Mancinelli R L
NASA-Ames Research Center, Moffett Field, CA 94035, USA.
Life Support Biosph Sci. 1996;3(1-2):17-24.
Most of the nitrogen available to the biosphere exists as N2 in the atmosphere, and is not useful to most organisms until it is "fixed" either biologically or abiotically (by lightning or aurorae, or industrially). Once it is fixed into NH3, usually it is either assimilated and transformed into organic N or nitrified into NO3-. Organic N can be transformed back into NH3 by ammonification. Nitrate can be converted into N2O by nitrification and denitrification, and to N2 by denitrification. Such N2O and N2 production results in nitrogen loss from ecosystems and a nitrogen gain to the atmospheric nitrogen reservoir. The different steps of the nitrogen cycle require different environmental conditions. These differences result in a spatial distribution pattern of the different nitrogen transformation reactions. Biological nitrogen fixation occurs universally in soils, sediments, fresh water, and marine systems that are both aerobic and anaerobic. Nitrification occurs primarily in aerobic habitats, whereas denitrification predominates in anaerobic habitats such as sediments and water-logged soils. In closed systems, such as plant growth chambers for CELSS, denitrification and nitrification result in a loss of fixed nitrogen available to plants.
生物圈可利用的大部分氮以大气中的N₂形式存在，在通过生物或非生物方式（通过闪电、极光或工业过程）“固定”之前，对大多数生物无用。一旦它被固定为NH₃，通常要么被同化并转化为有机氮，要么被硝化成为NO₃⁻。有机氮可通过氨化作用再转化为NH₃。硝酸盐可通过硝化作用和反硝化作用转化为N₂O，并通过反硝化作用转化为N₂。这种N₂O和N₂的产生导致生态系统中的氮损失以及大气氮库中的氮增加。氮循环的不同步骤需要不同的环境条件。这些差异导致不同氮转化反应的空间分布模式。生物固氮普遍发生在有氧和无氧的土壤、沉积物、淡水和海洋系统中。硝化作用主要发生在有氧生境中，而反硝化作用在沉积物和渍水土壤等厌氧生境中占主导。在封闭系统中，如用于受控生态生保系统的植物生长室，反硝化作用和硝化作用会导致植物可利用的固定氮损失。
