Soil Potassium Fractionations, Release and Fixation in a Cadmium Contaminated Soil Treated with Plant Residue and Biochar

s Potassium (K) equilibration, release and fixation in the cadmium (Cd) contaminated soils may be affected by organic amendments application. The effect of 1.5% corn, cotton and wheat residues and their produced biochars and 150 mg kg –1 Cd application on different soil K forms, K release to 0.01 M CaCl 2 and 1% wood vinegar and K fixation of a coarse-textured calcareous soil was studied. Plant residues and their produced biochars increased soluble K content by 24–69 and 22–99 mg kg –1 , respectively. The exchangeable K content was also increased 57–191 mg kg –1 after organic amendments application. Contamination of the soil with Cd increased the soluble K content by 11–23 mg kg –1 . Cadmium application also increased exchangeable K and decreased non-exchangeable K of the biochar-treated soils. Exchange of Cd with exchangeable K and Ca and non-exchangeable K and subsequently exchange of soluble Ca with non-exchangeable K may be the main mechanisms for changes in K fractionations. CaCl 2 and wood vinegar extracted 500 and 502 mg K kg –1 , respectively, with non-significant difference. The main mechanism of K release with both solutions is the exchange reaction of non-exchangeable K with soluble Ca (originated from CaCl 2 or dissolution of CaCO 3 by wood vinegar) and it seems that acidic wood vinegar is not able to dissolve K-bearing minerals (micas and feldspars). Potassium released from the soil increased 32–338 mg kg –1 in the uncontaminated soil samples and 58–408 mg kg –1 in the Cd-contaminated soil samples after organic compounds application. Potassium fixation in soils treated with CaCl 2 and wood vinegar was 124 and 113 mg kg –1 , respectively and decreased with organic amendments application. Organic acids in wood vinegar may enter to the interlayers of clay minerals or adsorbed on the mineral edge and decrease layer collapse and K fixation. Generally, it is concluded that contamination of soil by Cd and remediation of soil with organic compounds application may increase the K availability of K-depleted calcareous soils via K addition to soil and release of native non-exchangeable K. In addition, an increase in soil soluble K by 21–230% may increase the risk of K leaching, soil salinity and cations equilibration of soil solution in long-term periods and this should be considered in K fertility management strategies of K-depleted soils.