Analysis Of The Accumulation Difference Of Pd And Cd In Wheat Grains Under Different Air‐Soil Pollution Levels Based On Stable Isotope Technology

With the rapid development of urbanization,industrialization and agricultural intensification,economic improved greatly accompanied by the destruction of ecological environment to varying degrees.Soil is an important part of natural ecosystem and a basic resource for human survival and development.Heavy metal elements are enriched and difficult to be degraded in the environment.Their harm to the soil environmental system is directly related to the safety of agricultural products.As the staple food of people in northern China,the excessive accumulation of Pb and Cd in wheat grains has also aroused our continuous concern.Effective identification of the sources of Pb and Cd enriched in wheat grains and their migration and transformation patterns in the air-soil-wheat system is one of the preconditions to prevent and control the pollution.In this paper,we selectet an area with a decades of lead and zinc smelting history as the research object to examine the accumulation differences of lead and cadmium in wheat grains under different air and soil pollution level.Heavy metal elements in total suspended particulate matter(TSP)were introduced to the modelling of Pb and Cd in wheat grais.Qualitative and quantitative analysis of the grain Pb accumulation under different TSP and soil pollution exposure conditions were further conducted by means of of Pb stable isotope tracer technique to reveal the contribution of TSP Pb and soil Pb to wheat grain Pb.Cadmium isotope fractionation in the soil-root-straw – grain was systematically discussed under different air and soil pollution level so as to provide available information to control the Cd contamination in wheat grains.The main research contents and conclusions are as follows:(1)Studies on Pb and Cd in wheat grains are mostly focused on the soil-wheat system.In this study,the concentration of Pb and Cd in TSP was continuously observed during two growth cycles of wheat.Combined with the concentration of Pb and Cd in soil,a field system was constructed which contained different air pollution levels at similar soil pollution levels and different soil pollution levels at the same air pollution levels.The accumulation differences of Pb and Cd in wheat grains under different exposure conditions were analyzed.The results showed that Pb in ambient air might be the main source of Pb in wheat grains.However,Cd in wheat grains may derived from the transfer of soil Cd to wheat,and soil p H is the key factor affecting the accumulation of Cd in wheat grains.(2)Most existing research of the grain of Pb and Cd in the modeling inclined to select the total or available content of of Pb and Cd in the soil and soil organic matter,p H and soil physical and chemical properties as influencing factors,the impact of ambient air on wheat grain Pb and Cd accumulation was often neglected.This study made an attempt in the modeling of Pb and Cd in wheat grains,and soil Pb and Cd,soil p H,especially the TSP Pb and Cd were all taken into consideration.The results showed that the modeling effect was significantly improved of the grain Pb with the introduction of TSP Pb into the influencing factors rather than the modeling effect of Cd.This also confirmed that the grain Pb mainly comes from the air,and the grain Cd mainly come from the conclusion of soil Cd from the side.(3)The stable isotope tracing technology of Pb has been proved to be an effective tool in the source apportion of wheat grain Pb.While most of these researches were conducted in soil and air polluted areas.Based on the system consisting of different soil and air pollution levels at field,this study examined the contribution of TSP Pb and soil Pb to wheat grain Pb qualitatively and quantitatively.The results showed that the contribution of ambient air Pb to wheat grain Pb was as high as 82.65%-91.91% at the samples with high TSP Pb concentration,while the contribution of ambient air Pb to wheat grain Pb was 7.76% and 33.20% depending on the difference of soil Pb concentration at the samples with very low TSP Pb concentration.It is necessary to control the concentration of Pb in ambient air to reduce the Pb content in grains.(4)Although the isotopic composition of Cd in wheat,soil and TSP was different under different soil and TSP exposure levels,the isotopic composition of Cd in wheat,soil and TSP showed the same trend at their respective levels,that is,the heaviest in grains,followed by TSP and the lightest in soil.The fractionation of soil-root-straw-grain system also showed a trend from light to heavy under different soil and TSP exposure conditions.The value of δ 114/110Cdgrain-soil ranged from 0.415‰ to 0.539‰,indicating that the fractionation of soil-root-straw-grain system also showed the universality of Cd isotopic fractionation in soil-wheat system with a trend from light to heavy under different soil and air Cd exposure levels and even different soil physical and chemical properties.It is a little limited to use Isosource model to analyze the source of Cd in grains with the obvious Cd isotopic fractionation in soil-wheat system.Studies on the source apportion of Cd enrichment in plants should be carried out under the premise of full consideration of the fractionation coefficient of Cd.