A significant part of Se is associated with the organic pool (Tolu et al. , 2014) of soils but some organo-Se compounds still remains unidentified (Di Tullo et al. , 2015). Although the biogeochemical Se cycle is well documented and studied, the mobilization processes in this organic Se pool are still poorly understood as well as the biotic and abiotic factors controlling this mobilization and the temporal scale involved. We propose an original method that consists i) in labelling a non-accumulator ryegrass with 4 isotopes (78SeIV, 82SeVI, 15N and 13C) to study uptake, assimilation and speciation of SeIV and SeVI species in this plant (Versini et al. 2016) and ii) in examining during 10 months the decomposition of this isotopically labelled biomass added to a grassland soil either as shoot litter or as root litter. This original method allows us to monitor Se, C and N in the same fractions extracted from the first centimeters of soils, the residual litter and the vegetation.