Limits to nitrogen use on grassland

Data from nitrogen (N) response experiments on grassland in Belgium and the Netherlands were analysed with the help of a descriptive crop N response model, to identify permissible doses below which no accumulation occurs of residual mineral soil N in autumn, Nmin. Using different years as separate sets, a total of 29 data sets were obtained from eight locations on various soil types. A large variation was found in Nmin base levels (unfertilised) between locations and between years at a given location. For doses low enough not to affect crop N recovery, every 100 kg N applied was associated with 3–4 kg residual Nmin in autumn. This is considered very low compared to Nmin base levels, but values differed significantly from zero. After normalising N-doses from different sources (mineral fertiliser and cattle slurry) with the help of a coefficient expressing effectiveness based on crop N uptake, no difference was found between fertiliser and slurry in terms of their effect on residual Nmin. The above also holds for nitrate leaching as measured. The sources do differ, however, with respect to long-term effects and these are quantified with a first-order approximation. It it shown that, also after incorporation of long-term effects, much higher N-doses on grassland are justified than the 170 kg N per ha per year in animal manures currently proposed by the European Commission. On normal productive cut grassland as in the analysed experiments, total N doses in cattle slurry up to 400 kg per ha per year have very little effect on residual Nmin, if not accompanied by high fertiliser doses. Introducing limits to the use of animal manures on grassland without limiting the input of mineral fertiliser-N lacks any scientific ground.