Review Iodine Fractions in Soil and Their Determination Eva Duborská 1, *, Michaela Matulová 1 , Tomáš Vaculoviˇ c 1,2 , Peter Matúš 1 and Martin Urík 1   Citation: Duborská, E.; Matulová, M.; Vaculoviˇ c, T.; Matúš, P.; Urík, M. Iodine Fractions in Soil and Their Determination. Forests 2021, 12, 1512. https://doi.org/10.3390/f12111512 Academic Editor: Robert G. Qualls Received: 13 October 2021 Accepted: 29 October 2021 Published: 2 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Institute of Laboratory Research on Geomaterials, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina, Ilkoviˇ cova 6, 84215 Bratislava, Slovakia; michaela.matulova@uniba.sk (M.M.);vaca@mail.muni.cz (T.V.); peter.matus@uniba.sk (P.M.); martin.urik@uniba.sk (M.U.) 2 Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 62500 Brno, Czech Republic * Correspondence: eva.duborska@uniba.sk Abstract: Iodine is an essential micronutrient for most living beings, including humans. Despite efforts through various iodine deficiency disorders elimination programs, such as salt iodization, this problem still persists. Sub-clinical deficiencies have often an endemic character, and they are characteristic to certain areas, where iodine is low or inaccessible in the geochemical environment. Thus, the monitoring of iodine speciation and fractionation—especially in soil, drinking water, and food—is needed. Therefore, the aim of this work is to review the iodine distribution in most common soil fractions which relate to its bioavailability; and briefly summarize the available methods for their identification in the environmental matrices as well. Keywords: iodine; soil; iodine fractionation; sequential extraction; iodine determination 1. Introduction Iodine is an essential trace element for both humans and animals, and a necessary component for the synthesis of thyroid growth hormones. Therefore, the World Health Organization (WHO) recommends a daily iodine dose for an adult of approximately 150 μg. Insufficient or extensive iodine intake is attributed to various diseases, such as goiter, and hypo- and hyperthyroidism. Iodine deficiency can also cause miscarriages, endemic cretinism, and other diseases [1]. The occurrence of iodine deficiency disorders is often linked to locations where the iodine is deficient in the geochemical environment. The average iodine content in topsoil of inland areas is estimated at ~2.5 mg·kg −1 , comprising of 10% iodine in water-soluble form. Since soils usually contain more iodine than the parent rocks, the atmospheric deposition of marine aerosols, dust, and flying ash particles from volcanic activity is considered a major source of iodine in topsoil [2,3]. Some studies have shown a significant correlation between iodine concentrations in soil and associated vegetation. However, it may not necessarily imply a direct soil-to-plant transfer [4]. Dead plants’ biomass with accumulated iodine of atmospheric origin has been suggested as a potential source of iodine in soil [5]. Additionally, Roulier et al. [6] assumed that iodine soil concentration is influenced by litterfall, indicating humus represents its temporary storage. Roulier et al. [7] determined iodine content in various forest compart- ment. The main iodine reservoir is hummus with more than 1800 μg·kg −1 iodine. Trees contain 161 μg·kg −1 iodine from which green leaves and stembark accumulate the most of iodine. Lichens and mosses are also considered great reservoirs. Rainfall can cause slow migration of iodine downwards the soil profile [8]. Still, since iodine shows high upwards mobility due to capillary forces, the water level fluctuations may affect its redistribution in soil layers [9]. Besides its essentiality, the reason why the environmental mobility of iodine is a scientific concern is the potential risks of exposure to radioactive iodine isotopes that are considered dangerous for human health. Shaw et al. [10] reported that the full isotopic equilibrium between the native 127 I and 129 I has not been achieved even after 29 years Forests 2021, 12, 1512. https://doi.org/10.3390/f12111512 https://www.mdpi.com/journal/forests