Study On Controlling The Availability Of Mercury In Soil-plant Ecosystem And Its Mechanisms

Mercury is very harmful for human being and propagation. It is well known that the injury of the minamata disease in Japan to humans was produced by mercury pollution. Mercury pollution has been world widely recognized as a global environmental problem since 1 990s. More than 90% of mercury emitted to the atmosphere by the combustion of fossil fuels may deposit into the terrestrial ecosystem, and mercury originated from industrial and agricultural production may also enter the terrestrial ecosystem via various ways. As a result, the terrestrial ecosystem has become a recipient of mercury pollutant. Mercury in the polluted soils can be transported into the bodies of man and animals through food chain, causing potential damages. However, no too much attention has been paid to this aspect due to the concealment of Hg pollution in the terrestrial ecosystem. It has been shown recent years that Hg content in the edible part of the vegetable has already been close to or exceeded the Food Standard Limit in some areas. In the acid precipitation regions, this may occur even the Hg content in the atmosphere is low, indicating that polluted soil becomes an important Hg pollution source. The accumulation of Hg in the soil seriously threatens the health of human being. In order to nip in the bud, it is thus imperative to control the cycling of soil Hg in the terrestrial ecosystem and its accumulation in the crops so as to prevent soil Hg from contaminating the food chain. The strategies of precaution and restoration of the Hg contaminated soil are included by reducing the quantity of Hg in terrestrial ecosystem through the reduction of the use of fossil fuels, removal of Hg from the soils, and controlling the transferring and activating of the soil Hg pool etc. Engineering, biological and agricultural measures are taken for the purpose. Among these the agricultural measure is more practically suitable, and is in accordance with the farming habit and cultivation tradition in our country. However, studies on the regulation of the cycling of soil Hg in terrestrial ecosystem are not sufficient, and no effective measures have been put forward for preventing soil Hg from entering the food chain. On the other hand, many research have been done, for example the cycle of Hg in the terrestrial ecosystem, the bio-availability of soil Hg, and the relations between the activation of soil Hg and the environmental factors etc. These studies may be taken as beneficial references for the establishment of effective measures for preventing soil Hg from entering the food chain. For such a purpose, complete studies on the selection of controlling methods and the relevant mechanisms are necessary. Mercury occurs in various forms in soils after it enters into the terrestrial ecosystem, among these forms the soluble and exchangeable soil Hg and the gaseous Hg volatized from soil are the two phyto-available species. The former species is easy to be absorbed by plant root system, and transported to the upper parts of the plant, and the later species may get into plants through the stomas of leaves. Thus the main approach to preventing soil Hg from accumulating in plant is to reduce the transformation of soil Hg into these two forms. Many studies show that the more organic matter soil contents, the more Hg are accumulated in soil, indicating that the increase of soil organic matter is beneficial for Hg to be immobilized by soils. It has been proved that the ability of humic acids to absorb Hg is stronger than that of minerals in soils, and the adsorption capaci...