Kinetics and equilibria of cadmium in selected Kenyan soils

Abstract

Cadmium (Cd) is ubiquitous in agricultural soils and is of concern because of its potential toxicity to the ecosystem. The objectives of this study were to investigate (1) the Cd status and speciation and the fate of applied Cd in selected tropical soils in Kenya and (2) the effect of soil properties, low-molecular-weight organic acids (LMWOAs), and the application of monoammonium phosphate (MAP) and potassium chloride (KCI) fertilizers on the kinetics of Cd in the soils. The total Cd and the Cd availability index (CAI) which was taken as arrnnonium acetate-acetic acid-ethylene diamine tetraacetic acid (AAAc-EDTA) exaactable Cd generally decreased with depth and varied with the soil type. Exchangeable and carbonatebound Cd were not detected in the natural tropical soils studied. Metal-organic complex bound-Cd was the most predominant in the surface soils, whereas in the subsurface soils, Cd was present mainly in the residual form. Metal-organic complex-bound Cd significantly contributed to the CAI of the soils studied. With increasing residence time, the Cd added to the soils in the form of the Idaho MAP-fertilizer was transformed to a series of pamculate-bound Cd species. The residual Cd species steadily increased, whereas the rest of the particulate-bound Cd species decreased. The M WC1 extractable Cd also decreased with the residence time and the extent of the decrease depended upon the soil type. The LMWOAs influenced the Cd released from the natural soils and the soils treated with the Idaho MAP-fertilizer to solution through the formation of Cd-LMWOA mmplexes. The LMWOAs commonly present in the soil rhizosphere were effective in mobilizing the soil Cd, especially in the phosphate treated soils. The KC1 fertilizer promoted the release of Cd fkom the soils particularly after treatment with the Idaho MAP-fertilizer. The release of Cd to solution by KCl was attributed to the combined effects of Cd-cMoride complexation and ionic strength. The application of Idaho MAP-fertilizer gnatly enhanced Cd released fmm the soils Ammonium taranalcite was identified as a reaction product in the MAP treated acidic tropical soils using X-ray diffraction analysis. In the fertilizer granule-soil interface, the forhation of m-taraoakite may be pemrrbed by Cd, if the applied MAP fertilizer contains sufficiently high Cd as an impurity. The fmdings obtained in the present study are hndamental for establishing a Cd database and are of significance in interpreting the importance of speciation, dynamics and fate of Cd in its bioavailability in tropical soils. The impact of fanning practices such as crops and cropping systems and application of fertilizers on Cd contamination of the fwd chain deserves atten tion.