| Heavy metal pollution in farmland soil will directly affect the quality of crops and threaten human beings’ health. In order to study the heavy metal pollution status and distribution characteristic of farmland in Guanzhong Plain, soil and plant samples were collected in the mining area, sewage irrigation area together with greenhouse concentrated area, respectively. The heavy metal distribution characteristic and sources were analyzed, and the livestock manure pollution risk were evaluated. Meanwhile, laboratory simulation experiment and maize pot experiment were conducted to study the activation effect of calcareous soil acidification on heavy metals. The conclusions as follows:(1) The soil samples from farmlands in Guanzhong Plain mining area were collected and analyzed for concentrations of Cd, Cr, Cu, Pb, Zn. The results indicated that heavy metal concentrations in the topsoil(0~10cm) were highest and decreased with the depth of soil layers. Cd, Cu, Pb contents of topsoil were higher than those of Shaanxi Province soil background, while Cr, Zn contents were lower. The geostatistical analysis results suggested that heavy metal concentrations in topsoil decreased from west to east, and the highest concentrations were all around cement factory. There were significantly positive correlation among Cd, Cr, Cu, Pb, Zn. The principal component analysis was applied to estimate the sources of heavy metals. Cd in factor 1 was associated with anthropogenic activities, whereas Cr in factor 2 was mainly controlled by parent material, Cu, Pb and Zn were affected by both two factors. The quality of soils were evaluated with “Pollution Load Index”(PLI) and“Potential Ecological Risk Index”(PER), respectively. The results showed that coal mine area was clean or slightly to moderately polluted, while the cement factory area was moderately polluted. Heavy metals in soil were ranked by pollution index as Cd>Pb>Cu>Cr>Zn. In the study areas, Cr and Pb exceeding rates of apple samples were up to 1.3 % and 31.3 %,respectively, which indicated that coal mining and cement production have polluted the apple fruits.(2) The concentrations of Cd, Cr, Cu, Pb, Zn in soil, wheat and vegetable samples from sewage irrigated farmland in Xi’an, Shaanxi Province, were determined. The results indicated that the average contents of Cd and Pb were higher than those of Xi’an and Shaanxi Province soil background, respectively, and the average contents of Cu and Zn were higherthan those of Shaanxi Province soil background, respectively. There were significantly positive correlation between Cd and Pb in soils, while significantly and highly significant positive correlation among Cr, Cu and Zn, respectively. The principal component analysis showed Cd and Pb in soils were associated with anthropogenic activities, whereas Cr, Cu and Zn were predominantly controlled by parent material. The quality of soils were evaluated with potential ecological risk index and Nemerow pollution index, however, both of the results showed that Cd was the major pollution element in soil. In the study areas, Cr and Pb exceeding rates of wheat samples were up to 70.00 % and 80.00 %,respectively; Cd, Cr, Pb exceeding rates of fruit vegetable samples were 66.67%,100% and 66.67%,respectively;100% of flower and leaf vegetable samples exceeded standards on Cd, Cr, Pb. As a result, it is suggested that farmers should avoid cultivating high transfer factor plants, such as flower and leaf vegetable, meanwhile, sewage with Cd must be purified strictly before irrigation.(3) Over ten-year-old greenhouse soils and field soils from factory concentrated regions in Guanzhong Plain were collected, and the heavy metal contents of soil samples were analyzed. The results showed that the concentrations of Cd from greenhouse soils and field soils were all higher than Shaanxi province soil background, further more, 50% and 16.7% of soils exceeded the “National soil environmental quality standard”, while the concentrations of Cu, Pb, Cr were lower than the standard. The quality of greenhouse soils and field soils were evaluated with “Potential Ecological Risk Index” and “Index of Geo-accumulation”,respectively. The results indicated that 75% of greenhouse soils and field soils were moderately to severely polluted on Cd, respectively. The pollution status of Cu and Pb in soil were slighter, and there was no pollution on Cr.(4) The concentrations of Cr, Cu, Pb, Zn in livestock manures and feeds from livestock farm concentrated regions in Guanzhong Plain, Shaanxi Province, were determined to calculate the heavy metal accumulation rates and the manure safe service period. The results indicated that 76.00% and 8.00% samples of pig manure exceeded the limits of Cu and Zn,respectively, 7.69%, 4.35% and 8.00% of cattle, chicken manure and pig manure samples were out of limit on Cr, respectively. The concentrations of Cu and Zn in livestock feeds were more than those of Cr and Pb, and had a significantly positive correlation between those in manure. The principal component analysis results showed that the concentrations of Cu and Zn in manure primarily controlled by livestock feeds, while Cr and Pb contents in manure were affected mainly by soil and atmosphere. The morphological analysis of Cu and Zn showed that the availability of Cu and Zn in manure were very high, especially in chicken manure. The concentrations of Cu and Zn would exceed the safe limits from soil background values in 24.4~131.0 and 69.7~91.9 years under the manure fertilization situation,respectively. As a result, it’s urgent to control the manure overuse in agriculture and reduce the addition of Cu and Zn in livestock feeds.(5) The laboratory simulation experiment and maize pot experiment were conducted to study the dynamic change of p H, available-Cd, available-Cu, available-Zn, available-Mn and available-Fe after acidification in calcareous soil, and the relationship among p H and the availability of trace metals, meanwhile, to estimate the suitable acidification range on calcareous soil. The results of laboratory experiment showed that, soil p H decreased at the initial stage of acidification, then increased gradually. There were significant relationships(P<0.01)between p H and available trace metal concentrations. When soil p H is lower than7.11, the content of available-Cu, available-Zn, available-Mn and available-Fe in soil could be higher than the soil nutrient deficiency threshold. Maize pot experiment results indicated that there were significant relationships between p H and available-Cd, available-Mn and available-Fe, respectively(P<0.01), no significant relationships between p H and available-Cu,available-Zn. Due to growth of maize and root exudates, the available trace metal concentrations in soil decreased after elongation stage. The Cd concentration of maize plant was too low to be detected, while Cu, Zn, Mn, Fe were mainly concentrated in root and leaf of maize, and the concentrations of them ranked as Fe>Mn>Zn>Cu. According to the estimation,when the soil p H is in the range of 5.37~6.98, the contents of available-Cu, available-Zn,available-Mn and available-Fe in soil could meet the demand of crop growth, but simultaneously it would activate soil Cd and bring pollution risk. |
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