Impact of sulphur fertilisation on crop response to selenium fertilisation

UK wheat (Triticum aestivum L.) has a low selenium (Se) concentration and agronomic biofortification with Se is a proposed solution. A possible limitation is that UK wheat is routinely fertilised with sulphur (S), which may affect uptake of Se by the crop. The response of wheat to Se and S fertilisation and residual effects of Se were determined in field trials over 2 consecutive years. Selenium fertilisation at 20 g ha^-1 as sodium selenate increased grain Se by four to seven fold, up to 374 μg Se kg^-1. Sulphur fertilisation produced contrasting effects in 2 years; in year 1 when the crop was not deficient in S, grain Se concentration was significantly enhanced by S, whereas in year 2 when crop yield responded significantly to S fertilisation, grain Se concentration was decreased significantly in the S-fertilised plots. An incubation experiment showed that addition of sulphate enhanced the recovery of selenate added to soils, probably through a suppression of selenate transformation to other unavailable forms in soils. Our results demonstrate complex interactions between S and Se involving both soil and plant physiological processes; S can enhance Se availability in soil but inhibit selenate uptake by plants. Furthermore, no residual effect of Se fertiliser applied in year 1 was found on the following crop.