Revision of nitrogen critical loads for Natura 2000 Habitat types in The Netherlands.

Critical loads for atmospheric deposition of nitrogen are increasingly used as official standards in national legislation. This requires unique values (i.e., not ranges) based on state-of-the-art scientific knowledge. In The Netherlands such values have been derived in the past by combining the empirical critical load ranges for Europe with local simulated values. We describe this method in detail, resulting in a unique critical load value for each Habitat type of the European Habitats Directive that occurs in The Netherlands. As the empirical values have been adjusted downward several times in recent decades while the simulation model was not updated since 2004, there is an increasing gap between the empirical and the simulated values. Here we use the new, empirical response model DOREN to bridge this gap, but we argue that a new simulation model is urgently needed. Our results show critical load exceedance in 61 % of the total area of nitrogen-sensitive Habitat types in The Netherlands. The most threatened Habitat types are oligotrophic freshwater, bog, species-rich grassland, and beech forest; either because they have a low critical load, or because they occur in areas where nitrogen deposition is high.