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, N-min. 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 N-min 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 N-min in autumn. This is considered very low compared to N-min 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 N-min. 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 N-min, 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. (C) 2002 Elsevier Science Ltd. All rights reserved.