply farmyard manure and flue-cured tobacco planting systems. |
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