| The heavy metal cadmium(Cd) was listed as one of the most dangerous environmental pollutants which can easily accumulate in the human body and endanger human health,through the food chain.Nitrogen(N),phosphorus(P),potassium(K) fertilizers are widely used as the basic fertilizers for agricultural production.Application of these fertilizers may change some physical and chemical properties of soil,leading to conversion of speciations of heavy metals in soils,and enhanced or reduced absorption and accumulation of cadmium on plants.The objectives of this study were to investigate the effects of different NPK fertilizers on bioavailability of cadmium in soil,evaluate their effects on whether or not applications of these fertilizers would promote or inhibit uptake of Cd by wheat.The results generated from these studies are used to put forward the best fertilizer management practices for wheat production on soils suffering from cadmium pollution.In current studies on soil heavy metal pollution,the most commonly used methods are either by adding readily bioavailable inorganic compounds of heavy metals up to severely polluted levels to a targeted soil or directly using a heavy metal severely polluted soil. However,results generated from these studies cannot tell whether or not a soil with the heavy metal content just below the threshold of the permitted maximum level set by the State will certainly produce "safe" agricultural products free of heavy metals contamination. There is lack of such a report to date.For this reason,this paper aimed to examine the effects of different N,P,K fertilizers on availability of soil Cd that was just below the threshold of the permitted maximum level set by the State,on wheat growth and on Cd uptake by wheat.Incubation study was conducted to examine effects of different N,P,K fertilizers on availability of soil Cd;sand culture experiments were used to uncover effects of different Cd concentrations on growth of wheat roots,shoots,and Cd distribution in roots and shoots;pot experiments were carried out to to study effects of different N,P,K fertilizers on wheat growth and Cd uptake by wheat.Following achievements were made throught the studies.1.After applied in different N,P,K fertilizers in soil to culture,availability of soil Cd was significantly affected by different fertilizers.After 60 d incubation,all the N fertilizer treatments lowered soil pH,increased amounts of Cd extracted;while the high rate of urea and ammonium chloride treatments resulted in the lowest soil pH and the highest amounts of Cd extracted from the soil;increased Cd extracted from the ammonium sulfate treatment was minor compared to the CK.All the P fertilizer treatments caused the modest reduction in soil pH,while only the single superphosphate slightly increased the extractable Cd.All three K fertilizer treatments reduced soil pH,among which potassium chloride extracted the highest amounts of Cd at 0d and its superior extracting ability for Cd disappeared after 15d.The Cd extracted from all the K fertilizer treatments fell below the Cd extracted by the CK at end of the incubation.The Cd extracted from the soil increased with amounts of acid added(fall in pH) and decreased with amounts of base added(rise in pH).Peat treatments caused the reduction in soil available cadmium in a certain extent.2.With the cadmium concentration increased,wheat roots growth significantly were blocked,the greater the concentration,the slower the growth of roots,dry matter accumulation was inhibited,but the root system of absorption rate of cadmium become higher.Stems and leaves can promote its growth under the cadmium concentration of 0.3 mg·kg-1 the following,when it excess this concentration,its growth can be inhibited,but with the concentration of cadmium increased,stems and leaves absorption rate of Cd is down.Cd content of Root is always higher than the leaves.Follow the Cd concentration increased,Cd content of wheat roots increased at a slower rate,the Cd content of leaves increased faster than increasing rate of Cd concentration,the distribution of Cd between the two differences is gradually shrinking.3.In the wheat life,pre-absorption of the wheat absorb the total fertility Cd less,the latter significantly increase.But Plant concentrations of Cd performance for pre-high,late low growth in vivo,period with a progressive and gradual lowering of the trend,which is the dilution effect on the plant.The effects of different N,P,K fertilizers on grain yield,biomass,and wheat Cd are significant difference.In the all fertilizer treatment,the grain yield and biomass of wheat were highest with the treatment of ammonium sulfate and ammonium nitrate fertilizer respectively match fertilizer calcium superphosphate and potassium chloride.There were no significant differences between different amendments on grain yield and biomass of wheat.Follow the assay results of wheat Cd in vivo,All N fertilizers increased Cd uptake by wheat,among which nitrogen chloride was most capable and ammonium sulfate was least capable of enhancing Cd uptake.The Cd contents in wheat tissues were increased with an increase in urea rates.All P fertilizers increased Cd uptake by wheat,among which di-ammonium phasphate was most capable,next came high rates of superphosphate;low rates of superphosphate and mono-ammonium phosphate was least capable of enhancing Cd uptake.All K fertilizers increased Cd uptake by wheat, among which high rates of potassium chloride was most capable,next came low rates of potassium chloride;potassium nitrate and potassium sulfate was least capable of enhancing Cd uptake.Peat promoted Cd transfer from the straw to the grain,and lime prevented the Cd transfer from the straw to grain in base material handling.All fertilizer treatments show that wheat grain uptake the total amount of Cd more than straw.4.In agricultural production,application of different N,P,K fertilizers,to more or less extent,change the availability of soil Cd,and thus affect uptake Cd by wheat.In order to produce pollution free wheat on the soils suffering from Cd pollution or with Cd contents in the neighborhood of pollution,Cd resistant wheat cultivars should be adopted and use of ammonium chloride or overdose of urea and other ammonium N fertilizers should be avoided.In order to effectively reduce activity of soil Cd,application rates of di-ammonium phosphate and potassium chloride should not be overdosed,use of and mono-ammonium phosphate,potassium sulfate,along with alkaline matters such as lime, can be very helpful.5.Though the Cd content(0.283 mg·kg-1) of the soil in study is a little bit below the Cd pollution level(=0.30 mg·kg-1) of the acid and neutral soils set by the State,Cd concentrations in wheat grains of all treatments exceeded the permitted minimum value (0.1 mg·kg-1) set by the State.This result indicates that the wheat cultivar used in the studies may be a Cd-accumulator,suggesting that Cd pollution levels of a soil should be crop variety dependent,rather than a fixed value for all.
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