Fen soils in northwestern Germany have a high content of organic N (4% of organic matter) which gives rise to excessive levels of soluble N in the soil. In laboratory incubation studies, N mineralization and nitrate formation were very high in a calcareous fen soil at 60-80% water capacity, but low in an acid fen soil containing no nitrifying bacteria. During 29 days, 145 and 17 mg nitrate-N/l soil was formed in the calcareous and acid soils, respectively.