The Nitrogen Cycle

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	As plant residues and microbes are mineralized, nitrogen in the form of ammonium (NH4+) is excreted. When nitrogen is in this form plants and microbes easily assimilate it. A few bacteria oxidize ammonium to nitrate (NO3-) as part of their metabolism; this process is called ‘nitrification’. Nitrification is a 2 stage process; first ammonium is oxidized to nitrite (NO2-) by bacteria genera Nitrosomonas, then nitrite is oxidized to nitrate by bacterial group Nitrobacter. Nitrate can be used as a source of nitrogen by plants and microbes in a similar way to ammonium. Nitrate is progressively reduced in a series of microbially mediated processes until we return to N2 gas, which is then released back to the atmosphere, finally completing the nitrogen cycle. Essential Soil Science by M.R. Ashman and G. Puri (ISBN 0-632-04885-9) gives a clear and concise introduction to soil science(see Garden Pests section of the Library).

The Nitrogen Cycle

Nitrate is progressively reduced in a series of microbially mediated processes until we return to N2 gas, which is then released back to the atmosphere, finally completing the nitrogen cycle.