Role of Nitrogen
Nitrogen plays an essential role in the fertilisation of crops from both an economical and environmental perspective.
The different forms of nitrogen in the soil in a plant available form are Nitrate (NH₃-) and Ammonium (NH₄+). The behaviour of these two forms of nitrogen in the soil is very different.
Nitrates, by virtue of their high solubility, are easily leached through irrigation water or rainfall into deeper soil layers where the roots cannot absorb them. On the other hand, nitrogen in the form of ammonium can be retained in the soil (clay-humus complex) due to its positive electrical charge, and is not prone to leaching or denitrification (gaseous) losses. A balanced supply of ammonium nitrogen and nitrate nitrogen is important to optimise plant growth and deliver quality produce.
In soils, ammonium nitrogen can come from fertiliser or organic matter. It must be in the root zone to be taken up by plants. Soil bacteria convert ammonium nitrogen to nitrate nitrogen (NH₃-). This can happen quite quickly, given the right soil conditions and temperature. Due to this process, most of the nitrogen taken up by plants is nitrate nitrogen. Nitrate nitrogen is highly mobile in the soil and easily accessible to plants. Any nitrate nitrogen not taken up by plants is at risk of leaching or denitrification losses. The gases released include nitrous oxide.
Nitrification is a naturally occurring process in soils where ammonium (NH4+
) nitrogen from fertiliser or organic forms is oxidised to nitrate (NO3-
) nitrogen. Nitrification is carried out by select soil bacteria in two steps.
The first step is the oxidation of ammonium nitrogen to nitrite (NO2-) nitrogen - Nitrosomas spp is the most common bacteria associated with this step however there are other micro organisms like Archaea that drive nitrification in certain soils. The second step is the oxidation of nitrite nitrogen to nitrate nitrogen - this step is usually undertaken by the bacteria Nitrobacter spp. While most crop species can take up nitrogen in both the ammonium and nitrate forms, the uptake mechanism is different in that mobile nitrate can move to roots in soil water by mass flow, whereas roots have to go to less mobile ammonium nitrate which tends to bind to negatively charged clay and organic matter particles.
The rate of nitrification is dependent on a number of factors, in particular soil moisture and temperature - in a healthy soil with optimum moisture and temperature conditions for plant growth, an application of ammonium nitrogen would expect to be largely nitrified in the space of two weeks.
The benefits of retaining nitrogen in the ammonium form are numerous:
- ammonium nitrogen, while still generally available for plant uptake, is not subject to two key nitrogen loss mechanisms: leaching (the movement of mobile nitrate nitrogen out of the root zone in soil water) and denitrification (the loss of nitrogen in the form of nitrous oxide or nitrogen gas under waterlogged conditions)
- ammonium nitrogen has been demonstrated to stimulate additional root growth compared to nitrate nitrogen
- plant uptake of ammonium nitrogen is more energy efficient in the plant as nitrate nitrogen has to be reduced back to ammonium nitrogen as a precursor to amino acid production in the plant
To learn more about the nitrification process and other nitrogen loss issues, click on a journal below: