| Cadmium(Cd)pollution in rice fields has become one of the prominent problems of heavy metal pollution in acid soil in southern China.In 2014 China State of the Environment Bulletin,the over-standard rate of heavy metal in cultivated land accounted for 19.4%of the country,and heavy metal accounted for 16.1%.The over-standard rate of soil Cd elements was 7.0%,and the soil Cd was developing at0.004 mg/kg·y.Systematic research on the key input-output sources affecting the accumulation of Cd in paddy fields and the law of balance and growth under different agronomic control measures will facilitate the targeted prevention and control of Cd pollution in paddy fields and gradually solve the problem of heavy metal pollution in regional paddy fields.In this study,three representative soils of Cd content in Changsha County were selected,among which Beishan Town was a moderately polluted area,Kaihui Town was a mild pollution,and Ganshan Township was in critical pollution.Cd input-output annual monitoring was carried out simultaneously.This study also analyzed the annual fluctuation rule and influencing the mechanism of Cd in paddy soil in different polluted areas.At the same time,the moderately polluted Beishan Town was selected to carry out the micro-area field experiment to study the effects of different agronomic control measures on the migration,transformation and balance of rice-soil system Cd,and to explore the control measures for rice field pollution risk under different Cd environment.The main findings are as follows:1.Investigate the average annual input of Cd in paddy fields in the three monitoring areas of Changsha County is 10.86 g/hm~2·y.The total input of Cd in paddy fields in industrial and mining area,the aquaculture area and the general agricultural area are 9.39,7.86 and 15.21 g/hm~2·y respectively.Among the different input pathways,atmospheric dry and wet deposition is the primary source of Cd in paddy fields in Changsha County.The three monitoring areas account for an average of 87.1%,followed by irrigation water Cd input,with an average ratio of 10.3%.The lowest input source is fertilizer,fertilizer application accounted for only 2.6%of the Cd input source.Studies have shown that Cd pollution caused by atmospheric deposition with a high contribution rate can effectively reduce the Cd pollution of rice-soil system.Secondly,average Cd output of rice fields in different monitoring areas is 36.64 g/hm~2·y,the Cd output of paddy fields in the industrial and mining area,the aquaculture area and the general agricultural area was 66.78,12.04 and 31.11g/hm~2·y,respectively.The principal pathways include paddy field drainage with an average of 2.8%;rice with an average ratio of 17.1%;and straw accumulation with an average of 80.1%.The study found that effective control of straw removal can effectively decrease the accumulation of heavy metal Cd in soil.2.Setting up five agronomic control measures,including straw returning,straw removal expansion,the clean water irrigation,truncating atmospheric deposition and lime compatibilization,and study of different measures for Cd migration and transformation of rice-soil system and its dynamics influence.The result shows that,the Cd content of rice in the straw removal expansion treatment was reduced by 0.418mg/kg compared with the straw returning treatment,and the soil available Cd content decreased by 0.032 mg/kg per year,indicating that the straw was separated that can reduce the accumulation of Cd pollution in the rice-soil system.On the basis of the straw removal expansion,the clean water irrigation,truncating atmospheric deposition and lime compatibilization increase can further effectively decrease the Cd content of rice(p<0.05).The average annual decrease of grain Cd content is 0.138,0.280 and 1.189 mg/kg,respectively.After lime compatibilization,the soil pH value can be effectively increased,and effective Cd content in paddy soil can be reduced.Lime treatment significantly increased the pH value of the soil(p<0.05),which was increased by 1.23~2.20 units compared with the straw removal expansion treatment,which effectively reduced the soil available Cd content,and reduced by 0.101mg/kg on average.3.Whether the straw returning to the field can substantially affect the soil Cd balance in the plough layer,and the soil Cd showed a growth trend after the straw returning to the field.Annual flux of Cd input was significantly higher than the annual output flux.The net input equilibrium value of Cd in each region was14.20~49.01 g/hm~2·y;the annual flux of Cd input in straw removal is significantly lower than the annual output flux,and the net input equilibrium value of Cd in each region is-57.39~-4.19 g/hm~2·y.Therefore,effective straw-removal mode can be adopted in areas with high Cd content in straw,which can make the net input of Cd negative,and reduce the input of Cd pollution.When the current soil Cd pollution status is reduced to 0.300 mg/kg by rice straw,the rice paddy in the Beishan area of the industrial and mining area is continuously leaving the field for 31 years can reduce the Cd content of the plough layer to below 0.30 mg/kg.Generally,the Ganshan area of the agricultural area can be completed in 4 years,and the Kaihui area of aquaculture area can maintain the status of not exceeding the standard.Therefore,further analysis of the key influence mechanism on accumulation of heavy metal in soil and plant,and the balance of soil heavy metal balance with different agronomic regulation measures.Atmospheric deposition and straw returning are the two most important factors for Cd input and output into the soil.However,due to the use of the construction of truncating atmospheric deposition settlement,more production costs are required,and field operations are not convenient.Therefore,from the perspective of both production and economic benefits,the use of straw removal+lime compatibilization treatment which can effectively reduce Cd pollution in the Cd-contaminated paddy fields in the entire Changsha County is more popular. |
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