Modelling critical loads for the Solling spruce site

After an introduction to the historical development of the critical load concept, methods for calculating critical loads are presented. Besides empirical relationships three types of models are identified and evaluated: steady-state soil models, dynamic soil models and integrated forest-soil models. Critical load assessments are carried out with examples from each of the three types of models, using data from the Solling spruce site. The resulting critical loads of sulphur obtained with the dynamic and integrated models are close (within 30%) to the steady-state model calculations. For nitrogen, the spread in the model results is wider due to the very different criteria (soil solution chemistry vs. forest vitality) used for deriving the critical loads. This agreement lends confidence to the simplified critical load calculations carried out on a European scale. However, much work remains to be done, both in the modelling efforts and the data collection, before truly integrated forest-soil models can be used for assessing critical loads on a large regional scale.