Inorganic nitrogen (N) loading was simulated by the catchment scale INCA-N model from two large river basins with contrasting land use. The main aim was to analyze the timing and origin of inorganic N loading and the effectiveness of different water protection methods. Predicted changes in precipitation and temperature increases the nutrient load from catchments to water bodies in future climate. The total inorganic N load from the forested Simojoki river basin located in northern Finland was about 5% of that from the Loimijoki river basin in south western Finland. In the Loimijoki river basin agriculture dominated inorganic N loading. When applying realistic water protection methods (limits on manure spreading) the simulated inorganic N load from the river basin decreased by 11%. With more drastic methods (no manure spreading + catch crop) a decrease up to 34% was achieved. In the Simojoki river basin there were several equally significant sources, so suitable combinations of different water protection measures would be the most efficient way to decrease the inorganic N load. As the inorganic N load may be composed of very different sources, depending on land use in the river basin, efficient allocation of water protection measures requires detailed analysis of different sources of loading.