Study On Heavy Metal Deposition Mechanism In Ponds Of Forest-pond Complex System In Southern Hilly Region

In hilly regions of southern China,the most classic paddy field ecosystem is the forest-pond-rice agroforestry system,pond water is the main source of paddy irrigation water..In recent years,with the development of industrialization and urbanization,air pollution keeps increasing heavy metal input to the surface by atmospheric precipitation.Thus the amount of heavy metal from slope runoff has been increased rapidly which lead to higher heavy metal input into paddy field.Reducing the heavy metal content in the water can slow down the heavy metal accumulation in the paddy field;which is a key to reduce the risk of this system.Besides reducing the heavy metal content in the water of ponds,promoting the heavy metal deposition to the bottom of the pond is an another way.The dynamic transformation of heavy metals in various forms can reveal settlement rules of heavy metal including main existing form and how different forms translate.To figure out these rules,it can increase the heavy metal retention and decrease the heavy metal output,which provides theoretical and methodological support for mitigating the environmental threats of heavy metals to rice fields and reducing the ecological risks of hilly agroforestry complex ecosystem.This study was investigated in two types of forest-pond system in Dashanchong national forest park,Hunan Province of China,by laboratory simulation and laboratory simulation.The results showed that:(1)Cu,Zn,Pb and Cd in pond water are transformed from true solution form to collid form,and then get together to particle form,which accorded with"colloid pump" effect;the conversion process of Cd,Pb and Hg between colloid form and true solution form is reversible.(2)In ponds of Chinese fir-ponds complex system,Zn in particle form mainly exsisted with Fe2O3,Cu in particle form mainly exsisted with Al2O3 and organic carbon;In ponds of coniferous and broadleaved mixed forest-ponds complex system,Zn in particle form mainly exsisted with Al2O3,Cu in particle form mainly exsisted with Al2O3 and organic carbon;In ponds of bare land-ponds complex system,Cu in particle form mainly exsisted with Al2O3 and Fe2O3.(3)Heavy metal in pind water mainly come from atmospheric bulk deposition,which content is greatly affected by seasonality.The effect of environmental factors on heavy metal in different ponds are not the same,but Cu and Zn in the same pond are affected by the same environmental factors,and Pb and Hg are less affected by environmental factors.(4)Seasonal distribution dynamics of heavy metal in sediments all show that:Pb mainly existed in reduction state,Hg mainly exsisted in residual form,Pb and Hg in this form are relatively stable;Zn and Cu mainly exsisted in weak acid extraction state and residual form,and continuously transform to the residual form;Cd and As mainly exsisted in weak acid extraction state and reduction state.