| Heavy metal pollution of soil is becoming more and more serious in China,Hunan province has become the most prominent and typical area of heavy metal pollution in agricultural land and products.Hunan is the largest rice producing region in China,but it is also one of the provinces with the highest frequency of Cd pollution events,especially the Cd pollution of rice.It is urgent to explore Cd pollution control methods in Hunan paddy fields,however,to control heavy metal pollution in farmland,it is necessary to investigate the pollution situation first.At present,most studies on the source analysis of heavy metals are focused on the farmland soil in mining areas,and most of them are carried out based on soil monitoring,there is lack of studies on the source analysis of heavy metals in rice fields with different pollution conditions.Therefore,choose typical Cd pollution farmland to explore the mainly Cd sources of paddy fields in different functional areas,and then research for the control measures of farmland soil heavy metal pollution,it is of great significance to the remediation or prevention of heavy metal polluted farmland.This study systematically monitored the input of heavy metals of paddy field in typical polluted areas for five years,including atmospheric deposition,irrigation water and fertilizer,and we counted the contribution rates of heavy metal from different input sources.Finally,it was determined that atmospheric deposition was the most important source of heavy metal pollution in farmland soil of Hunan Province.The effects of Cd from different sources on the accumulation and absorption of Cd in soil and rice in farmland system were analyzed also,determined the correlation between Cd in soils and in atmospheric deposition.In the process of analyze about heavy metal output and dynamic equilibrium in farmland of typical polluted areas,it was found that crops straw accumulated a lot of heavy metals,which was the main way of Cd output in farmland system,therefore,rice straw removal was proposed as a remediation measure to reducing Cd pollution in farmland soil.Based on this idea,crop rotation-straw removal experiments were further carried out in moderately Cd contaminated areas,and it was found that with crops straw removal,oil sunflower-rape and peanut-rape rotation patterns could realize phytoremediation of Cd pollution effective in farmland soil.Finally,through the controlling measures experiments,it was confirmed that atmospheric deposition cut off and application of lime could effectively reduce the accumulation of heavy metals in rice,provide scientific basis for the treatment of cadmium-contaminated soil and the safe production of agricultural products.The main results of this study are as follows:（1）The mainly Cd input source of farmland soil in typical pollution areas was atmospheric deposition.Mining area（Zhuzhou）has the highest Cd input fluxes,followed by livestock breeding（Liuyang）,suburb（Ansha）and control（Baisha）,the Cd input fluxes was 29.89,12.13,8.73 and 7.68 g·ha-1,respectively,The Cd input fluxes of atmospheric deposition accounts for 66.66%-96.43%of the total input fluxes.The average annual Cd input fluxes from fertilizer in study areas ranged from 0.01 to2.56 g·ha-1,and the Cd input fluxes from fertilizer application accounted for 0.13%to14.10%of the total input fluxes,input fluxes from organic manure were far more than from commercial fertilizer,and mainly affect Cd accumulation about rice root.During the research period,the average annual Cd input fluxes in study areas brought by irrigation water was between 0.50 and 3.50 g·ha-1,and the contribution rate to the total Cd input from farmland was between 3.28%and 19.24%,which was significantly lower than atmospheric deposition.In addition,Cd deposition fluxes from the atmosphere has been decreasing year by year since 2017,indicating that Hunan’s atmospheric environmental quality has begun to improve,and the corresponding atmosphere environmental protection measures work effectively.（2）The effect of newly deposit Cd on rice leaves was the greatest,especially in mining area（Zhuzhou）where was the area with highest Cd deposition fluxes.The contribution of atmospheric deposition to Cd accumulation in rice leaves of Zhuzhou was 41.23%,while was 26.19%in Ansha.In addition,the contribution of atmospheric deposition Cd to rice grains at Zhuzhou study area was up to 28.30%,indicating that newly deposited Cd could be easily transferred to rice grains.The Cd in atmospheric deposition increased the mobility and bioavailability of Cd in soil,which resulted in the higher accumulation of Cd in the overground part of rice.Organic manure mainly affected Cd accumulation in rice roots.BCR distribution extraction results showed that the speciation distribution of Cd in deposition particles is related to the Cd in soil,and deposition particles may be the most direct factor affecting soil heavy metal pollution.（3）By analyzing the output fluxes of Cd in the farmland system,it was found that in the case of rice straw returning,the average annual Cd output fluxes in the farmland system of the study areas were Mining area（Zhuzhou）>livestock breeding（Liuyang）>suburb（Ansha）>control（Baisha）,with the fluxes of 7.64,4.32,1.67 and0.70 g·ha-1,respectively,however,in the case of rice straw removal,the average total output fluxes of Cd was 51.01,28.70,10.98 and 4.05 g·ha-1,respectively.The output Cd fluxes of straw removal accounted for more than 80%in all the study areas.The Cd content in soils of Zhuzhou,Liuyang,Ansha and Baisha increased by 0.010,0.006,0.003 and 0.003 mg·kg-1each year under straw returning.However,the soil Cd of Zhuzhou,Liuyang and Ansha will decrease at an annual rate of 0.008,0.004 and0.0004 mg·kg-1under straw removal from field,respectively.This result gives an important guidance for the controlling of cadmium pollution in farmland.（4）Based on the above conclusions,we further discussed the phytoremediation ability of straw removal after crop rotation on heavy metal in farmland soil.The Cd output fluxes of different crop rotation pattens after straw removal was about oil sunflower-rape(77.24 g·ha-1)>peanut-rape(62.09 g·ha-1)>rice-rape(46.46 g·ha-1)>rice-rape(41.03 g·ha-1)>single rice(23.85 g·ha-1),respectively.Considering the equilibrium value of Cd input fluxes and output fluxes under the five crop rotation patterns,it was found that with crops straw removal,the value of Cd equilibrium fluxes were:oil sunflower-rape(-50.72 g·ha-1)>peanut-rape(-35.77 g·ha-1)>rice-rice(-21.01 g·ha-1)>rice-rape(-18.24 g·ha-1)>single rice(4.79 g·ha-1),Using the new phytoremediation calculation method,the final phytoremediation efficiency of cadmium in soil by five crop rotation patterns were:oil sunflower-rape（1.73%）>peanut-rape（1.22%）>rice-rice（0.72%）>rice-rape（0.62%）>single rice（0.16%）,respectively.（5）Further study on the controlling measures of Cd pollution sources,it is found that the application of lime can effectively increase the soil p H value and reduce the content of available Cd in soil.Meanwhile,the accumulation of Cd in stem,leaf and grain of rice could be reduced by lime treatment and atmospheric deposition cut-off treatment.The Cd content in rice grains decreased by 22.70%and 28.90%after lime treatment in moderately Cd contaminated area（Zhuzhou）and mildly Cd contaminated area（Ansha）,respectively.However,cadmium content in rice grains decreased by28.3%and 11.28%under atmospheric deposition cut-off treatment,respectively. |
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