The nitrogen cycle

Plants require large amounts of nitrogen (N) for growth, as, along with other elements, it is a part of every living cell and is a major component in protein.

Unlike phosphorus, most of which is bound to soil minerals, nitrogen is found mainly in the soil organic matter fraction and as protein in plants and soil organisms.

Nitrogen is present in the soil in two major forms:

• organic nitrogen in soil organic matter and soil organisms (organic nitrogen is not available to plants)
• mineral nitrogen in soil as either ammonium (NH4+) or nitrate (NO3-) (mineral nitrogen is available to plants)

Only 2-3% of soil nitrogen is in the mineral form.

Organic nitrogen is converted to mineral nitrogen (mineralisation) during the breakdown of soil organic matter by soil organisms or through nitrogen fixation by legumes.  

Nitrogen is re-converted from mineral to organic forms (immobilisation) when it is taken up by plants.

Nitrification is the process of converting ammonium nitrogen to nitrate nitrogen. Ammonium nitrogen is rapidly converted to nitrate nitrogen under aerobic (containing air) conditions when soil moisture is adequate and the temperatures are not too hot or cold.

This is important for optimising pasture production because nitrates are the form most available to plants.

Nitrates can be lost through denitrification (lost to the atmosphere under anaerobic or waterlogged conditions) or leaching (because nitrate is freely mobile in the soil).

Nitrogen fixation

Rhizobia are one of the most important bacterial groups in agricultural soils because of their symbiotic (win-win) relationship with legumes.

Legumes can fix nitrogen in the soil only when the strain of rhizobium in the soil is compatible with that legume.  

In Australia, the most common causes of reduced nitrogen fixation in pastures are soil acidity and phosphorus deficiencies, reducing rhizobia activity.

Highly productive, perennial grass-based pastures can exploit the added nitrogen in the system, and reduce any sustainability problems associated with nitrogen loss.