Potential availability of fertiliser selenium in soils during flooding and subsequent aeration

Abstract

The partitioning coefficient (Kd) and potential availability of selenite (SeO3-2) or selenate (SeO4-2) as a percentage of Se applied (% Eadd values) were measured using isotopic dilution techniques in a study under variable redox conditions. Both Se species were added to a soil subjected to prolonged (30 d) submergence (low redox) followed by 7 d oxidising conditions (high redox). Even though applied Se had no effect on the SeO3-2 or SeO4-2 Kd values, time had a significant effect. Selenate Kd values were always lower than SeO3-2Kd values. Applied Se species had a significant effect on the SeO3 -2 %Eadd values. For both forms of applied Se <20% of applied Se was initially in the available pool as SeO3-2. However, after 14 d of submergence%Eadd values for SeO3-2 increased to 80%. Then availability decreased and did not increase during oxidation. Applied Se species, time and their interaction had a significant effect on the SeO4-2 %Eadd values. Selenate applied soils initially had more than 80% in available pool but this decreased with time and did not increase during oxidation. These results clearly show that irrespective of the species of Se applied to the soil, speciation changes depend on the changes in soil redox and pH. Adsorbed SeO3-2 and SeO4-2 were released into the soil solution with the reduction of Fe oxides and we possibly reduced to Se (0). Since there was no increase in the available pool of either Se species during the oxidation phase we can assume that fixed Se, as Se(0), did not oxidise readily during the 7 day oxidation period.