THE NITROGEN CYCLE

The flux of nitrogen through the global biogeochemical nitrogen cycle has undergone dramatic alterations in the past few decades (Galloway and Cowling, 2002, Vitousek et al., 2002). Over half of the fixed nitrogen that enters the terrestrial portion of the cycle annually now has its origins in anthropogenic processes including production of ammonia-based fertilizers via the Haber-Bosch process, cultivation of crops such as legumes and rice where biological nitrogen fixation provides fixed nitrogen for crop growth, and the combustion of fossil fuels leading to release of nitrogen oxides (Vitousek et al., 2002). Whereas all forms of life require nitrogen and hence participate in the nitrogen cycle, bacteria facilitate steps in the global biogeochemical nitrogen cycle that other organisms cannot. The three well established bacterial processes in the cycle include

    1. Biological nitrogen fixation, the nitrogenase-mediated reduction of dinitrogen into useable ammonia;
    2. Denitrification, in which bacteria carry out an anaerobic respiratory pathway with transfer of electrons to nitrate to form dinitrogen; and
    3. Nitrification, in which ammonia and nitrite serve as energy and reductant sources for growth of bacteria.

  • A fourth process, the anaerobic oxidation of ammonia, in which nitrite and ammonia are combined to form dinitrogen gas, is a more recently described bacterial contribution to the nitrogen cycle (Jetten et al., 2005).

  • To be considered are also the studies suggesting that aerobic ammonia-oxidizing Crenarchaea (AOA) have a role in the process of nitrification. (Konneke, et al 2005). AOA possess AMO-like genes (Hallam, et al. 2006), are widely distributed and are abundant in marine environments (Beman and Francis, 2006, Wuchter, et al. 2006, Treusch, et al. 2005). Current research is establishing whether - and if so - the extent as to which AOA contribute to nitrification (Nicol and Schleper, 2006).

  • In addition to the abiotic processes of mineralization and of dinitrogen oxidation to nitrite and nitrate by combustion (Mancinelli and McKay 1988), ammonification, the release of ammonia during decay of organic matter, as well as assimilatory and dissimilatory nitrate (nitrite) reduction are also biotic contributions to the N cycle that are performed by bacteria, fungi and plants.