| Since the 21st century,there have been many cases of heavy metal pollution of crops due to heavy metal pollution in farmland soil.Heavy metal pollution in farmland soil is a serious threat to human health and has attracted wide attention from scientific researchers.Inspired by industrial heavy metal pollution in the 20 th century,China has begun to pay attention to the discharge of heavy metals in livestock manure in the breeding industry.Currently,in many countries,due to the large output of livestock manure,there is an imbalance between the output of livestock manure and the capacity of farmland,the return of livestock manure to the field will lead to the problem of excessive heavy metals in farmland soil.This article gives the list of heavy metals Cu,Zn,Cr,Pb,Ni,Cd,Hg,As in the feces of the livestock breeding industry(including cattle,pigs and chickens)in China,and clarifies the development status of China's breeding industry and the characteristics of spatiotemporal changes of heavy metal emissions,and then,this article reveals the influencing factors and its impact mechanism in affecting heavy metal emissions of the breeding industry,and discusses the specific situation of farmland pollution caused by the return of livestock manure to the field.Conclusion as below:(1)The large-scale breeding industry has contributed to the continuous and steady growth of livestock and poultry manure production in my country,and there is no overall spatial and temporal difference in the content of heavy metals in livestock and manure.Different provinces and cities in China have a large span of heavy metal emissions.The total emissions of heavy metals in Sichuan,Shandong and Henan account for more than 25% of the country 's total,and the total emissions of various metals in Shanghai,Qinghai,Tibet and Ningxia account for only 1%-4% of the country 's total.(2)The main influencing factors of heavy metal emissions in China include GDP,total population,urbanization level and industrial structure.Heavy metal emissions in underdeveloped cities are positively correlated with GDP and industrial structure,of which Cu,Zn and As emissions are also negatively correlated with urbanization levels.Heavy metal emissions in developed cities are negatively correlated with GDP and positively correlated with the level of urbanization,besides,Cu,Zn and As emissions are also positively correlated with the total population.Each influencing factor has significant different influence mechanism in cities with different development levels.(3)The scenario which livestock manure replaces inorganic fertilizer needs to consider farmland fertility and the amount of nitrogen applied to the farmland.The research results show that the application of cattle manure will exceed the standard after 29 years,the heavy metal that is most likely to exceed the standard is Cd,and the application of pig manure will exceed the standard after 13 years,the heavy metal that is most likely to exceed the standard is Cu,The application of chicken manure will exceed the standard after 50 years,the heavy metal that is most likely to exceed the standard is Cd.(4)Nationwide,when the rate of returning livestock manure to the field is high,the Cu and Cd emissions will exceed the capacity of farmland soil in China after 60 and 72 years respectively.In order to ensure that the Cu content of farmland soil is at a safe level,it is necessary to achieve a negative increase in the amount of Cu element in livestock manure.At the provincial level,considering the total amount of livestock manure emissions,there are only half of the provinces and cities in China which their farmland has sufficient capacity to accommodate local livestock and poultry manure.The main pollution elements are Cu,Zn,and Cd,the content of Cd in farmland soil of Tibet and Qinghai Province will excess after 21 years.If the rate of returning livestock manure to fields in various provinces and cities reaches 60%,there will be heavy metal pollution in farmland soil in 8 provinces and cities in China. |
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