Iodine fractionation in agricultural and forest soils using extraction methods

Iodine is an essential micronutrient that enters the food chain from soil primarily via plant root absorption. However, only water soluble and exchangeable fractions are usually linked to the bioavailable iodine in soils. Therefore, an assessment of iodine pools in soils via sequential extraction can provide useful information on the availability of iodine for plant uptake. Moreover, studies that deal with iodine content and fractionation in non-contaminated agricultural and forest soils are scarce and unique. Thus, we have evaluated the total iodine content and its concentration in soil fractions extracted from agricultural and forest soil samples by single and sequential extraction methods followed by ICP-MS determination. Our results showed that the total iodine content in Slovak, non-contaminated soils is around the average value of referred inland soil iodine content. Most of the iodine was extracted from organically rich soils of the haplic cambisol rhodic (CMro), cutanic luvisol (LV) and stagnic cambisol (CM) type, with more than 2.5 mg.kg⁻¹ of iodine. Other soils contained less iodine, and its content decreased in the order of haplic chernozem (CH) > fluvisol (FL) > gleyic fluvisol (glFL). The correlation analysis highlighted that organic matter content plays the most important role in soil iodine retention, followed by concentration of amorphous and weakly crystalline forms of aluminum, clay fraction content, aluminum and iron minerals. Most of the iodine in the selected soils was determined in oxide-bound fraction, fraction bound in humic acids, and in soil residue. The other fractions contained less than 10% of the total iodine on average. The water-soluble fraction, which is theoretically the most bioavailable, contained only 4.4% of the total iodine on average. Furthermore, correlation analysis indicates that the changes in iodine fraction distribution are strongly dependent on soil organic carbon content and the content of metal oxyhydroxides. Our results highlight the significance of soil chemistry for iodine distribution and provide useful information for agricultural research and geochemistry, especially regarding the assessment of mobile and bioavailable fractions of iodine for plant uptake from agricultural and forest soils.