Abstract
Chemical forms of soluble selenium were studied in the cultivated soils (n=48) collected from Kameoka Basin, Kyoto Prefecture. Soluble selenium was extracted with a 0.1 mol L^<-1> Na_2SO_4 solution (soil : solution=1: 10) in a boiling water bath for 30 min, and was fractionated into three fractions : organically-bound selenium and inorganic seleniums (IV) and (VI). 1) The mean content of total soluble selenium was 21.8 μg kg^<-1> with a range of 7.8-75.3 μg kg^<-1>, amounting to 7.6% of the total selenium content in the soils. 2) The mean contents of soluble inorganic seleniums (IV) and (VI) were 4.8 and 1.9μg kg^<-1>, respectively. On the other hand, the soluble organically-bound selenium content was 15.0 μg kg^<-1>, which amounts to 69% of the total soluble selenium. This indicates that organically-bound selenium is the predominant form in soluble selenium. 3) Soluble selenium increased with increasing pH, especially, the correlation coefficients between pH and selenium contents in the forms of organically-bound and inorganic selenium (IV) were 0.70 and 0.80, respectively. It is believed that organic matter containing selenium and selenium (IV) adsorbed in the soils were released as pH increased. 4) Although a positive correlation was found between selenium (VI) and pH, the amount of selenium (VI) showed a slight increase with increasing pH. Most inorganic selenium (VI) in the soil was considered to be leached out from the soil because the solubility of selenium (VI) compounds is generally greater than that of inorganic selenium (IV) compounds.
