Soil Nitrogen Turnover — Mineralisation, Nitrification and Denitrification in European Forest Soils

Abstract

Mineralisation, nitrification and denitrification are major processes in the soil nitrogen cycle (see Schulze, Fig. 1.2, this Vol. Chap. 1). Nitrogen (N) mineralisation is by definition the biotic conversion of organic N into inorganic N, mainly NH4+ and NO3−. Many organism groups, bacteria, fungi and certain soil animals, possess this capability, and the product formed is generally ammonia/ammonium, depending on the soil pH. Ammonium (NH4+) can be oxidised via nitrite (NO2− to nitrate (NO3−. Both chemoautotrophic (using CO2 as a C source) and heterotrophic (using organic C as a C source) microorganisms can perform this nitrification process (Prosser 1986). Heterotrophic microorganisms can also produce nitrate directly from organic N sources (Killham 1986). Nitrate can be reduced via nitrite to nitric oxide (NO), nitrous oxide (N2O) and/or dinitrogen (N2) (denitrification). In addition to these processes, ammonium/ammonia, nitrite and nitrate can be assimilated (immobilised) by microorganisms, mycorrhizal fungi and roots. Further, ammonia and nitrite are known to react with certain organic compounds and, thus, be chemically immobilised (Nömmik and Vahtras 1982; Azhar El Sayed et al. 1986).