Pathways and kinetics of redistribution of cobalt among solid-phase fractions in arid-zone soils under saturated regime.

An adequate supply of Co in pasture is important to the health of grazing animals. Bio-availability of Co in soils is largely depended upon its distribution among solid-phase fractions. Distribution of cobalt in six arid-zone soils and its redistribution among the solid-phase fractions during long-term saturated paste incubation were studied. Cobalt was fractionated by a selective sequential dissolution procedure into six empirically defined fractions. Concentrations of total Co and Mn or Fe, and Co and Mn fractionation pattern were strongly correlated in the soils. During saturated incubation, Co in soils was redistributed mainly from the Mn oxide bound, and to some extent, Fe oxide bound and organic matter bound fractions into the carbonate bound fraction. During saturated incubation, significant correlations were found between concentrations of Co and Mn in the Mn oxide bound, Fe oxide bound and carbonate bound fractions. Also, significant correlations between concentrations of Co and Fe in the Fe oxide bound fraction were present. However, a negative correlation between concentrations of Co and Fe in the Mn oxide bound fraction was observed. The rates of redistribution of Co between these solid-phase components were initially high: major changes occurred in the first 3 days in the sandy soil and the first 18 days in the loessial soil. Afterwards, the rates of change slowed but changes in redistribution continued during the rest of the study period of one year.