| Sewage irrigated croplands has reached 1.4×10~6 hm~2 in the early 1990s inChina,and total of 6.3×10~5 hm~2 farmland suffering from pollution.Farmlands polluted by Cd was almost 1.3×10~4 hm~2,involving 25 regions 11 provinces,Zhang Shi irrigation and Shenfu irrigation were influential.Meanwhile,Cd pollution was exacerbated by atmospheric deposition,the mining industry and agricultural encroachment accumulation of solid waste material.As non-essential elements of plants,plant roots can accumulate Cd,and thus affect plant photosynthesis,antioxidant activity,as well as the absorption of nutrients,and then resulting in stress effects on plant growth.Plants on Cd contaminated soils can absorb soil and causing damage to the human body through the food chain.Therefore,the quality and quantity of agricultural products should beconsidered.On the one hand,in order to deal with the conflicts between the food needs and farmlands decrease,food yields should be increased.On the other hand,in order to prevent soil contaminants enter the body through the food chain and endanger people health,remediation should be considered.This study selected the typical soils,artificial mineral Cd was added to contaminated soils.Pot experiments were used to study the followings:1)the effects of different moisture and fertilizer conditions on rape growth and accumulation of Cd;2)characterization of chemical fraction of Cd in soils and rape.The main results were as follows:1.Rape can grow in Cd pollution soils.Under the experimental conditions,the rape yield between 46.0-255.2 g per pot,while lowest yield harvested in semi-arid and not fertilization treatments,and highest yield in conventional water and high amounts of fertilizer treatment.At the same irrigation,fertilization increased rape yield significantly,rape yield was 1.7-3.8 times higher in fertilization treatments than non-fertilization treatments,meanwhile,rape yield increased with fertilizer input increasing;under the same fertilization treatments,rape yield increased significantly when soil moistrue increased,rape yield was 1.2-2.3 times higher in sufficient water treatments than in semi-arid treatments,but there was no significant difference between the conventional water treatment and high-volume water treatments.Rape yield was obviously affected by water and urea,rape yield was not significantly affected by soil moisture when N fertilizer was absent(P>0.05);and the moisture will become limiting factor of rape growth when N fertilizer was sufficient,but there was no significant difference between regular soil moisture and high soil moisture.Therefore,soil moisture conditions and the amount of conventional fertilizer N is a necessary condition to ensure that rape yield when cultivated rape in Cd contaminated soils.2.Cd accumulation by rape changed in different moisture and fertilizer conditions.During the whole rape growth period,Cd accumulation was dominant in stems and leaves.Cd accumulation were 7.2-32.7,10.2-38.0,11.8-42.2 mg/kg in roots in early,middle and later stage period of rape,respectively.and coresponse Cd accumulation in stems and leaves was 14.4-41.1,20.2-53.9,23.9-58.5 mg/kg,respectively.The Cd accumulation ratio of roots and stems and leaves was between0.45-0.88,0.53-0.78,0.49-0.72,respectively,which indicated that rape stems and leaves have better accumulated effects of Cd than roots.The highest accumulation of Cd occurred in high soil moisture and high levels of fertilization treatments,and lowest accumulation of Cd in semi-arid and not fertilization treatments.Transfer coefficient was significantly higher in no fertilizer and halved fertilization treatments than conventional fertilization and high levels of fertilization treatments under the experimental conditions.Results showed that Cd was easier to migrate to the stems and leaves in the case of nutrient deficiencies,thus,resulting in greater food security risk.Rape can be used as ingredients in the whole growth period,but in 10 mg/kg Cd contamination soils,the edible parts of the Cd concentrations were much higher than the food in limits(0.2 mg/kg).Results showed that it was not suitable for cultivating rape in Cd contaminated soils.3.Soil pH change trend was early stage>mid stage>later stage period during the rape whole growing period,and soil pH were 5.23-5.67,5.20-5.41,4.59-5.14 in each growing period,respectively.Soil pH decreased 0.30-0.85 units when rape was harvested,and soil acidified obviously,the acidification trend increased with increasing urea fertilizer input.For non-fertilization,soil moisture did not have a significant impact on soil acidity in conventional fertilization treatments and high level fertilization treatments;for the half amount of fertilizer treatment,the conventional moisture treatment was the highest,the high moisture treatments was the lowest,and significant differences were observed.However,there was no significant difference between the semi-arid moisture treatment and treatments before.Results showed soil pH was obviously affected by different moisture and fertilizer input,soil acidification is bound to affect soil chemical fraction of Cd in soils.4.Within rapeseed growth period,Soil chemical fraction of Cd content followed the order:exchangeable>carbonate bound>residual>iron and manganese oxidation state>organic bound;where iron and manganese oxidation state and residual proportion fundamental changes little during the whole growing period,and organic bound Cd changed into exchangeable Cd and carbonate bound Cd when rape was harvested,and thus increased its bioavailability.Exchangeable Cd changed slightly during rape growing period,soil Cd concentration was 3.12-4.42 mg/kg when rape was harvested.Carbonate bound Cd changed slightly during rape growing period,soil Cd concentration was 2.35-3.31 mg/kg when rape was harvested.Bound to Fe-Mn oxides Cd changed slightly during rape growing period,soil Cd concentration was1.01-1.06 mg/kg when rape was harvested.Bound to organic matter Cd changed slightly during rape growing period,soil Cd concentration was 0.10-0.15 mg/kg when rape was harvested.Residual fraction Cd changed slightly during rape growing period,soil Cd concentration was 1.21-1.30 mg/kg when rape was harvested.Results showed Cd was affected by different moisture and fertilizer condition.5.In the early and middle growth stage of rape,a negative correlation between the soil acidity and the extent of exchangeable Cd in soil,the content of carbonate bound Cd,the content of iron,manganese oxidation state of Cd,the content of organic bound Cd;and a positive correlation between the soil acidity and the residual Cd content,but the correlation was not significant.In the latter growth of rape,the trend was similar to the early and middle growth stage,but the soil acidity and carbonate bound Cd was significantly negatively correlated and significantly positively correlated with the residual Cd content.It is shown that in acidic soils,it is helpful for residual Cd conducive to rest several chemical speciation changes and increasing its bioavailability when soil acidity increased.6.The chemical speciation of Cd in soil was closely related to the content of Cd in rape.There is little relationship between the Cd accumulation in rape and the iron and manganese oxidation state Cd content in soil,the content of organic bound Cd;and positively correlated between the Cd accumulation in rape and exchangeable Cd content,carbonate bound Cd content,furthermore the correlation with the latter reached a significant level that showed a significant negative correlation with the residual Cd content.It showed that high biological activity in soil chemical speciation of Cd is carbonate bound Cd and exchangeable Cd. |
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