Ecological Remediation Evaluation Of Heavy Metals Contaminated Agricultural Soils In The Downstream Of Minjiang River Affected By Different Planting Modes

Interplanting and rotation were the traditional agricultural planting forms of our country, under the interplanting and rotation,the absorption of heavy metals of crops could be improved and the heavy metal content of soil also could be changed,these results were suggested by the past researchs.Therefore,it may be possible to realize the repairment of mild heavy metal polluted agricultural soils by reasonable interplanting and rotation in the premise of guaranteeing the normal agricultural production activities and crop safety at the same time.This study was on the basis of previous work of our research team,field experiments were conducted in lower reaches of Minjiang River,southwest of China.Four typical agricultural planting mode composed of interplanting,rotation and monoculture (Ml:cor-n+ginger-cabbage;M2:corn-mustard;M3:rice-fallow;M4:ginger-scallion-lettuce) were sele-cted and cultivated from April in2010to November in2010.The variations of Pb,Zn and Cd in the soils/the migration and distribution of Pb,Zn and Cd in the soil—crop system and the variations of Pb,Zn and Cd in the organ of crops were investigated within the four agricultural planting modes.In the meantime,GRATS(gray relational analysis with time series) was used to evaluate the comprehensive ecological remediation efficiencies of four planting modes,including the remove ability of planting modes for Pb,Zn,Cd,and extraction capacity of inedible parts of crops and security of crops for Pb,Zn,Cd and so on.The results were as followed:(1) Before planting crops,the single contaminate index of Pb,Zn in the four planting modes were both less than1,and this indicated that the soil were not polluted by the Pb,Zn.The single contaminate index of Cd in the four planting modes were all more than l,and this suggested that the soil had been polluted by the Cd serevely.After a period of crop planting(from April2010to November2010),the soil pollution of Pb,Zn,Cd in the four planting modes were all decreased to some degree,because the different planting modes were composed of different crops,the pollution change feature is also different in each planting mode.The single contaminate index of Cd in the M1,M2and M3were all decreased largely,and the decreasing amplitude were39.72%,32.04%and36.94%respectively,the single contaminate index of Pb had a biggest drop in the M4(33.94%). The crop growth period in which the decreasing amplitude of the single contaminate index of Pb,Zn,Cd were largest in the four planting modes was as followed:M1:corn and ginger common growth period, M2:maize growth period, M3:whole growth period of rice, the M4:ginger growth period and spring onion growth. Single pollution index evaluation results show that only the pollution levels of Cd in the M2had a drop from heavy level to moderate level after the planting period,and others had no changes. Comprehensive pollution index evaluation results indicated that the comprehensive pollution index of soil in the M2droped from moderate level to mild level,droped from heavy level to moderate level in the M4,and it was not changed in the M1and M3.The results of this research showed that the four planting modes all had a certain effect on the decrease of heavy metal in the polluted soils,and the effects were depended on the planting modes, this indicated that it was feasible to use different planting modes to repair the soil in the mild heavy metal polluted farmland.(2)Except for ginger,the heavy metal were largely accumulated in the crop roots,the content of Pb,Zn,Cd in the organ of crops showed a overall performance as roots> stems>leaves>seeds.The content of Pb,Zn,Cd in the edible parts of crops were significantly less than these in the inedible parts,the biological accumulation coefficient of the crops’ edible parts in the different crops for vegetables> root-eating crops>cereal crops.In addition to the content of Zn in the seeds of corn and rice not exceed the standard,the heavy metal content in the edible parts of other crops all exceeded the health standard with a certain degree.The content of Pb,Zn,Cd in the edible parts of the cabbage severely exceeded the health standard,so the cabbage planting should avoid in the heavy metal polluted soils.The heavy metal pollution of the edible parts of the cereal crops was less than root-eating crops and vegegables,so planting cereal crops in the heavy metal polluted soils could alleviate the poison effect on human body to some degree.(3) GRATS was used to evaluate the comprehensive ecological remediation efficiencies of four agricultural planting mode,the results showed that the comprehensive repairing effect coefficiencies were0.7889,0.7380,0.7938and0.8590respectively.This indicated that the four planting modes in this study all had a good ecological remediation efficiencies and closed to the ecological restoration goals.Therefore choose different planting modes according to the local conditions in the heavy metal polluted plain and hilly aeras have ecological benefits and economic benefits,and this would be popularly accepted by local farmers,and this also would be a good way to achieve the sustainability and large-scale remediation for the local mild heavy metal contaminated agricultural soils.