Selenium fractionation and speciation in agriculture soils and accumulation in corn (Zea mays L.) under field conditions in Shaanxi Province, China.

Upland and paddy soils, as well as corn samples, were collected in the selenosis area of Naore Village, Ziyang County, Shaanxi Province, China. A five-step sequential extraction procedure was used for selenium (Se) fractionation, including soluble Se, exchangeable Se and carbonate-bound Se, iron and manganese oxide-bound Se, organic matter-bound Se, and the residual Se fraction. Species of soluble Se in upland soils included Se(-2), Se(4+), and Se(6+). The results showed that soluble Se and exchangeable Se fractions accounted for less than 1% of the total Se in the upland soil, but approximately 16.1% in the paddy soil. Concentrations of residual Se were lower than those of iron and manganese oxide-bound Se and organic matter-bound Se in both upland and paddy soils. Iron- and manganese oxide-bound Se was the dominant fractions in upland soil, whereas organic matter-bound Se abounded in paddy soil. Concentrations (mg kg(-1)) of Se in the corn samples ranged from 0.05 to 14.5 in seed, 0.31 to 12.3in root, 0.09 to 9.15 in stalk, and 0.16 to 36.15 in leaf. Path analysis indicated that soluble Se(6+) significantly (P<0.05) affected Se accumulation in corn tissues directly, whereas the organic matter-bound Se had a significant (P<0.05) indirect effect. In conclusion, corn did not readily absorb a major portion of soil Se. However, organic matter-bound Se was an important fraction and source of plant Se in agricultural soil.