**1. Introduction**

Selenium (Se) is an essential micronutrient for humans and animals, but can lead to toxicity when taken in excessive amounts. Plants are the main source of dietary Se, but the essentiality of Se for plants is still controversial, although the beneficial effects of low doses of Se on plants have been reported in several studies [1–3]. The concentration of Se in plants is directly related to the concentration and the bioavailability of the element in the soil and the plant species [4]. Selenium reactivity in soils depends not only on its total content but also on its chemical form [5,6]. The mobility and plant-availability of Se in soil is controlled by numerous chemical and biochemical processes, as follows: sorption, desorption, microbial activity, the formation of organic and inorganic complexes, precipitation, and dissolution and methylation to volatile compounds [6,7]. Depending on the oxidation state, Se is present in soil as selenide (Se2 <sup>−</sup>), elemental selenium (Se0), selenite (SeO3 <sup>2</sup>−), selenate (SeO2−) and organic Se. The main factors controlling Se solubility and availability in soils are considered to be pH, oxidation-reduction potential (Eh), metallic oxy-hydroxides and clays, organic matter, microorganisms, and the presence of competing ions [6,8]. Comprehensive information regarding Se geochemistry and Se behavior

in soil–plant systems is included in the extensive reviews of Winkel et al. [6], Etteieb et al. [8] and Schivaon et al. [9]

The total concentration of Se in soils varies spatially, and the average global value is quite low at 0.4 mg kg<sup>−</sup>1, ranging between 0.01 and 2 mg kg−<sup>1</sup> [9,10]; soils containing less than 0.5 mg kg−<sup>1</sup> Se are considered as deficient. In humans, Se deficiency occurs when a dietary intake of Se is <40 μg/day and chronic toxicity is observed above levels of >400 μg/day [11]. WHO has recommended 50–55 μg/day Se in human diet [12–14]. It has been estimated that more than 1 billion people all over the world are suffering Se malnutrition, which makes them susceptible to health problems such as growth retardation, impaired bone metabolism and abnormalities in thyroid function [7,9,12]. Selenium deficiency has been reported in countries such as Canada, China, Scotland, Japan, New Zealand, Spain and USA [6,7,15,16]. Thus, numerous studies have been carried out aiming to enrich agricultural products with Se [17–19], and to examine the behavior of added Se in soils. Greece is also considered as an Se deficient area (daily Se intake <55 μg) [20], and very low selenium concentrations were recorded in Greek agricultural products such as soft and hard wheat, barley, oat, rye and corn [21]. However, published studies reporting on Se concentrations or describing the geochemical behavior of the element in Greek soils are missing from the literature. Considering that Greek soils are Se deficient, it is highly possible that in the future Se addition by fertilization can be proposed in order to enrich edible agricultural products. Thus, the geochemical behavior of Se in soils with different physicochemical properties should be examined to ensure the availability of Se for plant uptake and to restrict Se leaching. It is worth to note that Greek soils can be regarded as representative of Mediterranean soils, and any information on the geochemistry of Se in these soils can be projected and used for soils of similar characteristics formed and developed under comparable environmental conditions.

The purpose of the present study was to obtain data on the behavior of freshly added Se(IV) in acid and alkaline Greek soils with different physicochemical properties, and to evaluate the potential environmental risks arising from Se(IV) application. Thus, a batch experiment was conducted to investigate (a) the adsorption of different Se(IV) concentrations in the selected soils, (b) the desorption patterns of sorbed Se(IV) by using 0.25 M KCl as a desorbing agent, as well as (c) to determine the soil properties that mainly affect the sorption/desorption processes.
