Effect of soil properties on time-dependent fixation (ageing) of selenate

Previous biofortification studies have established that the residual effect of added selenium (Se) fertilisers on second-season crops is minimal. To explore the fate of exogenous Se in soil, chemical and biological methods were employed to assess the change in Se bioavailability with time. Eight soils varying in physicochemical properties were spiked with sodium selenate (0.5 mg kg(-1) Se) and incubated at 25 degrees C for different periods (1, 30, 60, 90 and 300 d). At the end of the incubation, soil Se was fractionated by a sequential extraction procedure into 'soluble', 'adsorbed' and 'organically-bound' Se fractions. Simultaneously, wheat was grown in the Se-aged soils, under controlled conditions for six weeks, and uptake was determined. A general decrease in Se solubility over time was observed, but the rate of decrease varied depending on soil type. A reversible first order model fitted the Se ageing kinetics well, except in an Oxisol. The most pronounced ageing was observed in calcareous soils. Concentrations of Se in the shoots of wheat grown in freshly spiked soils ranged from 71.8 +/- 17.5 mg kg(-1) in calcareous soils to 110 +/- 31.6 mg kg(-1) in non-calcareous, low-OM soils. With ageing, shoot Se concentrations decreased to < 10 mg kg(-1), with the lowest concentrations (0.31 +/- 0.03 mg kg(-1)) measured in plants grown in calcareous soils. The Se concentrations in plants correlated well with the soluble Se fraction (extracted with 0.01 M CaCl2). The findings of this study can be used to make more informed decisions about the rate and frequency of Se fertiliser application in agronomic biofortification programs.