Treatments Of CaCO₃ For The Soils Complexly Polluted By Cd And/or Acid Rain

A pot experiment was conducted to study effects of CaCO3 application to the soils with three levels of Cd pollution and with complex pollution of Cd and simulated acid rain, including changes in soil property, plant availability of Cd and part nutrient elements in soil, growth of Clycine max. plants, chlorophyll contents and SOD (superoxide dismutase) activities, and MDA (malondialdehyde) contents in the plant leaves. The results are given as follows:Application of CaCO3 significantly increased soil pH values, decreased exchangeable Cd contents, and reduced Cd uptake in Clycine max plants. Through a competition with Cd, Ca inhibited Cd accumulation in the overground parts. CaCO3 significantly increased the biomasses, chlorophyll contents, and SOD activities, and decreased MDA contents in the Cd-poIIuted soils, indicating that suitable application of CaCO3 could effectively mitigate Cd toxicity to Clycine max plants. Application of CaCO3 influenced availability of part nutrient elements in soil, for example, greatly increasing exchangeable Ca contents and decreasing exchangeable contents of K, Mg, Zn, and Mn, but having no distinct effects on exchangeable Fe and Cu. Meanwhile, this treatment significantly increased Ca contents, and decreased K, Mg, Fe, Zn and Mn contents in Clycine max leaves. No special influences were observed for Cu in the leaves. Diagnosis of lack of nutrition elements in Clycine max plants indicated whether or not applying CaCO3 resulted in lack of K, but applying CaCO3 aggravated lack of K. Although applying CaCO3 decreased contents of Mg, Fe, Zn and Mn in the plants, it didn't result in lack of these four elements.There were two factors determining the suitable application amount of CaCO3 in Cd-pollution soils. The first was mitigative effect of CaCO3 on Cd toxicity to the plants and the second was effects of CaCO3on nutrition balance in the plants. From the point of reducing Cd availability, a greater application amount of CaCO3 would result in less contents of soil exchangeable Cd and less uptake of Cd by the plants. The biomass of Cfycine max plants was used as the index to decide the best application amount of CaCO3 (BAACa) in this experiment: BAACa was 0.5 g/kg for the unpolluted soil, 2 g/kg for the soil polluted with 5 mg Cd /kg, and 4 g/kg for the soil polluted with 20 mg Cd /kg.Simulated acid rain caused decreasing soil pH, increasing exchangeable acidity and exchangeable Cd contents in the Cd-polluted soils, enhancing Cd contents in the plant roots, but had little effects on Cd accumulating in the overground parts. With increasing acidity of acid rain, the height and biomass of Cfycine max. decreased, but no special patterns were found for changes of chlorophyll contents, SOD activities, and MDA contents. In this experiment, the simulated acid rain with pH 3.5 significantly aggravated Cd toxicity to Cfycine max plants, but that with pH 4.5 changed Cd toxicity only a little.CaCO3 applying reduced the differences among soil pH values and exchangeable Cd contents, and eliminated increasing accumulation of Cd in the plant roots resulting from different simulated acid rain solutions. This indicated that CaCO3 applying increased soil buffering capacity to acid rain, depressed plant availability of soil Cd, and effectively mitigated complex pollution of acid rain and Cd.