The nitrogen cycle in soil
Nitrogen (N) is the only plant nutrient that comes from the atmosphere. This complicates both its addition to a cropping system and its loss, but also makes it possible for some plants to obtain their own N.
N is absolutely essential to all life. In particular, it is an essential component of all proteins.
The following diagram shows the complexity of the N cycle. The main forms of N are shown in large black type. The stores of N are shown in small black type. The processes by which the N is transformed from one form to another are shown in pale blue. These processes are described below.
N fixation
N is fixed in plant roots by bacteria called Rhizobium. The bacteria live in nodules that grow on the plant roots. They convert atmospheric nitrogen gas (N2) to ammonium (NH4+), which they give to the plants in exchange for energy and a safe home:
N2 (nitrogen gas) → NH3 (ammonia) → organic N compounds
Legumes (e.g. clover, lucerne, beans, Acacia) are the most familiar plants that can do this, but some other species are capable, too, notably alders and Casuarina.
There are several species and strains of Rhizobium, and each is specialised to a particular host. This is why it is important to inoculate legume seeds with the correct strain.
The bacteria work best at a soil pH above 5, in moist aerated soil with an adequate supply of molybdenum. If the soil N content is already good, the bacteria reduce their activity, because taking N from the soil is easier for them.
Nitrification
The ammonium supplied in fertiliser or manure is converted first to nitrite (by one group of bacteria) and then to nitrate (by a second group):
NH4+ (ammonium) → NO2– (nitrite) → NO3– (nitrate)
Because the bacteria are sensitive to temperature, moisture and oxygen, nitrification is fastest when the soil is warm (20–30 °C), moist and well aerated.
Plant uptake
Plants will take up both ammonium and nitrate if the soil conditions are aerobic.
Immobilisation
When soluble N is incorporated into organic forms such as organic matter and soil microbes, it is said to be immobilised. It can remain bound up in this form for some time and therefore remain unavailable to plants. The incorporation of materials with a high carbon-to-nitrogen ratio, such as straw, will increase the microbial demand for N, thus locking up available N.
NH4+ (ammonium) / NO3– (nitrate) → organic N compounds
Eventually the microorganisms die and the organic matter is broken down, and the organic N is made available again by mineralization and nitrification.
Mineralisation
Mineralisation is the conversion of organic N sources such as plant residues and manure to ammonium by soil microorganisms as they decompose the material:
Organic N compounds → NH3 (ammonia gas) → NH4+ (ammonium)
Materials with a low carbon-to-nitrogen ratio, such as poultry manure, can release large amounts of ammonium. The ammonium that is not converted to nitrate (by nitrification) and absorbed by plants can be converted to ammonia gas and lost to the air (by volatilisation).
As in nitrification, the bacteria responsible for mineralisation work best at 20–35 °C in moist, well aerated soils.
Because soil particles are negatively charged, the positive ammonium ions can be bound to them and so be retained in the soil instead of being leached out or volatilised.
Denitrification
The conversion of nitrate (NO3–) to gaseous forms of N, particularly in anaerobic soil, is known as denitrification, because this N is lost:
NO3– (nitrate) → NO2– (nitrite) → NO (nitric oxide) → N2O (nitrous oxide) → N2 (nitrogen gas)
Most is lost as N2 gas, and the rest as N2O and NO, both important greenhouse gases.
Volatilisation
Volatilisation is the loss of ammonium N by its conversion to ammonia (gas):
(NH2)2CO (urea) → NH4+ (ammonium) → NH3 (ammonia gas)
Significant amounts of ammonia can be lost from manure and urea that are not promptly incorporated into the soil. Virtually all is lost in only 5 days if it is not incorporated.
Leaching
Under high rainfall or irrigation, soil nitrate can be lost to the deeper soil by leaching below the plant roots. Ammonium tends to be held on the soil particles and retained.
Further reading
Wikipedia: Nitrogen cycle.
Univ. WA: What are the major processes in the nitrogen cycle?
Pregelj L. 2006. Legumes and the nitrogen cycle. ARC Centre of Excellence for Integrative Legume Research, Univ. Qld.