Nitrification in acid soils: micro-organisms and mechanisms

Nitrification in acid soils was first reported in the beginning of the 20th century. Although this finding has been well substantiated by countless studies since then, it has until recently remained unclear which micro-organisms were responsible for nitrate production at low pH. Substantial evidence now supports the role of chemolitho–autotrophic bacteria as the main nitrifying agents in most acid soils. Heterotrophs may make some contribution to nitrification in acid soils, but this is difficult to demonstrate conclusively. Current insights in the phylogenetic position of autotrophic nitrifying bacteria in acid soils and the mechanisms that may enable them to be active at low pH are presented. In addition, the spatial variability of their activity and their contribution to the flux of the greenhouse gas N2O is discussed.