In the course of an integrated research project vegetation dynarnics were investigated on long-term and one year fallows with high nitrogen load caused by former deposition of farmyard manure and slurry. At the same time the effect of set-aside was studied on fields which bad been conventionally cultivated. In comparison to a conventional field, the abandonment of an N-loaded field led to a lower species number and evenness. With increasing age of the old-fields, the number of species declined. Since the population of the perennial Artemisia vulgaris crashed in the fourth year of abandonment and annuals (Galium aparine) came to dorninance again the progress of succession was slower on the N-loaded field. One year of set-aside caused a huge increase of the soil seed bank of weeds. In comparison to a crop rotation without fallow the biomass and coverage of weeds were greater in the following years. However, large species-specific differences occurred. On the N-loaded field, fluctuations in the population size of several species were higher. Finally, the contribution of set-aside to nature conservation and environmental protection was discussed considering zoological and abiotic effects.