| Soil is an important natural resource for human livelihoods.The enrichment and lack of soil elements will directly affect the quality and yield of agricultural products. Research of soil geochemical composition is helpful not only to recognize the vertical distribution characteristics and their origin, but also heplpful to reveal the pollution intensity of human activity and the impact depth. These are great significances to guide farmer to improve the soil quality and make policies for agricultural sustainable development. The Loess Plateau is one of high quality apple production bases in Chinese even in the world, but with the growing ages increasing, the environmental problem about declining orchard soil quality has been a serious threat to apple production.This thesis selects different planting years (5a,10a,15a,20a,25a) apple orchard soils in Baishui county of Shaanxi Province as the main objects, and takes natural soil section (less human influenced prairie soil) and farmland section (maize/wheat rotation land) as comparative sections. Firstly, the author analyzes the basic properties (such as soil texture, soil organic matter, pH value, carbonate, magnetic susceptibility, chroma) of these soil sections. Secondly, the author studies the spatial distribution of major elements, trace elements including heavy metals, and discusses the relationships between basic properties and the contents of elements. Thirdly, the author discusses migration and enrichment characteristics and their control factors. Fourthly, the author establishes stepwise regression prediction model based on the element contents in S0(B layer) and L1(C layer) layers, which are applied to identify the components of heavy metals in natural pollution (enrichment) and anthropogenic contribution. Finally, single factor and comprehensive index methods are used to evaluate soil environment, and the two rapid diagnosis and prediction models based on magnetic susceptibility and chroma to identify soil heavy metal contents are suggested.The results and new finding are listed as the following:(1) The soils are mainly composed by silt. The pH values in different soil range from7to8.3, a neutral to slightly alkaline. Soil organic matter (SOM) content is higher in the surface soil layer, and SOM contents of the natural soil are higher than those of the apple orchard and rotation farmland. Carbonate content is characterized by lower in middle and upper layer and higher in the lower layer in the soil section. Magnetic susceptibility show a low-high-low variation trend. The color results show that the clayey layer (paleosol, S0) are characterized by lower brightness and higher redness.(2) There are obvious differences of average contents and coefficient variations (CV) of trace elements such as Cu, Zn, Cr, Ni, As, Pb, Hg, P, S from different sections, The differences of average contens and CV of other trace elements such as Mn, Mo, Co, Sr, Rb, V are not significant. The average content of P element is the apple orchard> farmland> natural soil, which indicates that the increased P is mainly due to anthropogenic inputs. In comparison, average content and the CV of major elements in the various soils sections have no obvious difference. But the change patterns of major element content are different in various soil sections, These phenomena may be caused by upper L0horizon suffered different degrees erosion.(3) The vertical distributions of most of elements show a low-high-low patterns, which are corresponding with the L0-S0-L1structure of soil section. The natural accumulation of heavy metal such as the contents of Cr, Ni, Cu, As, Zn, Pb, Mn, Mo, Co, V are obviously high in So layer, which maybe become potentially contaminated source in apple orchard. There are no obvious regular of Cd, Hg in the soil sections.(4) Some heavy metal such as Cu, Zn, Ni in the soil sections enrich and migrate several times, which means that these phenomena are not only related with pedogensis in the L0, S0, L1formation, but also related to the modern human activities. At present, the surface layers in apple orchard soils specially with longer planting ages in Baishui county have accumulated Cr, Cu, Zn, As, Cd, Pb, V, P, S. The enrichments are closely related to a long-term use of pesticides and fertilizer with harmful elements. The natural enrichments of Cr, As, Pb, V in S0layer (B layer) are related to migration and leaching of other easy migrated and adsorption of clay minerals. The Cu, Zn, Ni of the S0layer also showed enrichment in some apple orchard soil sections. Migration depth of heavy metals is generally0.3～0.7m, maximum depth migration will not be greater than1.5m. Based on the observation of CaCO3white film and migration of Ca, the author presents the migration depth of heavy metals in the soil mainly be controlled by gravity water.(5) Correlations between soil basic properties and the contents of elements show that clay, magnetic susceptibility, redness, yellowness are positively related with most trace elements, and soil sand content, brightness, carbonate content are negatively correlated with most trace elements. That is, clay, magnetic susceptibility, redness, yellowness, brightness, sand content, carbonate content are the main influence factors for elements distribution in soil sections. Whereas, the correlations between soil organic matter, pH value, silts and trace elements are uncertain. Based on the basic soil properties of So (B layer) and L1(C layer) from7sections, the author try to establish the elements regression equation. The predictive values of Pb, Cr, Cu, Ni, Zn, Mn, As, Ga are well correlated with the measured values. Then the regression equation was applied to predict the elements contents of A layer of soil, which attempts to identify the contribution of natural pollution and human activity in topsoil A layer contribution. The results show that the anthropic inputs are very limited to the enrichments except Cu, As, and the enrichments of heavy metals in So layer are natural pollution.(6) The analyses of elements contents from surface apple orchard soil indicate that the content change of P, K, Ca, Mg, S, Mn, Zn, Cu, Mo, Pb, Cd is closely related with the planting ages, the content of these elements showed the distribution characteristics of the function three times with the increasing ages.(7) The results of evaluation of the current situation of heavy metal pollution in Baishui county show that the contents of heavy metals are lower than the standard of green food producing area. The contents of Cr, As in So layers of5a,15a,20a,25a apple orchard soil sections approach to critical values. The Cr pollution index in25years apple orchard soil tillage layer is greater than0.7, which caused by anthropic input. However, heavy metal pollution contents in the surface soil are near to the critical level. Cd pollution index of all the apple orchard soil sections are higher than those of other’s heavy metals. This may be related to high Cd background value in this study area, not anthropic input just as the "natural pollution elements ". Based on the correlation between P elements and heavy metals in apple orchard, the author infers that effects of phosphate fertilizer have greater effects on Pb, Cd, followed by Zn, Cu, Cr, Ni, and little influences on As, Hg elements. The author carries out the regression analyses of heavy metal content and color, magnetic susceptibility of in the surface soil, it shows that the soil magnetic susceptibility, brightness, the redness can predict the content of Cr, Pb, Zn, Ni, As, which provide new approaches for rapid diagnosis of heavy metal content. |
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