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Abstract Pb， Cd and As are main heavy metal elements in tobacco leaves in China. Through the application of organic fertilizers in the tobacco area of Puer City， the effects on tobacco growth and heavy metal contents were investigated. The results showed that the tobacco plants in the treatments applied with organic fertilizers as the base fertilizer grew at a lower rate and had relatively smaller stem girth and leaf number than the treatment applied with the compound fertilizer special for tobacco in early growth period， and the differences between various treatments were not significant. After applying the organic fertilizers， they had a positive effect in reducing heavy metal contents of fluecured tobacco， and the effect of applying oil cake was better， followed by the treatment of chicken manure plus oil cake. The sole application of chicken manure had a certain effect on the control of Hg， As and Pb， but the Cd content increased. For the areas with high heavy metal levels， the selection of appropriate organic fertilizers has a practical significance to the reduction of heavy metal contents in tobacco leaves.
Key words Fluecured tobacco; Organic fertilizer; Heavy metals; Health
In recent years， with the progress of industrialization and urbanization and the development of agricultural intensification， heavy metal pollution has become increasingly serious[1]. Heavy metals have low degradability and poor migration in soil， and can be easily absorbed by plants， and excessive heavy metals not only affect the growth and development of plants， but also threaten human health through the food chain. Tobacco is a plant that accumulates heavy metals easily[2-3]， and heavy metal elements in tobacco can enter human body through mainstream smoke in the form of aerosols or metal oxides， causing potential harm to human body[4-6]. When fluecured tobacco develops rapidly， controlling heavy metal contents in fluecured tobacco leaves has gradually become an urgent issue for tobacco research. At present， there have been many related studies on the relationship between organic fertilizers and fluecured tobacco yield and quality， but few studies have been conducted on the correlation between organic fertilizers and heavy metal contents. In this study， the effects of common organic fertilizers on the growth of fluecured tobacco and the control of heavy metal contents in tobacco leaves were investigated， with an attempt to find a relatively reliable method for promoting the growth of fluecured tobacco and controlling heavy metal contents in tobacco using organic fertilizers. General Situation of Experiment Field
This experiment selected such three areas in Puer City with relatively high soil heavy metal contents as Xiazhai， Puyi Village， Puyi Town， Ninger County （the soil is red loam with moderate fertility， and the previous crop was wheat）， Huiwo Group， Mangla Village， Yongping Town， Jinggu County （this area has flat terrain， convenient transportation， convenient irrigation and drainage and sufficient sunshine， and the soil is light sandy soil with moderate fertility， and was not planted with any plant in previous season）， and Pingzhang Village， Longtan Town， Simao District （this area has flat terrain， convenient transportation， convenient drainage and irrigation and sufficient sunshine， and the soil is light red soil with moderate soil fertility， and was planted with wheat previously）. The background values of heavy metals in the various experimental areas are shown in Table 1.
Experimental Materials and Methods
Experimental materials
The tested organic fertilizers included fermented farmyard manure （chicken manure， containing 2.03% of nitrogen）， fermented farmyard manure （oil cake， with a nitrogen content of 3.25%） and compound fertilizer special for fluecured tobacco （containing 15% of nitrogen）. The applied nitrogen， phosphorus and potassium ratio was 1≥1≥2， and the nitrogen application rate was 120 kg/hm2. All treatments were carried out according to this ratio， and the insufficient part was supplemented with the compound fertilizer special for fluecured tobacco， and the control （CK） was applied with the compound fertilizer special for fluecured tobacco at the same ratio and quantity.
Experimental methods
The experiment was set with four treatments in randomized block arrangement. The four treatments were treatment 1 （f1， the same below）： applying fermented chicken manure， treatment 2 （f2， the same below）： applying fermented oil cake， treatment 3 （f3， the same below）： applying 50% fermented chicken manure + 50% fermented oil， and control （CK）： applying the compound fertilizer for fluecured tobacco， without organic fertilizer. After harvesting the previous crop， the field was cleaned， deeply ploughed and sunned for 15 d， and the base fertilizer （organic fertilizer + fluecured tobacco special compound fertilizer） was applied， followed by plowing and flattening the land. The base fertilizer accounted for 70% of the total nitrogen application rate. The organic fertilizers were all applied as the base fertilizer， and the application rate was calculated based on 20% of the total nitrogen application rate. The insufficient nitrogen content of the base fertilizer was supplemented with the compound fertilizer special for fluecured tobacco. Each treatment had three replicates， and each plot was planted with 70 plants at a density of 18 000 plants/hm2， with guard rows around. In the growth period of tobacco plants， the growth stages and field agronomic traits were investigated. After harvesting， the soil samples and tobacco samples were taken from each treatment for the determination of heavy metals. Heavy metals in soil and tobacco were extracted by DTPA， and the contents of heavy metals were determined by atomic adsorption spectrometer.
The investigation of various items was all implemented in accordance with the statespecified standards. Data reduction and statistical analysis were performed using Excel software and spss data processing and analysis software.
Results and Analysis
Effects of different organic fertilizers on the growth stages of tobacco
It could be seen from Table 2 that after the application of organic fertilizers， the rosette stage， vigorous growing stage and bud stage of tobacco in different areas were slightly delayed compared with the CK. Especially， The oil cake treatment showed more delayed days than the CK， which might be due to the slow release of organic fertilizer， which further slightly delayed the supply of fertility. In terms of the entire field growth period， the field growth period of organic fertilizer treatments was also several days longer than the CK. From the respective of vegetative growth， the prolongation of the vegetative growth is conducive to accumulating more dry matter， which has a positive effect on the improvement of fluecured tobacco yield and quality[7-9].
Effects of organic fertilizer on the agronomic traits of tobacco plants
Effect of different treatments on plant height of tobacco plants
It could be seen from Table 3 that the application of organic fertilizer had a greater effect on the growth of tobacco plants in early period. At rosette stage， there were significant differences in plant height of tobacco plants between treatment 1 and treatments 2 and 3， treatment 2 and treatment 4， and treatment 3 and treatment 4， while there were no significant differences in plant height between treatment 1 and treatment 4 and between treatment 2 and treatment 3. It could be seen from Table 2 that when comparing the plant height， the differences between all treatments were not significant， indicating that the organic fertilizer treatments did not fully exert the fertilizer efficiency in early period， making the growth rate relatively slow， and later， the organic fertilizers played their role to promote the rapid growth of tobacco plants[10]， and finally made the differences in growth not obvious. In terms of the stem girth of tobacco plants， at rosette stage， there were significant differences in stem girth between treatment 2 and treatment 4 and between treatment 3 and treatment 4， and the CK showed a stem girth smaller than the treatments applying chicken manure and oil cake+chicken manure. After topping， only the difference in stem girth between treatments 3 and treatment 4 was significant， and the stem girth of tobacco plants applied with 50% oil cake and 50% chicken manure was smaller than the CK.
For the effect on the number of remaining leaves of tobacco plants， at rosette stage， it could be seen from Table 2 that there were significant differences in the number of leaves between treatment 1 and treatment 2 and between treatment 2 and treatment 4， and treatment 1 and treatment 4 had more leaves than treatment 2; and after topping， there were no significant differences in the number of leaves between various treatments， which was similar to the differences in plant height.
The application of different organic fertilizers had a greater effect on the growth of fluecured tobacco in early period， and especially for the treatment applying oil cake， the growth in the early period was relatively slow， which might be due to that after the application of organic fertilizers， the efficiency of the organic fertilizers was released relatively slow， and in later period， the fertility of the organic fertilizers effected and provided sufficient nutrients for plants， which grew rapidly in later period， which eliminated the differences[11]. From the respective of the whole growth period， tobacco seedlings grew slowly， which was conducive to the occurrence of a large number of roots， which laid a solid foundation for the late growth of tobacco plants. Meanwhile， the prolongation of the growth period was conducive to the accumulation of more dry matter in the tobacco leaves， which had a positive effect on the improvement of the yield and quality of tobacco leaves[12-13].
Effects of different organic fertilizers on heavy metal contents of fluecured tobacco
Effects of different organic fertilizers on heavy metal contents of fluecured tobacco
It could be seen from Table 4 that heavy metal contents in tobacco soil and tobacco leaves were controlled and reduced to some extent after the application of organic fertilizers， and the reductions were especially more obviously in heavy metals. The comparison of heavy metal contents in the soil before and after harvesting showed that， heavy metal contents in the soil all decreased greatly after harvesting and baking except As of Yongping and Puyi. For instance， the Pb content in the soil of Yongping after harvesting and baking decreased by 60% compared with that before transplanting， and the Cd content in the soil of Yongping after harvesting and baking decreased by about 62% compared with that before transplanting， which indicated that the application of organic fertilizers had an obvious effect in reducing heavy metal contents in the soil. The Hg content was detected at a low level after transplanting， indicating that Hg was mainly introduced into the soil through fertilization and drug application; and the lowering of As in Yongping and Puyi might be caused by tobacco farmers fertilization and drug application habits. Effect of different organic fertilizers on Hg content in fluecured tobacco
It could be seen from Fig. 1 that the comparison of different experimental areas showed that the Hg contents in the soil of Longtan and Puyi were relatively high， and the Hg content in Yongping soil was relatively low; and the Hg contents in the tobacco leaves of different experimental areas were relatively close， all relatively low. The comparison of different treatments showed that the oil cake treatment had the lowest Hg content in the soil and tobacco leaves， followed by the oil cake+chicken manure treatment， while the Hg content in the CK was relatively higher in the soil and tobacco leaves， which can indicate that the application of oil cake had a positive effect on the reduction of Hg content in the soil and tobacco leaves.
Effect of organic fertilizers on As content in fluecured tobacco
Fig. 2 shows that the As content in the Longtan soil was relatively higher among the three experimental areas. After different treatments， it was found that the As content in the soil was relatively lower when oil cake was applied， while in the tobacco leaves， the As content was relatively lower in different treatments in different experimental areas， indicating that the ability of tobacco plants to absorb As from soil might be relatively lower， and As might not become the most important factor causing heavy metal pollution in tobacco leaves[14].
Effect of different organic fertilizers on Pb content in fluecured tobacco
It could be seen from Fig. 3 that the Pb content in the soil was still the highest in Longtan， and the Pb contents in various treatments were relatively closer， indicating that the effects of different treatments on the Pb content in the soil were not significantly different. It was found from the analysis of Pb content in tobacco leaves that the Pb contents in the tobacco leaves of different treatments were very close， and were very low. It could be considered that the tobacco plants absorbed less Pb from the soil[14-15]， which is different from the research result of Lu et al.
Effect of different organic fertilizers on Cd content in tobacco leaves
It could be seen from Fig. 4 that the Cd content in the soil was the highest in Longtan among the three experimental areas， and lower in Yongping and Puyi; and the differences between the treatments were not significant. In tobacco leaves， the Cd content was relatively higher in the chicken manure treatment， followed by the CK， and the lowest in the oil cake treatment， which might indicate that the Cd content in chicken manure was relatively higher， and can be absorbed easily by tobacco plants after being applied to the soil[16]， and the application of oil cake was beneficial to the reduction and control of Cd content in tobacco leaves. Conclusions and Discussion
Many studies have been conducted on the effects of organic fertilizers on the growth and development， yield and quality of fluecured tobacco， and the research results are relatively concentrated on that organic fertilizers have no obvious effect on fluecured tobacco in early growth period， and can better promote the formation of tobacco biomass in middle and late periods， and promote the improvement of tobacco yield and smoking quality[2，7-11，13]. After applying different organic fertilizers to tobacco， the several treatments all showed relatively slower growth of tobacco plants and had smaller stem girth and leaf number in early period， which might be due to that organic fertilizers slowly released its efficiency on the growth of tobacco plants in early growth period. In the middle and late growth periods of tobacco plants， the differences between the treatments were not significant， which is consistent with the research results of Dong et al.[7]and Wang et al.[10]. It was found through the discussion of the differences in the effect on fluecured tobacco between fermented oil cake and fermented chicken manure that oil cake promoted the prolongation of the field growth period of fluecured tobacco， which might have a good effect on the formation of fluecured tobacco yield and quality， which is of great practical significance to the improvement of the combined application of fluecured tobacco fertilizers and the development of fluecured tobacco production. This result was different from the research by Qiu et al.[17]. The actual effect needs to be demonstrated by more studies.
At present， few studies have been conducted on the correlation between organic fertilizer application and heavy metal contents in fluecured tobacco. Fu et al.[18]studied the effect of organic manure on tobacco response to soil heavy metals. It is believed that organic fertilizer inhibits the absorption of Cd and Pd by fluecured tobacco and reduces the content of Cd and Pd in the leaves. This study conducted a systematic analysis and achieved some newer results. The results showed that the organic fertilizer treatments had a more positive effect on the control and reduction of heavy metal contents of fluecured tobacco， and especially， the effect of applying oil cake was more obvious， followed by the treatment of applying chicken manure. There were two main findings in the study： firstly， the contents of heavy metals such as Cd， Hg， As and Pb in tobacco leaves were lower than the CK; and secondly， the sole application of chicken manure had a good effect on the control of Hg， As and Pb， but the Cd content increased. The results provide some scientific guidance for the improvement of the safety of tobacco leaves by improving fertilization methods and fertilizer types and for fertilizer selection and pretreatment. In general， the application of organic fertilizers has a positive effect on promoting the formation of fluecured tobacco quality， improving the economic benefits of tobacco leaves and improving the safety of tobacco leaves， and is of great significance in practical production. Due to the slow efficiency of organic fertilizers， it can be appropriately considered to apply organic fertilizers to fields half a month to one month before transplanting in practical production， so that the fertilizer efficiency is gradually released in the soil. And after the transplanting of tobacco seedlings， the root system can get enough nutrient supply， which can promote the early growth of tobacco seedlings and play an important role of promoting later growth of tobacco plants. Therefore， the yield and quality of tobacco leaves can be ensured， and heavy metal contents in tobacco leaves can be controlled and reduced.
References
[1]MI YH， LU L， ZOU BL， et al. Interaction and correlation of heavy metal compound pollution in soilfluecured tobacco system[J]. Acta Agriculturae Jiangxi， 2012， 24（1）： 154-157.
[2]CAO XL， SUN JG， LU HL， et al. Research progress of the effects of heavy metals on yield and quality of fluecured tobacco[J]. Journal of Hebei Agricultural Sciences， 2009， 13（9）： 3-6， 9.
[3]LANG SM， WANG LX， XU XC， et al. Absorption and distribution of heavy metal in tobacco and its influencing factors[J]. Acta Agriculturae Jiangxi， 2012， 24（11）： 93-99.
[4]XU ZL， WU YP， YANG YH， et al. Accumulation and distribution of different heavy metals in fluecured tobacco[J]. Environmental Science Survey， 2007， 26（1）： 7-10.
[5]ZHANG XH. Absorption and distribution of heavy metals in fluecured tobacco under different fertilization levels[J]. Agriculture Network Information， 2005， 11： 144-146.
[6]ZHANG YL， YIN QS， ZHOU HP， et al. Contents and distribution of Pb， Cd and As in Chinese tobacco leaves[J]. Tobacco Science & Technology， 2006， 11： 49-52， 57.
[7]DONG HT， CHEN X， JIAO YS， et al. Effect of agaric bran organic fertilizer on dry matters of fluecured tobacco[J]. Heilongjiang Agricultural Sciences， 2014， 4： 51-54.
[8]WANG JS， LI ZY， ZHAO C， et al. Effect of bioorganiclnorganic compound fertilizer on soil and nitrogen nutrition of fluecured tobacco[J]. Guizhou Agricultural Sciences， 2012， 40（6）： 69-71.
[9]ZHOU WL， LAI HM， LIN BS， et al. Effects of active organic manure on growth and quality of fluecured tobacco[J]. Southwest China Journal of Agricultural Sciences， 2013， 26（2）： 647-652. [10]WANG JS， LI ZY， ZHAO C， et al. Effect of bioorganiclnorganic compound fertilizer on soil and nitrogen nutrition of fluecured tobacco[J]. Guizhou Agricultural Sciences， 2012， 40（6）： 69-71.
[11]LIANG W， TIAN ZF， WEI JY， et al. Effects on organic manure to soil physical and chemical properties， yield， quality of fluecured tobacco[J]. Tianjin Agricultural Sciences， 2013， 19（8）： 68-71.
[12]ZHANG HJ， CHEN G， WANG SS， et al. Reducing the chemical fertilizer rate with bioorganic fertilizer applied on tobacco production[J]. Chinese Tobacco Science， 2015， 36（1）： 48-53.
[13]ZHANG WJ， LIU H， XIE YJ， et al. Effect of organic fertilizer on development and internal quality of fluecured tobacco[J]. Mod Agric Sci Technol， 2012， 3： 69-71.
[14]LU LM， GU HZ， PENG Y， et al. Analysis of accumulation and distribution characteristics of heavy metals lead， chromium and arsenic in tobacco[J]. Acta Agriculturae BorealiSinica， 2013， 28： 366-370.
[15]CUI WW， WANG XL， DUAN JJ， et al. Effects of organic fertilizers on several physiologic indices of tobacco in combinative pollution soils of cadmium and lead[J]. Journal of Mountain Agriculture and Biology， 2012， 31（1）： 009-013.
[16]LI XT， CHANG SR， KONG NC， et al. Effects of combining application of various organic and chemical fertilizers on the growth and lead and cadmium contents of fluecured tobacco[J]. Chinese Tobacco Science， 2013， 34（5）： 37-41.
[17]QIU XP， DENG Q， HUANG ZP， et al. Effect of rapeseed cake replaced inorganic base fertilizer on yield and quality of tobacco in Shuicheng county[J]. Guangdong Agricultural Sciences， 2012， 12： 68-70.
[18]FU S， WANG KQ， HU ZS. Effects of organic fertilizer on tobacco response to soil heavy metals[J]. Anhui Agricultural Science Bulletin， 2010， 16（5）： 91-93，135.
Key words Fluecured tobacco; Organic fertilizer; Heavy metals; Health
In recent years， with the progress of industrialization and urbanization and the development of agricultural intensification， heavy metal pollution has become increasingly serious[1]. Heavy metals have low degradability and poor migration in soil， and can be easily absorbed by plants， and excessive heavy metals not only affect the growth and development of plants， but also threaten human health through the food chain. Tobacco is a plant that accumulates heavy metals easily[2-3]， and heavy metal elements in tobacco can enter human body through mainstream smoke in the form of aerosols or metal oxides， causing potential harm to human body[4-6]. When fluecured tobacco develops rapidly， controlling heavy metal contents in fluecured tobacco leaves has gradually become an urgent issue for tobacco research. At present， there have been many related studies on the relationship between organic fertilizers and fluecured tobacco yield and quality， but few studies have been conducted on the correlation between organic fertilizers and heavy metal contents. In this study， the effects of common organic fertilizers on the growth of fluecured tobacco and the control of heavy metal contents in tobacco leaves were investigated， with an attempt to find a relatively reliable method for promoting the growth of fluecured tobacco and controlling heavy metal contents in tobacco using organic fertilizers. General Situation of Experiment Field
This experiment selected such three areas in Puer City with relatively high soil heavy metal contents as Xiazhai， Puyi Village， Puyi Town， Ninger County （the soil is red loam with moderate fertility， and the previous crop was wheat）， Huiwo Group， Mangla Village， Yongping Town， Jinggu County （this area has flat terrain， convenient transportation， convenient irrigation and drainage and sufficient sunshine， and the soil is light sandy soil with moderate fertility， and was not planted with any plant in previous season）， and Pingzhang Village， Longtan Town， Simao District （this area has flat terrain， convenient transportation， convenient drainage and irrigation and sufficient sunshine， and the soil is light red soil with moderate soil fertility， and was planted with wheat previously）. The background values of heavy metals in the various experimental areas are shown in Table 1.
Experimental Materials and Methods
Experimental materials
The tested organic fertilizers included fermented farmyard manure （chicken manure， containing 2.03% of nitrogen）， fermented farmyard manure （oil cake， with a nitrogen content of 3.25%） and compound fertilizer special for fluecured tobacco （containing 15% of nitrogen）. The applied nitrogen， phosphorus and potassium ratio was 1≥1≥2， and the nitrogen application rate was 120 kg/hm2. All treatments were carried out according to this ratio， and the insufficient part was supplemented with the compound fertilizer special for fluecured tobacco， and the control （CK） was applied with the compound fertilizer special for fluecured tobacco at the same ratio and quantity.
Experimental methods
The experiment was set with four treatments in randomized block arrangement. The four treatments were treatment 1 （f1， the same below）： applying fermented chicken manure， treatment 2 （f2， the same below）： applying fermented oil cake， treatment 3 （f3， the same below）： applying 50% fermented chicken manure + 50% fermented oil， and control （CK）： applying the compound fertilizer for fluecured tobacco， without organic fertilizer. After harvesting the previous crop， the field was cleaned， deeply ploughed and sunned for 15 d， and the base fertilizer （organic fertilizer + fluecured tobacco special compound fertilizer） was applied， followed by plowing and flattening the land. The base fertilizer accounted for 70% of the total nitrogen application rate. The organic fertilizers were all applied as the base fertilizer， and the application rate was calculated based on 20% of the total nitrogen application rate. The insufficient nitrogen content of the base fertilizer was supplemented with the compound fertilizer special for fluecured tobacco. Each treatment had three replicates， and each plot was planted with 70 plants at a density of 18 000 plants/hm2， with guard rows around. In the growth period of tobacco plants， the growth stages and field agronomic traits were investigated. After harvesting， the soil samples and tobacco samples were taken from each treatment for the determination of heavy metals. Heavy metals in soil and tobacco were extracted by DTPA， and the contents of heavy metals were determined by atomic adsorption spectrometer.
The investigation of various items was all implemented in accordance with the statespecified standards. Data reduction and statistical analysis were performed using Excel software and spss data processing and analysis software.
Results and Analysis
Effects of different organic fertilizers on the growth stages of tobacco
It could be seen from Table 2 that after the application of organic fertilizers， the rosette stage， vigorous growing stage and bud stage of tobacco in different areas were slightly delayed compared with the CK. Especially， The oil cake treatment showed more delayed days than the CK， which might be due to the slow release of organic fertilizer， which further slightly delayed the supply of fertility. In terms of the entire field growth period， the field growth period of organic fertilizer treatments was also several days longer than the CK. From the respective of vegetative growth， the prolongation of the vegetative growth is conducive to accumulating more dry matter， which has a positive effect on the improvement of fluecured tobacco yield and quality[7-9].
Effects of organic fertilizer on the agronomic traits of tobacco plants
Effect of different treatments on plant height of tobacco plants
It could be seen from Table 3 that the application of organic fertilizer had a greater effect on the growth of tobacco plants in early period. At rosette stage， there were significant differences in plant height of tobacco plants between treatment 1 and treatments 2 and 3， treatment 2 and treatment 4， and treatment 3 and treatment 4， while there were no significant differences in plant height between treatment 1 and treatment 4 and between treatment 2 and treatment 3. It could be seen from Table 2 that when comparing the plant height， the differences between all treatments were not significant， indicating that the organic fertilizer treatments did not fully exert the fertilizer efficiency in early period， making the growth rate relatively slow， and later， the organic fertilizers played their role to promote the rapid growth of tobacco plants[10]， and finally made the differences in growth not obvious. In terms of the stem girth of tobacco plants， at rosette stage， there were significant differences in stem girth between treatment 2 and treatment 4 and between treatment 3 and treatment 4， and the CK showed a stem girth smaller than the treatments applying chicken manure and oil cake+chicken manure. After topping， only the difference in stem girth between treatments 3 and treatment 4 was significant， and the stem girth of tobacco plants applied with 50% oil cake and 50% chicken manure was smaller than the CK.
For the effect on the number of remaining leaves of tobacco plants， at rosette stage， it could be seen from Table 2 that there were significant differences in the number of leaves between treatment 1 and treatment 2 and between treatment 2 and treatment 4， and treatment 1 and treatment 4 had more leaves than treatment 2; and after topping， there were no significant differences in the number of leaves between various treatments， which was similar to the differences in plant height.
The application of different organic fertilizers had a greater effect on the growth of fluecured tobacco in early period， and especially for the treatment applying oil cake， the growth in the early period was relatively slow， which might be due to that after the application of organic fertilizers， the efficiency of the organic fertilizers was released relatively slow， and in later period， the fertility of the organic fertilizers effected and provided sufficient nutrients for plants， which grew rapidly in later period， which eliminated the differences[11]. From the respective of the whole growth period， tobacco seedlings grew slowly， which was conducive to the occurrence of a large number of roots， which laid a solid foundation for the late growth of tobacco plants. Meanwhile， the prolongation of the growth period was conducive to the accumulation of more dry matter in the tobacco leaves， which had a positive effect on the improvement of the yield and quality of tobacco leaves[12-13].
Effects of different organic fertilizers on heavy metal contents of fluecured tobacco
Effects of different organic fertilizers on heavy metal contents of fluecured tobacco
It could be seen from Table 4 that heavy metal contents in tobacco soil and tobacco leaves were controlled and reduced to some extent after the application of organic fertilizers， and the reductions were especially more obviously in heavy metals. The comparison of heavy metal contents in the soil before and after harvesting showed that， heavy metal contents in the soil all decreased greatly after harvesting and baking except As of Yongping and Puyi. For instance， the Pb content in the soil of Yongping after harvesting and baking decreased by 60% compared with that before transplanting， and the Cd content in the soil of Yongping after harvesting and baking decreased by about 62% compared with that before transplanting， which indicated that the application of organic fertilizers had an obvious effect in reducing heavy metal contents in the soil. The Hg content was detected at a low level after transplanting， indicating that Hg was mainly introduced into the soil through fertilization and drug application; and the lowering of As in Yongping and Puyi might be caused by tobacco farmers fertilization and drug application habits. Effect of different organic fertilizers on Hg content in fluecured tobacco
It could be seen from Fig. 1 that the comparison of different experimental areas showed that the Hg contents in the soil of Longtan and Puyi were relatively high， and the Hg content in Yongping soil was relatively low; and the Hg contents in the tobacco leaves of different experimental areas were relatively close， all relatively low. The comparison of different treatments showed that the oil cake treatment had the lowest Hg content in the soil and tobacco leaves， followed by the oil cake+chicken manure treatment， while the Hg content in the CK was relatively higher in the soil and tobacco leaves， which can indicate that the application of oil cake had a positive effect on the reduction of Hg content in the soil and tobacco leaves.
Effect of organic fertilizers on As content in fluecured tobacco
Fig. 2 shows that the As content in the Longtan soil was relatively higher among the three experimental areas. After different treatments， it was found that the As content in the soil was relatively lower when oil cake was applied， while in the tobacco leaves， the As content was relatively lower in different treatments in different experimental areas， indicating that the ability of tobacco plants to absorb As from soil might be relatively lower， and As might not become the most important factor causing heavy metal pollution in tobacco leaves[14].
Effect of different organic fertilizers on Pb content in fluecured tobacco
It could be seen from Fig. 3 that the Pb content in the soil was still the highest in Longtan， and the Pb contents in various treatments were relatively closer， indicating that the effects of different treatments on the Pb content in the soil were not significantly different. It was found from the analysis of Pb content in tobacco leaves that the Pb contents in the tobacco leaves of different treatments were very close， and were very low. It could be considered that the tobacco plants absorbed less Pb from the soil[14-15]， which is different from the research result of Lu et al.
Effect of different organic fertilizers on Cd content in tobacco leaves
It could be seen from Fig. 4 that the Cd content in the soil was the highest in Longtan among the three experimental areas， and lower in Yongping and Puyi; and the differences between the treatments were not significant. In tobacco leaves， the Cd content was relatively higher in the chicken manure treatment， followed by the CK， and the lowest in the oil cake treatment， which might indicate that the Cd content in chicken manure was relatively higher， and can be absorbed easily by tobacco plants after being applied to the soil[16]， and the application of oil cake was beneficial to the reduction and control of Cd content in tobacco leaves. Conclusions and Discussion
Many studies have been conducted on the effects of organic fertilizers on the growth and development， yield and quality of fluecured tobacco， and the research results are relatively concentrated on that organic fertilizers have no obvious effect on fluecured tobacco in early growth period， and can better promote the formation of tobacco biomass in middle and late periods， and promote the improvement of tobacco yield and smoking quality[2，7-11，13]. After applying different organic fertilizers to tobacco， the several treatments all showed relatively slower growth of tobacco plants and had smaller stem girth and leaf number in early period， which might be due to that organic fertilizers slowly released its efficiency on the growth of tobacco plants in early growth period. In the middle and late growth periods of tobacco plants， the differences between the treatments were not significant， which is consistent with the research results of Dong et al.[7]and Wang et al.[10]. It was found through the discussion of the differences in the effect on fluecured tobacco between fermented oil cake and fermented chicken manure that oil cake promoted the prolongation of the field growth period of fluecured tobacco， which might have a good effect on the formation of fluecured tobacco yield and quality， which is of great practical significance to the improvement of the combined application of fluecured tobacco fertilizers and the development of fluecured tobacco production. This result was different from the research by Qiu et al.[17]. The actual effect needs to be demonstrated by more studies.
At present， few studies have been conducted on the correlation between organic fertilizer application and heavy metal contents in fluecured tobacco. Fu et al.[18]studied the effect of organic manure on tobacco response to soil heavy metals. It is believed that organic fertilizer inhibits the absorption of Cd and Pd by fluecured tobacco and reduces the content of Cd and Pd in the leaves. This study conducted a systematic analysis and achieved some newer results. The results showed that the organic fertilizer treatments had a more positive effect on the control and reduction of heavy metal contents of fluecured tobacco， and especially， the effect of applying oil cake was more obvious， followed by the treatment of applying chicken manure. There were two main findings in the study： firstly， the contents of heavy metals such as Cd， Hg， As and Pb in tobacco leaves were lower than the CK; and secondly， the sole application of chicken manure had a good effect on the control of Hg， As and Pb， but the Cd content increased. The results provide some scientific guidance for the improvement of the safety of tobacco leaves by improving fertilization methods and fertilizer types and for fertilizer selection and pretreatment. In general， the application of organic fertilizers has a positive effect on promoting the formation of fluecured tobacco quality， improving the economic benefits of tobacco leaves and improving the safety of tobacco leaves， and is of great significance in practical production. Due to the slow efficiency of organic fertilizers， it can be appropriately considered to apply organic fertilizers to fields half a month to one month before transplanting in practical production， so that the fertilizer efficiency is gradually released in the soil. And after the transplanting of tobacco seedlings， the root system can get enough nutrient supply， which can promote the early growth of tobacco seedlings and play an important role of promoting later growth of tobacco plants. Therefore， the yield and quality of tobacco leaves can be ensured， and heavy metal contents in tobacco leaves can be controlled and reduced.
References
[1]MI YH， LU L， ZOU BL， et al. Interaction and correlation of heavy metal compound pollution in soilfluecured tobacco system[J]. Acta Agriculturae Jiangxi， 2012， 24（1）： 154-157.
[2]CAO XL， SUN JG， LU HL， et al. Research progress of the effects of heavy metals on yield and quality of fluecured tobacco[J]. Journal of Hebei Agricultural Sciences， 2009， 13（9）： 3-6， 9.
[3]LANG SM， WANG LX， XU XC， et al. Absorption and distribution of heavy metal in tobacco and its influencing factors[J]. Acta Agriculturae Jiangxi， 2012， 24（11）： 93-99.
[4]XU ZL， WU YP， YANG YH， et al. Accumulation and distribution of different heavy metals in fluecured tobacco[J]. Environmental Science Survey， 2007， 26（1）： 7-10.
[5]ZHANG XH. Absorption and distribution of heavy metals in fluecured tobacco under different fertilization levels[J]. Agriculture Network Information， 2005， 11： 144-146.
[6]ZHANG YL， YIN QS， ZHOU HP， et al. Contents and distribution of Pb， Cd and As in Chinese tobacco leaves[J]. Tobacco Science & Technology， 2006， 11： 49-52， 57.
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