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The role of nitrogen in agricultural production systems

This material addresses aspects of the following syllabus outcomes:

A student:

H1.1 explains the influence of the physical, biological, social, historical and economic factors on sustainable agricultural production
H2.1 describes the inputs, processes and interactions of plant production systems.

The work presented in the following section contributes towards understanding the following syllabus content areas:

• the role of soil nutrient cycles in Australian agricultural systems
• the role of microbes and invertebrates in the decomposition of organic matter and the fixing of atmospheric nitrogen via their association with legumes.

Source: Board of Studies NSW (1999) Stage 6 Agriculture Syllabus.

Nitrogen

Nitrogen is one of the most important nutrients for crop growth, second only to water and is the major nutrient the producer can control. Nitrogen exists in many different chemical forms and passes around natural and agricultural ecosystems in a cycle. The various forms of nitrogen determine its availability to plants or whether nitrogen escapes and is no longer available to plants. The presence of useable nitrogen and its losses affects the sustainability of production. Mismanaged it can result in economic loss to the producer, environmental repercussions, or both.

How plants use nitrogen

Nitrogen is one of the main chemical elements required for plant growth and reproduction. Nitrogen is in chlorophyll and therefore essential for photosynthesis. It is also the basic element of plant and animal proteins, including the genetic material DNA and RNA, and is important in periods of rapid plant growth.

Plants use nitrogen by absorbing either nitrate or ammonium through the roots. Most of the nitrogen is used by the plant to produce protein (in the form of enzymes) and nucleic acids. Nitrogen is readily transported through the plant from older tissue to younger tissue. Therefore, a plant deficient in nitrogen will show yellowing in the older leaves first due to the underdevelopment or destruction of chloroplasts and an absence of the green pigmented chlorophyll.

Chemical forms of nitrogen

• Nitrogen gas (N2) makes up 78 per cent of atmosphere. It is not directly available for use by plants but is directly used in nitrogen fixation and industrial fertiliser manufacture.
  
  
• Ammonia (NH3) is the basic form of nitrogen fertilisers. In this form the nitrogen is unavailable to plants.
  
  
• Nitrate (NO3ˉ) is the most common form available to plants. In this form, nitrogen is mobile, leachable and usually the end product of mineralisation.
  
  
• Ammonium (NH4+) is a less common plant-available form of nitrogen when compared to nitrate but is a preferred nitrogen source. Plants use less energy for uptake in this form. There are many beneficial side effects and nitrogen in this form is less likely to be lost than in other forms.
  
  
• Nitrite (NO2ˉ) is an intermediate in the conversion of ammonium to nitrate. It is a less common plant-available form but more toxic to plants and more prone to gaseous losses than nitrate.
  
  
• Nitrous Oxide (N2O) is a greenhouse gas and is lost through denitrification.
  
  
• Nitric Oxide (NO) this form is lost through denitrification and may be harmful to the ozone layer.

All of these above forms of nitrogen are known as inorganic forms of nitrogen.

Organic nitrogen compounds are complex and unavailable to plants. They are the end products of immobilisation.

The nitrogen cycle

Below is diagram showing the nitrogen cycle, the various forms of nitrogen, the processes that convert one form of nitrogen to another and the part of the environment in which these processes occur.

Nitrogen may be found in air, water or land; nitrogen may exist in several different chemical forms; it will undergo many changes in form and location throughout the cycle due to processes which occur as a result of weather, plants, animals and humans. In some cases nitrogen may be lost to the soil nitrogen cycle. It is important to determine whether human activity has accelerated the rate of natural losses and whether these losses pose an environmental threat.

Nitrogen cycle

Activity 1

Visit the University of Western Australia/Soil health-Nitrogen cycle web site and complete the following.

1. Write notes in your own words to summarise each of the five conversion processes that are described.
2. Identify each process on the nitrogen cycle diagram above, using the appropriate numbers 1-5.
3. Include the processes of leaching and volatilisation to your notes, giving them the numbers 6 and 7 respectively.
   Leaching: downward movement of dissolved nitrogen through the soil profile carried by infiltrating water (6).
   Volatilisation: losses of ammonia (especially in fertilisers) from the soil surface to the atmosphere (7).
4. Colour code the arrows on the nitrogen cycle diagram to show and differentiate between the inputs to and losses of nitrogen from the cycle. Make sure you include the harvest of plant and animal material.

Answers

To further your knowledge and understanding of the processes and effects of nitrogen you can complete the work in the tutorial Detrimental effects of nitrogen.

Farm managers manipulate the nitrogen cycle to maximise the availability of nitrogen and hence improve production. To complete an activity about manipulation of the nitrogen cycle go to the tutorial Manipulating the nitrogen cycle to maximise production.