ABSTRACT Sorption of selenate (SeO 4 2- ) and selenite (SeO 3 2- ) is poorly understood in Brazilian agroecosystems, especially in soils from agricultural areas containing different contents of competing anions, such as sulfate (SO 4 2- ). This study aimed to assess the sorption behavior of selenate and selenite at different soil layers of a tropical soil treated with different rates of agricultural gypsum (thus, containing different contents of sulfate), collected under a coffee plantation. Soil samples from an experimental area where phosphogypsum has been previously applied at different rates (0, 7, 14, and 56 t ha -1 ) were taken at the following soil layers: 0.15-0.25, 0.35-0.45, and 1.25-1.35 m. Adsorption experiments were carried out adding 20 mL of solutions containing 100 and 500 μg L -1 of selenate and 10 and 15 mg L -1 of selenite to 2 grams of soil. Desorption experiments were also performed using a soil:solution ratio of 1:10. Adsorption of selenate increased with soil depth and decreased upon increasing sulfate contents in the soil, by contrast, selenite was consistently adsorbed at higher contents - when compared with selenate - at any soil depth and its sorptive behavior was not affected by the presence of sulfate. Furthermore, selenite was less desorbed than selenate under all conditions. In conclusion, selenite is much more retained in tropical soils and less available to plants than selenate. Also, although sulfate has shown to be able to hinder selenate retention, it has no substantial effect on the sorption behavior of selenite in tropical agroecosystems.

RESUMO A sorção de selênio (Se) em solos pode ser influenciada por fatores que são alterados pelas práticas agrícolas, as quais são geralmente realizadas para a produção agrícola, tais como o pH do solo, anions competitivos e conteúdo de matéria orgânica, entre outros. Este estudo teve como objetivo avaliar a influência do manejo do solo e da força iônica (FI) na retenção de Se em solos brasileiros, o que é importante para prever a disponibilidade de Se em ambos os sistemas, solos nativos e cultivados. Para isso, as reações de adsorção e dessorção de Se foram avaliadas em 16 amostras de solo (oito de solos cultivados e oito de solos nativos), utilizando soluções contendo 100 e 500 μg L-1 sob a forma de selenato de sódio e em duas FI, 15 e 150 mM. A adsorção variou em função da FI para a maioria dos solos, o que pode indicar que o mecanismo de adsorção por complexo de esfera externa é importante para a retenção de selenato nos solos estudados, mas, estudos futuros envolvendo outras técnicas, como técnicas espectroscópicas, são requeridos para confirmar claramente o mecanismo de adsorção. Em geral, a adsorção variou em relação às características do solo, sendo que os solos nativos adsorveram maiores quantidades de Se que os solos cultivados. Em termos de dessorção, os solos que apresentaram as maiores quantidades adsorvidas de Se também apresentaram as maiores quantidades dessorvidas. Finalmente, a FI e o manejo do solo influenciaram a disponibilidade de selenato nos solos estudados.

ABSTRACT Sorption of selenium (Se) on soils may be influenced by factors that are changed though agricultural practices usually performed for crop production, such as soil pH, competing anion and organic matter contents, among others. This study aimed to evaluate the influence of soil management and ionic strength (IS) on Se retention in Brazilian soils, which is important to predict Se availability in both systems, native and cultivated soils. For that, adsorption and desorption reactions of Se were evaluated in 16 soil samples (eight from cultivated soils and eight from native soils), using solutions containing 100 and 500 µg L-1 Se in the form of sodium selenate and in two IS, 15 and 150 mM. Se adsorption varied as a function of IS for most soils, which may indicate that the adsorption mechanism by outer-sphere complex is important for selenate retention in the studied soils, but future studies involving other techniques, such as spectroscopic studies, are needed to more clearly confirm the adsorption mechanism. In general, adsorption varied in relation to soil characteristics, and native soils adsorbed larger Se amounts than the cultivated soils. In terms of desorption, soils which had larger Se adsorbed amounts also presented larger desorbed amounts. Finally, IS and soil management influenced the availability of selenate in the studied soils.