| As a sustainable land-use type and business model, agroforestry system not only can deal with the contradiction between agriculture and forestry, but also improve resource efficiency. The research on soil heavy metal spatial variability in agroforestry ecosystem is an effective way to study the temporal and spatial regulation of soil heavy metal, which can insure the safety of agricultural product and the health of rural ecosystem. This paper focuses on the slash pine-paddy ecosystems and conifer-Paddy agroforestry system of the Dong-ting Lake. By traditional statistics and Geo-statistics analysis, writer researches the content of heavy metal in surface soil (0-20cm), spatial variability, the origin of heavy metal, variation tendency and cause of formation, the environmental effect of agroforestry ecosystem could be analyzed by comparing with the soil heavy metal content in plain paddy field ecosystem, which can provide a significant way to reduce the heavy metal pollution in paddy soil. The result shows as follows:(1) the mean content of Cd in conifer-Paddy soil is bigger than that of the two ecosystems, which is 2.33 times than the background value in Hunan and 3.33 times than that in China. The content of Cd in three ecosystem are higher than the second level criterion of standard of soil environment (GB15618-1995). And the Cu and Cd in soil present a continuous accumulation trend during a short time. The content of Pb in soil of slash pine-paddy ecosystem and plain paddy ecosystem is qualified for the second level criterion of soil. The other heavy metal content don’t beyond the soil background value.(2) The parameters of soil heavy metal content or their logarithms obey normal distribution. At transplanting stage, Gaussian function is used to simulate the semivariograms of Cu and Cd in slash pine-paddy ecosystem; spherical model is used to simulate the semivariogram of Pb; exponential model is used to simulate the semivariogram of Zn and Mn. The results of semivariance analysis showed that Cu and Pb in conifer-Paddy agroforestry system are fit for the Gaussian function, and Cd, Zn and Mn fitted for exponential model. At tillering stage, the result of semivariance analysis showed that Cu and Cd in slash pine-paddy ecosystem are fit for the spherical model; Pb and Zn are fit for the Gaussian function; Mn is fit for the exponential model. The result of semivariance analysis showed that the Cu in conifer-Paddy agroforestry system is fit for spherical model; Cd, Pb and Zn are fit for Gaussian function; Mn is fit for exponential model. At maturity stage, the result of semivariance analysis showed that Cu and Pb in slash pine-paddy ecosystem is fit for Gaussian function; Cd is fit for spherical model; and Zn, Mn are fit for exponential model. The result of semivariance analysis showed that Cu, Pb, Cd and Zn in conifer-Paddy agroforestry system are fit for Gaussian function; Mn is fit for exponential model. The scale of spatial autocorrelation of heavy metal are ranked as Mn>Zn>Pb>Cu>Cd. The piece of base of Mn in slash pine-paddy ecosystem and conifer-Paddy agroforestry system are less than 25%, showing significantly spatial correlation. The piece of base of Cu, Cd, Pb and Zn range from 25%-75%, showing medium correlation. However, affected by fertilization at tillering stage, the piece of base of Cd in slash pine-paddy ecosystem and Cu in conifer-Paddy agroforestry system increased to 79.2% and 78.2% respectively, showing the weaker spatial correlation(3) Ordinary Kriging The spatial pattern of Cu in slash pine-paddy ecosystem comes out as obvious patches; and the distributional characteristic of Cd is combing strips and patches.the mean content of Cu and Cd in hollow are higher than these in ridge. The spatial pattern of Cu and Cd in conifer-Paddy agroforestry system comes out as strips. And the concentrations of Cu and Cd in southeast are higher than other place. In the two complex ecosystems, the contents of Pb and Zn in lower place are inferior to these in upper place. The spatial patterns of Pb and Zn in conifer-Paddy agroforestry system are more stable than these in slash pine-paddy ecosystem. The spatial distribution of Mn comes out as combing strips and patches, and the spatial of Mn is more stable than other heavy metals.(4) the origin of heavy metal in the two complex ecosystems could be concluded by Pearson related coefficient and geo-statistics. The Cu and Cd come from fertilization; Pb and Zn come from mineral production, industrial waste gas and automobile exhaust. Mn comes from soil parent material. There a negative correlation between Pb in soil and elevation. The Cu and Cd have a highly negative correlation with altitude in slash pine-paddy ecosystem. And the Cu and Cd have a negative correlation with altitude in conifer-Paddy agroforestry system. The Cu, Pb and Cd in soils are enriched to lower place.(5) The mean contents of Cd, Pb, Zn and Mn in the two complex ecosystems are lower than these in paddy ecosystem at the same growth stage, which showing obvious reduction on soil heavy metal. But the enrichment of heavy metal to lower area lead to increasing risk of heavy metal pollution on paddy. Depending on the analysis of the origin, distribution and transportation law of heavy metal in two complex ecosystems, some measurements can be used to control soil heavy metal pollution. First of all, rational application of fertilizer could decrease the heavy metal inputs.secondly, changing the understory layer and litter in ecosystems to control surface runoff, which can reduce the transportation from forest to paddy. Thirdly, digging slub ditch or pond to keep soil in paddy ecosystem from heavy metal pollution. |
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