Study On The Distribution Characteristics Of Heavy Metals In Soil Profile Under Different Plastic Film Mulching

In view of the fact that reclaimed water is used for farming for a long period in party irrigation district,the problem of heavy metals pollution and the state of film mulching is widely used are questioned.In this paper,the joint project with water resources department of Shanxi Province of study on the effect of agronomic water saving irrigation on crop yield and ET was taken as the carrier,agricultural soils for different landscape units in Fenxi irrigation district in Shanxi and other areas were selected as research object.Based on annual tracking tests in different mulching conditions of bare land,mulched ordinary plastic film and water-osmosis plastic film,the spatial and temporal distribution characteristics of heavy metals in three plastic films,transport characteristics of them in soils,migration law of them in groundwater fluctuant belt and the effect of plastic films on vertical transport process of heavy metal were analyzed.The main achievements are as follows:(1)The vertical distribution characteristics of different heavy metals are similar but different while the physiognomies are different.In Fenxi irrigation,the highest content of heavy metals is distributed in 0~40cm soil profiles,the content decreases greatly below 40 cm soil layer and remain a low level in 40~160cm.The content increases slowly and remain stable below 160 cm in the experimental plot in the plain and below 200 cm in alluvial plain and it is the result of the difference of groundwater levels of the experimental plot under different landform.The problem of heavy metal pollution does exist but the pollution is slight and Ni and Cr are the major pollutants reference to national soil environmental standard.(2)The vertical migration of heavy metals in soil is closely related to convection transport of soil moisture.Heavy metals migrate upward with soil moisture and accumulate in surface soil during the non-rainy season and downward under the action of leaching during the rainy season or under heavy irrigation conditions.In addition,heavy metals migration is affected by the soil environment.The migrations of Cu and Pb are influenced by the content of soil organic matter because of the adsorption between organic matter and them.(3)The migration ability of different heavy metals in soil is different and the order is Cd>Cu>Pb>Zn>Ni>Cr from strong to weak.Because the migration abilities of Cu and Pb are affected by the content of organic matter in the soil,they increase in the soil with high organic matter.(4)The application of film mulching in the experimental field can effectively reduce the heavy metal pollution in the surface soil,but it will aggravate the accumulation of heavy metals in the bottom plow soil in the depth of 20~40cm,and the phenomenon is more obvious under the condition of covering the permeable film.However the film mulching would increase the content of Cd in the farmland soil causing the Cd contamination of the soil.In addition,the plastic film mulching has little effect on the distribution of heavy metals in the groundwater fluctuant zone and will not increase the heavy metal pollution in groundwater.(5)The mulching conditions affect the movement of heavy metals in the soil by influencing of soil water migration.At the same time,the mulching films would accelerate the growth process of the plant and affect the physical and chemical properties of soil,such as temperature,bulk density,organic matter content and so on.They also affect the oxygen content in the soil and microbial activity,which affect the migration of heavy metals in the soil.Especially,the water permeable film with the characteristics of light rain resources would directly affect the movement of soil water and play a great role in the agricultural production in dry areas.(6)The content of heavy metals in the groundwater fluctuated zone is relatively high,the variation between layers is small and it is more obvious below groundwater level.Contrary to the process of water migration,heavy metals migrate towards the groundwater level in the groundwater fluctuation zone.The groundwater level drops and the heavy metals in the groundwater fluctuation belt move downward when the soil moisture moves upward.Therefore,the heavy metals pollution in groundwater is aggravated when the groundwater level falls down.Combining the two research hotspots of plastic mulching and heavy metal pollution,this paper achieves the research of vertical distribution and migration of heavy metals in soil in the depth of 0~300cm(including underground fluctuation zone)under the conditions of film mulching.In particular,the effect of permeable permeability of new plastic film on the transport of heavy metals in soil is studied.However,heavy metal pollution is only one of the soil inorganic pollutants.In order to clarify the influence of the film on the soil environment and maintain the sustainable utilization of the farmland,the study of the influence of the different mulching conditions on the distribution of inorganic salt and organic matter in the soil is also needed.In addition,only the total amount of heavy metals in the soil was measured.In the future research,it is necessary to clarify the existing forms of heavy metals in soil,so as to further study the distribution of heavy metals,bioavailability and the harm of heavy metals to the soil environment.