Uptake competition between selenium and sulphur fertilizers in sequential harvests of ryegrass: A stable isotope study

The demand for sulphur (S) fertilizers has increased due to declining S emissions from coal burning and subsequent deposition on agricultural land. Slow-release S may only be beneficial for grass Selenium (Se) status initially and less beneficial for the later harvests. Applied selenate is more phytoavailable than selenite, however, its residual availability is reduced to a very low level within a short time. Besides, applied S seems to enhance residual selenate phytoavailability, especially when soil environmental S status is decreased in later harvests. In contrast, applied selenite is rapidly adsorbed in soil and can be a stable long-term Se pool available for plant uptake albeit at a comparatively low level of availability. The Se isotopic composition was analyzed in the harvested grass samples. Slow-release S may only be beneficial for grass Se status initially and less beneficial for the later harvests.