| Heavy metal pollution is a phenonmenon that the amount of heavy metals in the system is beyond its carrying capacity, therefore, and causes ecological damage and environmental degradation. With the rapidly developing of industry and urbanization, the pollution in farmland ecosystems, especially heavy metal pollution becomes increasingly serious. In the context of exploring the amending technology of heavy metal pollution in the farmland ecosystems, it has an important practical and theoretical necessary to study the input, output, flux and distribution of heavy metal. It is necessary to understand the flow and distribution of heavy metal in farmland. It is also important to establish a systhesis theory to prevent the input from source, to control the distribution in flow direction, to rehabilitate the pollution.Here the data was obtained from survey and experiment to estimate the monthly average flux of the heavy metals'input, including air deposition, fertilization, irrigation and output, including surface runoff, seepage, and crop harvesting under different cropping patterns (i.e. M1:Zingiber officinale+Lactucapsativap sativa L.var.aspparagina+Z. officinale, M2:Zen mays+Z. officinale+Brassica rapa chinensis, M3:Z. mays+Ipomoea batatas+ B. juncea). Spatial and temporal distribution was also analyzed. Then, with the principal component analysis, the factors that affect the source and sink of heavy metal form the input and output flux of heavy metal in the farmland were identified. These included input from atmosphere deposition(X1), fertilization(X2), irritation(X3) and output from crop harvest(X4), seepage(X5), runoff(X6). Finally, a model was constructed to capture the transfer process of heavy metal in the complex field ecosystem. The results were summarized as follows:(1)In M1, the annual input magnitude of Pb, Cd and Zn were 87.0411,5.0418 and 112.441 mg·m-2·a-1 respectively. The annual output/input ratio was 75.897%,53.989% and 8.676% respectively. The Pb had the best just input-output, following Cd, and the last Zn with the worst. This crop pattern would cause Zn, Cd and Pb pollution in farmland.In M2, the annual input magnitude of Pb, Cd and Zn were 50.0054,1.7648 and 817.3761 mg·m-2·a-1 respectively. The annual output/input ratio was 24.305%,60.436% and 8.832% respectively. The annual fluxes of all heavey metals were positive. This indicated that farmland system absorbed lots of heavey metal. The flux of output/input was small. All these suggest that this pattern has a high risk of pollution of heavy metal in the long term.In M3, the annual input magnitude of Pb, Cd and Zn were-8.5073,-8.5378 and 416.3522 mg·m-2·a-1 respectively. The annual output/input ratio was 111.84%,321.954% and 33.876% respectively. The result revealed that it could rehabilitate the Cd pollution effectively and prevent Pb pollution to some extent, but not for Zn pollution.The annual output/input ratio of Pb showed the ranked order of M3>M1>M2, and Cd and Zn showed the similar trend. M1 and M2 have no rehabilitation effects on Pb, Cd and Zn pollution. On contrast, they would cause heavy metal pollution in the long run. M3 can rehabilitate Pb,Cd pollution, eapacially Cd, but not Zn.(2) The Pb content in soil showed M1>M3>M2, Cd showed M1>M2>M3, Zn showed M1>M3>M2. Pb and Zn inMl were significantly higher than M2 and M3.The Pb content in crop showed M1>M3>M2, Cd showed M3>M2>M1, and Zn showed M3>M2>M1 Pb and Zn in M3 were significantly higher than M1 and M2.The transfer and enrichnment ability depend on crop indentity. L. sativa L.var.aspparagina has a high potential enrichment for Pb. The enrichnment potential for Cd was:Z. mays> B. juncea> B. chinensis >Ipomoea batatas> Z. officinale> L. sativa L. var. aspparagina. The content of Cd in Z. mays showed leaf>stem>grain. The content of Zn in crops showed leafy vegetables> rhizome vegetables >grain crop.(3) The factors of the source and sink analyzed by principal component analysis can be attributed to two principal components, natural factors and human factors. Combineing with the results of regression model, we know that Pb and Cd is mainly affected by natural factors and Zn affected mainly by human activities.(4) We put up the regression analysis, using the contents of metal in the soil and the scores of two factors of the principal componnent analysis at different time-series got the relationship model between soil metal contents and each variables: YPb=6.101X1+2.211 X2+4.783 X3+2.301 X4+5.411 X5+6.285 X6; Ycd=0.110 X1-0.034 X2-0.007 X3+0.055 X4+0.073 X5+0.111 X6; YZn=-0.345 X1+6.846 X2+6.501 X3+1.900 X4-0.039 X5+1.351 X6.However, more study in-depth must be progressed to validate the fit of the model. |
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