This thesis is the result of a study on the production of nitrate in Dutch heathland soils. Most of the heathlands are located on acid, sandy soils. Therefore , it has dealt mainly with the occurrence, nature and mechanisms of nitrification in acid soils. In the Netherlands, the production of nitrate in acid soils is a matter of great concern because it has become obvious that this process contributes to a continuous acidification of such soils that are exposed to high levels of deposition of ammonia and ammonium. Continuous acidification of sandy soils has a negative impact on the vitality of trees. Moreover, the groundwater may become polluted with nitrate and aluminum.As almost all Dutch heathlands are semi-natural in origin, management practices, such as sod- cutting, are needed to maintain the original dwarf-shrub vegetation. At present, the replacement of dwarf-shrubs by grasses is a big problem for management. This change in dominancy coincides with the increase in the availability of mineral nitrogen for plants which, in turn, is attributed to high levels of deposition of NO x and NH x originating from industry, traffic and intensive animal husbandry. Grasses may become dominant under conditions of relatively high nitrogen availability.In this study, it was indicated that the high availability of mineral nitrogen has also resulted in the general occurrence of nitrification in Dutch heathland soils (Chapter 2 and 4). Nitrification was not restricted to heathlands that were dominated by certain plant species. Nitrification in Dutch heathland soils appeared to be regulated by the availability of ammonium and oxygen whereas the availability of phosphate may also be important (Chapter 2 and 4). The presence of nitrification was not dependent on the pH of the heathland soils. Most of the production of nitrate in Dutch heathland soils was shown to take place in the organic horizon (Chapter 2 and 3).Using inhibitors it could be concluded that chemolithotrophic bacteria are the main nitrifying micro- organisms in ...