Studies On Characters Of Fe-Mn Nodules And Pernicious Effect Of Dryland Crops In Litang

Addressed to soil,a key element in the field ecosystem,to find out the relationship of elements contents in Fe-Mn nodules,soil and dryland crops,and solve the problems like soil fertility, soil environmental quality and the quality of edible parts of dryland crops,we carried out sample survey,indoor experiment and theoretical analysis(take the karst area in Litang for example).The results were as follows:Firstly,the characteristic of the soil in the study area was high silicon and low sodium and manganese.The soil was mostly neutral.The fertility indicators of the organic matter,total N,available N,total P,avaliable P,total K and CEC in A layer soil were higher than those in B layer.The fertility indicators of available P and total K were quite lack,its contents can not meet the normal growth of crops and it has become the mail limiting factors.Secondly,the surface of Fe-Mn nodules were mostly yellow brown and smooth,the shape was sphere,spherical or globular ellipsoid. SiO2,Al2O3 and Fe2O3 were still the main chemical elements in Fe-Mn nodules,compared with soils, the Fe-Mn nodules'enrichment effect on Fe2O3 and MnO was obvious and the degree of enrichment of Fe2O3 was higher than that of MnO, Vertical,the enrichment effect on Fe2O3 and MnO in B layer was higher than that in A layer. Elements of Si,Mg,Ca,Na,K,Ti were weaking and P was specific adsorption effect in Fe-Mn nodules.Heavy metal contents in A,B layer of Fe-Mn nodules such as Pb,Co,Cr,Ni,Zn,Li were higher than clark value,it's order of average concents was Cr (â…¥)> Zn> Pb> Ba> Co> Ni> Li> Hg,However,the same heavy metal contents were obviously difference in depth,it's contents in B layer were 1.5 times as much as in A layer.The enrichment coefficient of heavy metals in A,B layer of Fe-Mn nodules had some differences vertical, its coefficient of concentration is generally higher in B layer than that in A layer,and its order and magnitude was also different in some extent. But iron group elements (Cr,Co,Ni) and the pro-sulfur elements (Zn,Pb) were dominating status,rock-forming elements (Ba,Li) were following status. Among them,Cr (â…¥),Co and Zn indicated high enrichment, Pb was high or moderate enrichment,Ni was moderate enrichment,Ba was moderate or slight enrichment,Li was slight enrichment.Correlation matrix analysis showed that enrichments of heavy meatal were related to Fe2O3,there existed nuclear enrichment,specific adsorption and leaching features by factor analysis.Thirdly,the average contents of the six kind of heavy metals in A layer soil were like this:Pb 53.55 mg / kg,Co 15.47 mg / kg,Cr (â…¥) 243.30mg/kg,Ni 20.34mg/kg,Zn 116.14mg/kg,Cu 12.01mg / kg,while B layer were like this:Pb 51.95mg/kg,Co 24.94mg/kg,Cr (â…¥) 208.28mg/kg,Ni 33.95mg/kg,Zn 159.86mg/kg,Cu 14.99mg/kg. Compared with background values of elements in Guangxi soil,Pb,Co,Cr (â…¥),Ni were higher and Ni,Cu were lower in A layer soil, while Pb,Co,Cr (â…¥),Ni,Zn were higher and Cu was lower in B layer soil; When compared with background values of elements in country soil,Pb,Co,Cr (â…¥),Zn,Cu were higher and Ni was lower in A layer soil, while Pb,Co,Cr (â…¥),Ni,Zn,Cu were higher;When compared with quality standard of gree food environment, heavy metals such as Pb,Cr (â…¥) and Zn in A,B layer were more than alert level;According to quanlity standards(â…¡) of country ,Cr (â…¥) in A,B layer soil had already exceeded warning level.Ways of single-factor pollution index,the comprehensive index pollution index,pollution load index and potential ecological risk index were used to evaluate heavy metal of the soil,the results showed that the soil were polluted by heavy metals such as Pb,Co,Cr,Ni,Zn, among them ,the pollution caused by element of Cr was particularly serious,pollution degree of every plots were slight,moderate , even heavy pollution. With environmental quality of national soil standards as evaluation criterion ,quality degree of Cr in A,B layer was medium,comprehensive quality degree was medium in A layer soil of 1# and A,B layer soil of 3#,others were alert level.With environment quality of green food as evaluation criterion ,Pb,Zn were medium and Cr was bad in A layer,Pb,Cr,Zn were medium in B layer, comprehensive quality degree in every plots are medium.Fourth,compared with heavy metal contents in the edible parts of dryland crops, contents of As,Cd,Pb were the highest in white radish ,contents of Cr,Cu,Zn were the highest in the cucumber ,contents of Cd,Cr,Cu,Pb were the minimum in the maize,contents of As and Zn were the minium in the sugar cane. Enchrichment coefficient was all less than 1,among them Zn,Pb,Cu were more high while Cd,As,Cr were low.Results of quality evaluation in dryland crop showed that the quanlity critieron of the six heavy metals in maize seed were good and comprehensive quality level was safe.The quality level of Pb,Cd were very very bad and Zn was medium in sugar cane,white radish and cucumber,it's comprehensive quality critieron was very bad;Ecological risk assessment showed that Pb was greatly exceeding the PTWI level and As was slightly high than PTWI level in the carrot and cucumber, it would be harmful to human health if we ate them as food for a long time.