Purpose The contamination of agricultural soils by heavy
metals is a worldwide problem. Organic amendments can
be used for the immobilization and binding of heavy metal
ions in soils by complexation, adsorption, and precipitation.
A field trial was carried out to evaluate the influence of
some low-cost organic materials such as rice straw (RS),
green manure (GM), and pig manure (PM) on the
distribution of Cu and Cd and the retention of these metals
by organic matter fractions in heavy metal-polluted soils.
Materials and methods The experiment was conducted in
Miaoyunao Village, Daye County, Hubei province, China.
PM, GM (peanut plants), and RS were obtained from a farm
close to the village. Sixteen treatments with three replicates
were designed. Soil chemical properties such as soil pH,
electrical conductivity (EC), organic matter (OM), and
available P were measured by standard methods. Soluble/
exchangeable, organic-bound, inorganic precipitates and
residual Cu and Cd in the soil were sequentially extracted
and analyzed. The amounts of Cu and Cd bound with soil
particulate organic matter (POM) fractions and humic
substances were also determined.
Results and discussion The addition of organic amendments
declined significantly the concentrations of soluble/
exchangeable Cu and Cd, but increased the amounts of
these metals in organic-bound and inorganic precipitate
forms in the soil. RS was more effective than GM and PM
in diminishing the solubility of Cu and Cd. The largest
retention for Cu and Cd by humic substances and POM was
noticed in RS treatments, whereas the lowest was found in PM
treatments. Humic substances showed higher potential in the
fixation of Cu and Cd than POM fractions. The conversion of
soluble/exchangeable Cu and Cd to other insoluble forms
after the application of organic amendments may be ascribed
to the increases of soil OM, pH, EC, and available P contents.
The highest binding of Cu and Cd with POM fractions and
humic substances after the incorporation of RS mainly
resulted from the greatest increase of soil OM contents.
Conclusions RS, GM, and PM can be employed as good and
cheap substances for the immobilization of Cu and Cd in
heavy metal-polluted soils. RS was the best amendment in
decreasing the solubility of Cu and Cd, and also in enhancing
the retention of these metals by humic substances and POM
fractions in the soil. Futures studies should focus on the
influence of these organic amendments or their mixtures on
the phytotoxicity of Cu and Cd for different plants in heavy
metal-contaminated soils. |
