Impact And Mechanism Of Organic Amendments On Cadmium Bioavailability In Soils

Cadmium contamination has great potential thread to ecosystem, food safety and human health. It has been an important environmental problem widely concerned. There are abundant organic materials in China. Application of organic materials to reduce mobility of cadmium not only adapts to national conditions of China but consists with the demand of sustainable agriculture. However, the effect of organic materials on the availability of cadmium and the related mechanisms is still unclear and inconsistent. Straw and manure are the main organic materials in China. So, it is necessary to study the effect of straw or manure on the availability of cadmium in soils and the related mechanisms in order for appropriate application of organic materials in reducing the availability of cadmium in contaminated soils.In this study, crop straw or pig manure was applied in Cd contaminated soils to investigate: the effect of straw or pig manure amendment on the bioavailability of cadmium in soils; the effect of straw or pig manure amendment on changes of cadmium fractions in soils; the mechanisms of the effect of straw or pig manure on cadmium availability. The main findings were obtained as follows:1. Pot experiments with cherry-red radish and winter wheat were conducted to study the effect of wheat straw or compost pig manure amendment on phytoavailability of cadmium in Cd contaminated soils and the mechanisms. Results showed that cadmium concentrations in radish tissues and the distribution of cadmium to edible part increased for the first harvest when straw was applied to calcareous soil and acidic red soil. But cadmium concentrations in radish tissues and the distribution of cadmium to the edible part decreased for the second harvest after the application of straw. Application of pig manure reduced cadmium concentrations in radish tissues and the distribution of cadmium to the edible part in both soils. Straw or pig manure amendment in Fluvo-aquic soil and yellow brown soil significantly reduced cadmium concentrations in wheat tissues and Cd distribution to the edible part (grain). The reduced effect of pig manure amendment on cadmium concentrations in plants was more significant than that of straw application. The reduced effect of manure on cadmium concentrations in plant tissues in red soil and yellow brown soil was more significant than in Fluvo-aquic soil.2. A soil incubation experiment and pot experiments were conducted to study the effect of straw or pig manure amendment on changes of cadmium fractions in soils. In soil incubation experiment, exchangeable cadmium in Fluvo-aquic soil and red soil increased significantly and other fractions decreased during the first 45 days after the application of straw. This increasing effect decreased with the incubation time and was not significant after incubation for 90 days. During the whole incubation period of 90 days exchangeable cadmium decreased and other fractions increased when pig manure was applied. When planting cherry-red radish exchangeable cadmium increased and other cadmium fractions decreased by the first harvest when straw was applied to Fluvo-aquic soil and red soil. Whereas exchangeable cadmium decreased and other cadmium fractions increased by the second harvest after the application of straw. While application of pig manure decreased exchangeable cadmium and increased other cadmium fractions by both harvests. When winter wheat was planted the exchangeable cadmium decreased and other cadmium fractions increased whether straw or pig manure was applied to Fluvo-aquic soil and yellow brown soil. Stepwise multiple regression analysis indicated that cadmium concentrations in radish and wheat tissues could be explained by the variations of exchangeable and/or organic bound cadmium in soils. Cadmium concentrations in radish and wheat tissues were positively correlated with the content of exchangeable cadmium and negatively correlated with the content of organic bound cadmium in soils. These results showed that straw or pig manure could affect cadmium concentrations in plants by changing cadmium fractions in soils. The reduced effect of pig manure on exchangeable cadmium in red soil and yellow brown soil was more significant than in Fluvo-aquic soil.3. Soil pH was affected by the application of straw or pig manure. The change of soil pH in Fluvo-aquic soil and yellow brown soil was not significant and the pH in red soil significantly increased after the application of straw. The pH in Fluvo-aquic soil significantly decreased, whereas the pH in red soil and yellow brown soil significantly increased when manure was applied. When straw was applied to Fluvo-aquic soil, red soil and yellow brown soil, a significant positive relationship existed between soil pH and exchangeable cadmium fraction. But when composted pig manure was applied to red soil and yellow brown soil, there was a significant negative relationship between soil pH and the exchangeable cadmium fraction. These results suggested that cadmium availability be reduced by changes of soil pH only when pig manure was applied to red soil and yellow brown soil.4. Although humic acid and fulvic acid increased when straw or pig manure was added to the soils, the changes of soil HA/FA ratio were different. The HA/FA ratio of soils without planting decreased when straw was applied to Fluvo-aquic soil or red soil but this reduced effect diminished with incubation time. When cherry-red radish was planted soil HA/FA ratio decreased by the first harvest and increased by the second harvest after application of straw to Fluvo-aquic soil and red soil. When wheat was planted soil HA/FA ratio increased when straw was applied to Fluvo-aquic soil and red soil. Amendment of pig manure increased soil HA/FA ratio in soils with or without planting. The effect of pig manure on the increase of soil HA/FA ratio was more than that of straw and this effect in red soil and yellow brown soil was more than that in Fluvo-aquic soil. Stepwise multiple regression analysis indicated that exchangeable cadmium fraction which was regarded as the most available form in soils was negatively correlated with HA/FA ratio and can be explained by the variation of HA/FA ratio in soils. Soil incubation experiment by the addition of HA or FA in soils further demonstrated that the exchangeable cadmium decreased while carbonated bound and oxide bound cadmium increased by increasing soil HA/FA ratio.In summary, the main mechanism for the effect of organic material on phytoavailability of cadmium in soils was that amendment of straw or pig manure changed soil HA/FA ratio which affected cadmium fractions especially the exchangeable Cd fraction. This result suggested that organic materials should be composted to increase the ratio of HA/FA before using to reduce the availability of cadmium in soils.