Study On Monitoring The Change Of Heavy Metal Content In Soil-Crop System Of Farmland By Hyperspectral Remote Sensing

With the rapid development of economy,a variety of heavy metal elements have been released into the soil of farmland,resulting in more serious heavy metal pollution of farmland soil.Due to the non-degradability and persistence of heavy metals,it is easy to accumulate in plants and animals through the food chain.This poses a huge threat to human health.Wheat is the main food of the people’s life in China.Its planting range is very wide.Copper(Cu)is an essential element for wheat growth,but excessive Cu can adversely affect wheat health.Excess Cu can inhibit the synthesis of chlorophyll or cause chlorophyll damage,leading to the peroxidation of lipids in photosynthesis biofilms,which affects photosynthesis of wheat and leads to a decline in biological yield.Moreover,it will pose a serious threat to human health when the Cu migrates into grain.Traditional methods of detecting heavy metal content have many disadvantages,such as complicated steps and soil damage.The emerging hyperspectral remote sensing technology can quickly acquire hyperspectral data information of ground objects with high resolution and multiple continuous spectra.It is more in line with modern fast,non-destructive,real-time monitoring the changes of heavy metal content in soil and crop.Since the soil is the growth substrate of the crop,when the heavy metal content in the soil exceeds the environmentally tolerable range,it will interfere with the synthesis of the chlorophyll of the crop,resulting in changes in the spectral information of the leaves or canopy,so the hyperspectral data can be determined by measuring the canopy or leaf of the crop.Then we can establish an estimated model to invert the heavy metal content of the soil using the hyperspectral data and spectral index.Therefore,in order to ensure food security and human health,it is important to real-time and quickly monitor the changes of heavy metal content in farmland soils and crops using hyperspectral remote sensing.In this study,framland soil and wheat were used as research objects,and the following studies were carried out by using hyperspectral remote sensing technology combined with correlation analysis,partial least squares regression method and geostatistical method,(1)Analysis of the correlation relationship of Cu content between wheat and soil,studying the feature of transport and enrichment for Cu in the soil-wheat system,and exploring the effect of adding different amounts of Cu on the total biomass and yield of wheat;(2)Using correlation analysis and partial least squares regression to screen the spectrum sensitive to available Cu content of soil,and PLSR model was established to estimate the available Cu content of soil based on the spectral characteristics of wheat leaves;(3)The hyperspectral reflection and absorption bands of wheat leaves were analyzed.The spectral information was pre-preprocessed in various methods.Based on the correlation analysis between spectral index and Cu content of grain,a PLSR model was established to estimate the content of Cu in wheat grains based on the hyperspectral characteristics of wheat leaves;(4)Based on Kriging interpolation,combined with single factor pollution index,Nemerow comprehensive pollution index and Geoaccumulation Index,analyzing the spatial distribution and pollution conditions of As,Cr,Cu,Pb and Cd of farmland soil in Zhangjiagang City,which would provide reference for improving the ecological environment of Zhangjiagang City and protecting food safety.The main research progresses were summarized as follows::(1)Characteristic of migration and transformation of Cu in soil-crop systems.When the amount of Cu added in the soil was 200 mg/kg,the total biomass and yield of wheat were the highest,and the total biomass of wheat was the lowest when the amount of Cu added in the soil was 600 mg/kg.There was a significant difference between the total biomass of wheat with a Cu addition of 200 mg/kg and other treatments.However,for adding different concentrations of Cu in soil,the effect on the yield of wheat was not significant.There was a significant positive correlation between Cu content in stems,leaves and grains of wheat and Cu concentration in soil,and there was also a significant positive correlation between stems,leaves and grains,indicating that heavy metals in grains were not only transported by soil,stems and leaves also provide a source of heavy metals.The different parts of wheat have different absorption capacity for heavy metals,the root has the strongest absorption capacity.The root>seed>leaf>stem was present when the concentration of Cu in the soil is low,and the root>leaf>stem>grain was present when the concentration of Cu in the soil is high.The enrichment coefficient of Cu in different organs of wheat did not exceed 1,and the enrichment order was root>stem>seed,and the transport coefficient was also less than 1.(2)Monitoring the change of available Cu content in soil by hyperspectral spectrum.The results showed that the original spectral reflectance of leaves treated with different Cu concentrations was consistent in all wavelength ranges in anthesis stage.Moreover,the original spectral reflectance of the leaves with high Cu addition in the soil was higher than the low added leaves in the range of each band.And there was no significant difference in plant height between different concentrations of Cu in soil.After various forms of spectral preprocessing,among the PLSR models established by spectral index,the PLSR model established by the reciprocal first-order differential spectrum had the highest accuracy,R2 is 0.83,RMSECV is 1.71 and close to RMSEP.The R2 of internal verification and external verification are 0.666 and 0.839,respectively.In summarize,the prediction accuracy was high.Therefore,the research results proved that we could use hyperspectral remote sensing technology to effectively monitor the change of heavy metal content in soil,determine whether the concentration of heavy metals exceeds the standard.It provids a research direction for large-area and in-situ stereo monitoring the change of Cu content in farmland soil.(3)Monitoring the change of Cu content in wheat grain by hyperspectral spectrum.There was no significant difference in wheat height between different treatments during the booting stage.And the Cu content in wheat grains increased with the increase of Cu concentration in soil.In addition to the treatment of 200 mg/kg and 400 mg/kg of Cu in the soil,there were significant differences among other treatments.What’s more,the original spectral reflectance of wheat leaves increased with the addition of Cu in the soil in the visible and near-infrared range,and each treatment had a similar trend.The precision for estimating Cu content in wheat grain was quite different for different spectral transformation forms.Among them,the PLSR model established by continuum removal spectrum had the highest accuracy,which was the best model to estimate the Cu content of wheat grain.The R2 of the beast calibration model was 0.859,and the internally verified R2 was 0.787,RMSECV was 1.03,the externally verified R2 was 0.828 and the RMSEP was 0.87.(4)The results of spatial distribution of heavy metals and pollution assessment of farmland soil in Zhangjiagang City.The results showed that the average values of Cu and Cd in the farmland soil in Zhang Jiagang City exceeded the background value of soil elements in Jiangsu Province,and the accumulation was the most serious,which was 1.26 times and 2.21 times of background value in soil of Jiangsu Province respectively.According to Kriging interpolation,the accumulation of heavy metals in Yangshe Town was the most;the As of soil was the least polluted,and its high value area was the least;the high value areas of Cr and Cu in soil were more distributed;and the high value area of Cd mainly distributed in the junction area of Yangshe Town and Jinfeng Town,there may have pollution sources of Cd in this area.The single factor pollution index indicated that the four elements of As,Cr,Cu and Pb in farmland soil of Zhangjiagang City belonged to the clean category,while the Cd in most areas belonged to the scope of clean and still clean(about 90%of the area),while 5%of the areas reached mild and moderate pollution levels.The Nemerow comprehensive pollution index indicated that Yangshe Town had the most serious comprehensive pollution,followed by Daxin Town and Jinfeng Town.The comprehensive pollution index of Zhangjiagang City indicated that the farmland soil in this city generally reached a light pollution level.The geoaccumulation index indicated that the Cd pollution of farmland soil in Zhangjiagang City was the most serious,followed by Cu,and then Pb.The least polluted were As and Cr.The innovations of this article were:(1)Exploring a fast,non-destructive,real-time and dynamic stereo monitoring method.The hyperspectral data information of ground objects can be obtained directly by using ground hyperspectral remote sensing technology,which has the advantages of high resolution and continuous spectrum.After establishing the inversion model of heavy metal content,the change of heavy metal content can be monitored without damaging soil and crops by acquiring the hyperspectral data of the monitored objects,which makes up for the shortcomings of traditional detection methods for heavy metal content,such as complicated steps,damaged detection objects,and unable to real-time detection.(2)Using hyperspectral remote sensing technology to indirectly monitor the change of heavy metal content in soil.Previous studies always used spectral data of heavy metals in soil to directly monitor the changes of heavy metals content in soil.However,it is difficult to detect the spectral information because the spectral information of heavy metals in soil is weak,so the monitoring accuracy is low.The hyperspectral remote sensing technology can indirectly retrieve the heavy metal content in soil through the spectral data of crop canopy or leaf combined with spectral index.The accuracy of the model is higher than that of direct monitoring.In summary,in order to monitor the changes of heavy metal content in soil and crop in a fast,real-time and non-destructive manner to ensure soil environmental quality and food security,this study analyzed the characteristics of migration and enrichment for Cu in soil-wheat system.And then the PLSR models were established to quantitatively estimate the content of available Cu in soil and Cu in wheat grain using hyperspectral remote sensing technology combined with leaf spectral data.At last,combined with geostatistical method and soil pollution index,the spatial distribution characteristics and pollution status of heavy metals in farmland soil of Zhangjiagang City were analyzed.This study provided the theoretical support for large-scale and rapid monitoring heavy metal pollution of soil and crop in the future.