Abstract
This article presents the data obtained from a field experiment in which grass-clover leys were fertilized with increasing N rates applied in either mineral N fertilizer and/or cattle slurry forms. The leys were composed of a 2-species mixture of white clover (Trifolium repens L.) and ryegrass (Lolium perenne L.) and a 4-species mixture of white clover, red clover (Trifolium pratense L.), festulolium (Festulolium braunii) and ryegrass. In total, eighty fields were established at two farm sites in the western part of Denmark on sandy soils and monitored for two herbage seasons (2018-2019). Dry matter yield, botanical composition, N concentration and the proportion of N derived from the atmosphere using the 15 N dilution method were recorded in the harvestable biomass after each cut. Furthermore, the specific growth, N uptake and quantitative biological N fixation of the species were determined. The dataset can be used to establish the N balance, to calculate the optimal economic fertilization rate based on grass-clover composition and to predict N leaching and residual effect. The data presented were thoroughly used and discussed in the research article "Contrasting effects of slurry and mineral fertilizer on N(2) fixation in grass-clover mixtures".