Cumulation, Spatial Distribution And Evaluation Of Soil Heavy Metals In Apple Orchards Of Weibei Area, The Loess Plateau

Apple is one of the world’s four fruits of great production and apple tree is the mostimportant cultivated tree in China and even the world. Weibei area of the Loess Plateau is oneof the high-quality apple production bases in China. Safety of apple quality is the key toensure continued development of apple production in the area. As chemicals existing in soilenvironment naturally, heavy metals’ cumulation is one of the potential factors affectingsafety of apple quality. With green food apple development and requirement of ecologicalconstruction, investigation and evaluation of soil heavy metals contents and pollutionsituation in apple orchards of Weibei area comprehensively and systematically become anurgent basic work. In this paper, soil samples of0-40cm layers soil were collected to analysiscontents, cumulation, possible sources and spatial distribution of6heavy metals (Hg, Cd, Pb,As, Cr and Cu) with the help of multivariate statistics, geo-statistical method and GIStechnology. Soil environmental soil environmental quality and ecological risk caused byheavy metals was evaluated. This study is able to provide scientific bases for optimizing appleproduction, improving apple quality and protecting environment of apple orchards. Mainresults are in the following:(1) Average contents of soil Hg, Cd, Pb, As, Cr and Cu are0.06,0.11,20.6,12.0,59.1,24.4mg/kg, respectively, and are higher than their corresponding background values.Cumulation of all6heavy metals is in the following order: Hg> Cd> Pb>As> Cu>Cr.Contents of Hg and As in most samples exceed their background values and cumulation ofthese metals is more widespread, when compared with other elements. Coefficient ofvariation(C.V.) of heavy metals was in the order of Hg＞Cd＞Pb＞Cr＞Cu＞As. C.V valuesof Hg and Cd are more than50%, with scores of67.24%and50.44%. The C.V. of Pb, As, Crand Cu are ranged from19.10%from23.70%.(2) Correlation analysis shows that Soil organic matter has significant correlations withCd and As at0.05significance level, indicating the strong adsorption effect of organic matteron these elements. Total P shows a strong correlation with Cd, Hg, As, Pb (p <0.05),suggesting that it may have some relationships between application of P fertilizers and thecumulation of these elements. Although pH exhibit positive correlations with Hg, Cd, Cr and Cu and exhibit negative correlations with Pb and As, these correlations are not at significantlevels. Pb shows a significant correlation with As (0.47*) and Cr is very significantlycorrelated with Cu (0.41**), which could indicate resources of these metals may be the same.Hg-Cd, Hg-Pb, Pb-Cr and Hg-As have relatively strong correlations suggesting cumulation ofthese elements is affected by common influential factors. Principal component analysis ofheavy metals demonstrate that cumulation of Cd is due to long-term application of Phosphatein apple orchards. Use of pesticides is related to cumulating of Pb and As in the soil. Hgcontents in the soil are improved by the used of Pesticides, fertilizers and firing of coal. Thecontents of Cr and Cu are mainly affected by natural properties rather than human activities.(3) The semivariograms of Hg, Pb, As and Cr are fitted for an exponential model, whileCd is best-fitted for a Gaussian model and Cu is fitted for a spherical model. Nugget/Sillratios of six heavy metals are all less than25%, indicating distribution of heavy metalspresent a stronger spatial relevance. Random factors play a relatively greater role in spatialvariability of Cd than others listed as Cd> As> Hg> Pb> Cr> Cu.(4) The result of cross validation shows the best interpolation models of Hg and Pb areordinary Kriging, while Cd and As are best-fitted for spline with tension and Cr and Cu arefitted for inverse distance weighting model with the distance coefficient of1. Interpolationmaps of heavy metals show that the high contents of Hg and Cd are in the soil of appleorchards in Midwest and northern of Weibei area. Soil in apple orchards in the central andsouthwest of studied area has high contents of Pb and As. Cr and Cu have low cumulation inmost parts of Weibei area.(5) Soil environmental quality evaluation based the first limit values of ChineseEnvironmental Protection Standard for Soils shows that single factor index of all metals areless than1and at the clean level. As plays a potential negative role to soil environmentbecause its average contents is the most close to the evaluation criteria. Multi-factor index of6metals has reached the alert level and cumulation of heavy metals in apple orchards soil hasaffected soil environment to a certain extent. Evaluation according to the environmentalquality standards of green food production base shows that single factor index of all metalsare less than1and their multi-factor index are less than0.7, indicating that most studied appleorchards meet the needs of the green food production base for further development. On theother hand, the content of As may have a potential negative influence to the green foodproduction, therefore, real-time monitoring and protection measures are required to prevent itsfurther enrichment.(6) Single potential ecological risk for six heavy metals is in the order of Hg> Cd> Pb>As> Cu> Cr. Hg has a high potential ecological risk and its single potential ecological risk index in more than80%samples are at moderate or severe risk level. Cd, Pb, As, Cu and Crare in low degree of potential ecological risk. RI of6heavy metals is in medium degree ofpotential ecological risk and the values in more than80%samples are at moderate or severerisk level. Hg and Cd are the main potential ecological risk factors and agricultural inputs ofApple production require to be regulated in order to prevent continuedincrease of ecological risk caused by heavy metals because they have lager contribution to thecomprehensive potential ecological risk than other elements.