Cadmium fractions in soils as influenced by application of organic materials

A soil incubation experiment was conducted to study the effects of fresh straw or composted pig manure application on the fraction of cadmium (Cd) in soils. Results showed that exchangeable Cd in soils decreased with the incubation time, carbonate and oxide bound Cd first increased and then decreased, whereas organic bound and residual Cd increased with incubation time. Application of straw increased the exchangeable Cd, but this increase weakened with the incubation time. Application of pig manure decreased exchangeable Cd. The variation of exchangeable Cd was mainly affected by organic bound or residual Cd. The effect of straw or pig manure on the transformation of Cd fraction in soils was related with the changes of soil humic acid (HA) and fulvic acid (FA). When straw was applied, the increase of FA in the soils was more than HA and therefore HA/FA ratio decreased. While when pig manure was applied the increase of HA in the soil was more than FA and HA/FA ratio increased during incubation period. Results also showed that straw or pig manure amendment significantly influenced soil pH, but only pig manure could reduce Cd solubility by increasing soil pH in acid Ferralsols. The transformation of Cd in soils applied with straw or pig manure from low mobility fractions to exchangeable fraction was negatively correlated with HA/FA.