ABSTRACT

The nitrogen cycle The nitrogen cycle refers to the movement of nitrogen through the food chain of living organisms (Figure 1). This complex cycle involves bacteria, plants and animals. All organisms can convert ammonia (NH3) to organic nitrogen compounds, that is compounds containing C-N bonds. However, only a few microorganisms can synthesize ammonia from nitrogen gas (N2). Although N2 gas makes up about 80% of the Earth’s atmosphere, it is a chemically unreactive compound. The first stage in the nitrogen cycle is the reduction of N2 gas to ammonia, a process called nitrogen fixation. Ammonia can also be obtained by reduction of nitrate ion (NO

reduction can be carried out by most plants and microorganisms. The ammonia resulting from these two processes can then be assimilated by all organisms. Within the biosphere, there is a balance between total inorganic and total organic forms of nitrogen. The conversion of organic to inorganic nitrogen comes about through catabolism, denitrification and decay (Figure 1).