Why Nitrogen Keeps Australian Plants and Ecosystems Alive

Nitrogen is an essential nutrient for plants because they use it to make chlorophyll, amino acids, proteins, enzymes and nucleic acids. Without enough nitrogen, leaves yellow, growth slows and reproduction becomes less successful. Yet about 78% of the air is nitrogen, a form that plants cannot use directly. A potted herb and a eucalyptus tree need different quantities, but both depend on the same nutrient supply.

The nitrogen cycle explains how atmospheric gas becomes available in soil. Fixation, assimilation, decomposition, nitrification and denitrification transfer nitrogen between air, water, organisms and minerals. It is a continuous circulation: plants absorb nitrogen, animals consume it, decomposers recover it from waste and dead material, and microbial processes return some of it to the atmosphere.

Nitrogen's essential plant roles

Plants need nitrogen to build cells and run chemical reactions. Chlorophyll gives leaves their green colour and captures light for photosynthesis, while nitrogen-containing enzymes control the reactions that convert carbon dioxide and water into sugars. Nitrogen is also central to DNA and RNA, which are required for cell division and genetic information.

A plant can often relocate nitrogen from older leaves to growing roots, shoots or fruits. This recycling explains why a deficient plant may develop yellow older foliage while young leaves remain green. However, the plant still needs a fresh nitrogen supply when recycled nitrogen cannot meet the demand for new growth.

Nitrogen is a material nutrient, not an energy source. This distinction matters when comparing plant nutrition with examples of roller-coaster energy changes, where an object repeatedly changes between gravitational and kinetic energy rather than being consumed.

Making atmospheric nitrogen usable

Nitrogen fixation converts atmospheric nitrogen gas, or N₂, into ammonia. Some ammonia reacts with water to form ammonium, which plants can potentially absorb. Certain bacteria and archaea perform this transformation using an enzyme called nitrogenase, but they require suitable conditions, energy and, for many organisms, low-oxygen surroundings.

Legumes provide a familiar example. Root nodules containing rhizobia protect these bacteria from oxygen and supply carbohydrates in exchange for fixed nitrogen. In natural grasslands, pastures and rotations, this partnership adds biologically available nitrogen without requiring a manufactured fertiliser.

Industrial fixation supports much of modern agriculture by converting N₂ into ammonia for fertiliser production. Atmospheric lightning also produces nitrate that enters soil through rainfall, although this route supplies only a fraction of global agricultural demand.

Roots absorb nitrate and ammonium

Plant roots take up two principal forms of mineral nitrogen: ammonium, written as NH₄⁺, and nitrate, written as NO₃⁻. Their availability depends on soil pH, moisture, temperature, organic matter and microbial activity. Drought can restrict nutrient movement, while waterlogging may reduce oxygen needed by roots and by nitrogen-transforming microbes.

After nitrate enters a root, it must be reduced before its nitrogen can be incorporated into amino acids. This process consumes energy from the plant and helps explain why nitrate assimilation is linked to carbohydrate supply. Ammonium can be used more directly, although excessive concentrations can become toxic.

A cross-curricular comparison with sound wave transmission reveals another role of physical structure. Both phenomena depend on a medium: soil pores, water films, particles and organisms control nitrogen movement just as material properties influence the transmission of pressure waves.

Microbial transformations reshape soil nitrogen

Decomposers begin the recovery of nitrogen from organic material. Bacteria and fungi break down proteins, wastes and dead organisms, releasing ammonium in a process called ammonification. This ammonium can be absorbed by plants, transformed into nitrate or retained by soil organisms. The speed of this process depends strongly on temperature, moisture and decomposable carbon.

Nitrification is a two-stage oxidation carried out by specialised microbes. Ammonium-oxidising organisms commonly convert ammonium to nitrite, and nitrite-oxidising organisms then convert nitrite to nitrate. Because these processes use oxygen, strongly waterlogged or compacted soil generally slows nitrification and may allow ammonium to accumulate.

Under oxygen-poor conditions, other microbes can convert nitrate into gaseous products. Denitrification may release nitrous oxide or nitrogen gas, removing mineral nitrogen from the soil and returning it to the atmosphere. Waterlogged pastures, wetland sediments and waterlogged riparian soils can therefore act as nitrogen-processing sites rather than simple stores.

Transfer through living communities

Animals obtain nitrogen by eating plants or other animals. Protein is digested into amino acids, absorbed by the gut and rebuilt into animal tissues. Nitrogen not retained in growth is returned through urine, faeces, sweat, milk, eggs and other products. Herbivores therefore connect plant nitrogen to predators, while their wastes return nitrogen to the soil surface.

Dead organisms, fallen leaves and animal products enter the same pathway. Decomposers recover mineral nitrogen from this material, plants absorb it, and consumers acquire it again. A food web thus participates in the chemical cycle: organisms move nitrogen between compartments, but they do not create the elemental nitrogen themselves.

Losses from soil to wider landscapes

Nitrate is highly mobile in water. Heavy rain can move it below plant roots through leaching, while erosion carries nitrogen attached to soil particles. Both processes may transport nitrogen into creeks, rivers, estuaries and coastal waters. Excessive fertiliser use can therefore affect water quality beyond the paddock, garden or field where it was applied.

Dry conditions slow plant uptake and microbial activity, but the next intense rainfall event can produce substantial losses if mineral nitrogen has accumulated. This is particularly important in northern Australian systems with distinct wet and dry seasons. Timing fertiliser or manure applications to crop demand can improve plant uptake and reduce runoff.

Nitrogen can also leave as ammonia gas, especially from warm, alkaline or over-fertilised soil. Soil cover, suitable application rates, legumes, drainage and nutrient testing all influence how much nitrogen is retained. These measures seek to balance crop productivity with losses to air and water rather than simply adding more fertiliser.

Australian farms and gardens

Melbourne’s backyard growers commonly purchase compost, manure and lawn fertilisers from garden centres and hardware stores. Composting recycles household nitrogen in kitchen and garden waste, while soil tests can prevent routine applications from exceeding a lawn’s requirements. Similar urban gardening customs exist across Australian cities.

In Sydney, suburban lawns receive nitrogen to maintain colour and recovery from wear. Rain can wash nutrients from compacted or over-applied lawns into drains and waterways. Gardeners can reduce this pathway by watering carefully, sweeping hard surfaces before storms and following fertiliser directions.

Queensland’s Wet Tropics sugar cane regions combine warm temperatures, high rainfall and long growing seasons. Rapid decomposition and repeated cropping can place heavy demands on soil nitrogen. Cover crops, crop rotations and carefully managed fertiliser programmes help maintain cane growth while protecting downstream water quality.

On inland NSW grazing properties, red soils, variable rainfall and nitrogen-limited pasture affect cattle production. Rotating legumes such as clover can add biologically fixed nitrogen and reduce dependence on fertiliser. On Perth’s sandy Swan Coastal Plain, irrigation makes crops possible, but sandy soil may allow nitrate to leach rapidly beneath the root zone.

For wider revision beyond this topic, the site's broader science resources can connect plant nutrition with physics, chemistry and earth systems. To revise the nitrogen cycle, draw air, soil, plants, animals, microbes and water, then connect them using arrows labelled fixation, uptake, feeding, decomposition, nitrification, denitrification, leaching and atmospheric return.