Take home messages

• N is a reactive element that cycles from the air to soil and into plant products, and losses can occur through microbially-mediated processes including ammonia volatilisation, immobilisation into soil organic matter, nitrification/denitrification, and nitrate leaching.
• Inputs of N to production systems can be from N fixation by free-living or symbiotic bacteria, deposition in water or from air, mineralisation from organic materials, and supplied from fertilisers, composts and manures.
• Matching the amount and timing of N supply to peak demand periods can both improve N recovery and minimise the potential for N losses. Understanding the pattern of N uptake for each species is important for developing efficient fertilisation strategies.

What is nitrogen and why is it important

Nitrogen (N) is one of six essential plant mineral macronutrients, along with phosphorus, potassium, sulphur, calcium and magnesium. It is taken up by plants in the mineral form as nitrate, or, less commonly, as ammonium. In addition, there are another nine or ten essential plant micronutrients. All these nutrients are required for healthy plant growth and are supplied from the soil.

Nitrogen is arguably the most important of the elements, as it is often the most limiting nutrient to plant and ecosystem function. It is a fundamental component of amino acids which make up the proteins in plants and animals, and the enzymes which control their metabolism. It is also present in nucleic acids, forming the DNA which makes each species unique and codes for the biosynthesis of the diverse proteins required for life.

Even though N makes up 78% of the atmosphere of the earth, to be available to plants and animals, dinitrogen (N₂) in the air must be converted to one of several chemically active N compounds including mineral forms, nitrate (NO₃^-)and ammonium (NH₄⁺). Along with the gases nitric oxide (NO) and nitrogen dioxide (N₂O), these forms of N are termed ‘reactive’ N because their presence has a range of environmental and biological consequences.

The supply of N fertilisers at an industrial scale produced through the Haber-Bosch process fuelled the ‘Green Revolution’, so doing reducing hunger, and, in 1999, it was estimated that almost two-fifths of the world's population would not be here without the food produced using N fertiliser (Ritchie 2017). Our dependence will only increase as the global count moves to nine or ten billion people (Smil 1999).

While N is vitally important for farm profitability, food production and a healthy diet, losses of N from production systems can result in environmental damage at a local and global scale. The European Nitrogen Assessment (Sutton et al. 2011) and ‘Our Nutrient World’ (Smil 1999) identified that leakages from the N cycle have resulted in effects of N pollution on water quality, air quality, greenhouse gas balance, ecosystems and biodiversity, and on soil quality.

The nitrogen cycle

As N cycles through the soil to plants, N transformations can result in leakage of N to the broader environment. Figure 1 shows a simplified view of an N cycle, with emphasis on the major pools of N in an agricultural soil and the transfer and loss processes occurring, namely leaching, volatilisation, denitrification and immobilisation. Each of these transformations is mediated by microbes or enzymes naturally present in soil. Ammonium derived from organic sources, such as biological fixation, decaying organic materials, animal excreta (manure and urine) and many fertilisers, is central to the N cycle. Nitrogen deposition from the atmosphere can occur from gases (dry deposition) and in precipitation as wet deposition. Enhanced reactive N deposition is a consequence of global emissions from fossil fuel combustion and ammonia mainly from agricultural sources (Dignon and Hameed 1989). Nitrate can also be supplied from synthetic fertilisers.

Figure 1. A simplified N cycle showing the inputs and pools of N, along with loss and transfer pathways in red (from International Plant Nutrition Institute). (Volat'n = volatilisation; Denit'n = denitrification). Gaseous N can be redeposited.

The process of nitrification then converts ammonium to plant available nitrate. Nitrate generally accounts for 95% of the total N uptake by crops (Subbarao et al. 2013), although plants can take up ammonium and some low molecular weight N compounds, such as urea and amino acids. Other than leaching, the processes that drive N losses are reversible, with wet and dry deposition the reverse of volatilisation, nitrification the reverse of denitrification, and mineralisation the reverse of immobilisation.

Surveys undertaken have shown large variations in the amount of N taken up and removed in crop products compared to the amount of N supplied from fertilisers (Norton and vanderMark 2016), and these differences are largely due to the different soil and environmental conditions operating in combination with the agronomic practices adopted.

Inputs of N to agricultural systems

When European systems were introduced into Australia, N was largely supplied to crops or pastures by mineralisation of soil organic matter. However, the stock of soil organic N is a finite resource, and a consequence has been the decline of soil organic matter across Australian agricultural soils (Luo et al. 2010).

Grain (pulse) and pasture legumes make substantial contributions to replenishing the pool of organic N through the active fixation of N₂ from the atmosphere into organic N in their tissues, much of which ends up as soil organic matter. This unique process operates through a symbiosis between specific bacteria and legume, forming root nodules where N fixation occurs.

On average, a good pulse or annual pasture legume crop will provide enough N for one non-legume crop, such as wheat, barley or canola, although the net N benefit depends on the growth of the legume and the demand by the non-legume. Some pulse crops (chickpeas, field peas, faba beans, lentils and lupins) are relatively poor N fixers due to either a weak symbiosis, low biomass production or high grain N removal (Peoples et al. 2001). A decline in the total area of pasture legumes, even legumes in mixed sward permanent pastures, and an increase in more intensive cropping systems, has resulted in an increasing use of fertiliser N to meet the N requirements for successful crop growth.

As a result of these changes, Australian agricultural systems, similar to those in other countries, have moved from extensive, low intensity and low productivity systems to more intensive, productive and water efficient systems. Around 80% of our agricultural production is exported, taking with it the nutrients embedded in the grain, meat and other products. For example, from 2019 to 2021, Australia exported an average of 19.5mt of wheat, with an embedded N content equivalent to 400,000t of N leaving our soils in that crop alone.

Rainfed grain production is the major user of N fertiliser, but even so, it has been estimated that in many cases current N management practices mean that growers are achieving around 60% of the water-limited yield (Hochman and Horan 2018). Even among leading growers in the Victorian Wimmera and Mallee regions, sub-optimal N rates limited wheat yields by more than 0.5t/ha in 15% to 42% of seasons (van Rees et al. 2014). Part of this under-use of N is a consequence of risk management strategies in variable environments, as well as the availability of agronomic advice on N decision making (Zhang et al. 2019).

The rapid take-up of N fertiliser parallels the development of conservation farming systems in the mid-1980s which reduced tillage, retained crop residues and improved soil conservation practices. To support increasing crop yields, and crop nutrition without further depleting soil organic matter, increased use of fertiliser N and other plant nutrients is required in many cases in Australia.

Fate of soil N

A consequence of the transfers between N pools in the soil and then into crop or pasture plants or into the soil organic matter pool, is that some N is lost through a range of pathways. Below is a summary of those pathways.

Losses of N gases

Four N gases are released from soil in appreciable quantities. Dinitrogen, ammonia (NH₃), nitric oxide (NO) and nitrous oxide (N₂O) (FAO 2001). Denitrification is the principal process where nitrateis biologically reduced by removing one or more of its oxygen atoms to create N₂, nitric oxide or N₂O – depending on soil conditions. Ammonia gas is produced when ammonium from manures or fertilisers decompose.

Nitrous oxide is the most environmentally important of these gases, as it is a potent and long-lived greenhouse gas with around 265 times the global warming potential of carbon dioxide (CO₂) (IPCC 2014). In addition, N₂O is recognised as causing depletion of tropospheric ozone (Ravishankara et al. 2009). It is produced from activities such as biomass burning, the consumption of fossil fuels, waste disposal and agriculture (Tian et al. 2020).

Nitrous oxide is produced naturally and in agriculture through nitrification, denitrification and dissimilatory nitrate reduction to ammonium (Stevens et al. 1998). All these in-soil reactions are biologically mediated, with the latter two processes occurring where the soil has little oxygen (anaerobic) in the soil, such as when the soils are waterlogged (Giles et al. 2012).

During the first step of nitrification, a small portion of ammonium may be converted to N₂O gas during the decomposition of nitrite. This process can occur in well-aerated soils with ammonia derived from organic matter, manures or fertilisers. Denitrification occurs in anaerobic soils when some microbes switch to using nitrate or nitrite (NO₂⁻) for respiration, producing N₂O as an end product. If anaerobic conditions in the soil are severe, the N₂O can be further reduced to inert N₂ gas and escape to the atmosphere (Figure 2). Denitrification only occurs when oxygen is depleted, the temperature is above 5°C, and where there are adequate supplies of nitrate and carbon substrates for microbes (Pan et al. 2022). Under particular conditions, there can be forty times more N lost as N₂ compared to N₂O (Schwenke 2022), but there is no specific ratio between N applied and N lost as either N₂ or N₂O.

In an agronomic sense, the losses of N due to denitrification to N₂O are small, generally less than 1% of the N supplied. In a field situation, where rates of application for crops and pastures are in the order of 40–150kg N/ha, losses of a few kilograms as N₂O would have little effect on productivity. However, the N lost as N₂ can be agronomically important, with up to 40% of the N lost as inert N₂ gas under certain conditions.

Figure 2. Relative contributions of nitrification and denitrification to N₂O production as a function of water-filled pore space (adapted from Davidson 1991).

Ammonia is an atmospheric pollutant, causing hazes and human health issues. Ammonia is a highly soluble gas that is produced from fires, fossil fuel combustion, manures and composts generated from high intensity agricultural activities, as well as from ammonium-based fertilisers.

Ammonia is not a greenhouse gas, but when volatilised into ammonia gas, it can be deposited back onto land or water where it may be converted to ammonium then to nitrate and so evolve N₂O. Atmospheric ammonia also plays a role in fine particle pollution, termed particulate matter PM_2.5 where airborne particles have a diameter of less than 2.5 mm (EPA Victoria 2021). These particles can result in atmospheric hazes and adversely impact human health when inhaled.

Reported ammonia volatilisation losses from broadcast urea applied to Australian cropping soils range from 0.1% to 34% (median of 6.7%) (Barton et al. 2022), which is similar to global averages of 6–19.5%, although the global range extends up to 65% (Ma et al. 2021). Ammonia volatilisation loss will be higher when urea is applied to a wet soil followed by dry, windy conditions with little or no follow-up rainfall and minimal ground cover.

Losses of N through water

Ammonium is a positively charged ion and attaches electrostatically to organic and soil colloids, and so, does not move down the soil like negatively charged nitrate ions. Preserving N in the ammonium form is one strategy to restrict the leaching of N into the subsoil and, also, into surface waters. However, the conversion of ammonia to ammonium and then to nitrate in soil, and subsequent leaching of nitrate through soil, can cause surface soils to become more acidic. Strongly acid soils have nutrient imbalances that restrict plant growth.

Nitrogen in the nitrate form is a negatively charged ion that moves freely with soil water as it moves through the soil profile. High nitrate concentrations in drinking water have been linked to human health concerns, and nitrate concentration is a regulated water quality parameter. Nitrate can move into surface and ground waters which are nitrate-limited, enriching those water bodies and enhancing the biological activity of aquatic plants and some photosynthetic micro-organisms. Some, such as cyanobacteria (blue-green algae), can produce a toxin rendering the water unusable. At a more general level, the biological bloom due to the added nitrate dies and then decays, which strips oxygen out of the water. These anoxic conditions can result in fish kills and dead zones in water bodies. Moreover, leaching of nitrate into the subsoil and then into surface waters can result in the evolution of N₂O from the water body.

Preserving fertiliser N in the ammonium form using nitrification inhibitors or slow-release coatings is one strategy to restrict the leaching of N into the subsoil and surface waters.

Losses of N to and from organic matter

Nitrogen can be released from (mineralised) or incorporated into (immobilised) organic matter. In warm, aerated and moist soils, the ratio of carbon-to-nitrogen (C:N) in organic residues is a major driver of whether N is mineralised or immobilised. When the C:N ratio of organic matter is low (high N content e.g. legume residues), microbial decomposition is favoured, leading to N mineralisation. In contrast, a high C:N ratio (low N content e.g. cereal residues) in organic matter can lead to N immobilisation as microorganisms utilise available nitrogen to decompose the organic matter, resulting in a temporary tie-up of N.

The cycling of organic matter is a natural process in all ecosystems, with mineralisation generally exceeding immobilisation under farming systems deploying cultivation, residue burning and long fallow periods. Less organic carbon also means less food for living organisms present in the soil, thus reducing soil biodiversity. The loss of soil organic carbon content can limit the soil's ability to provide nutrients for sustainable plant production, both from the mineralisation of N as well as acting as a store for N and other essential plant nutrients. This may lead to lower yields, soil degradation, desertification of farmlands and affect food security (Bot and Benites 2005). Modern conservation systems with minimal or no cultivation, retained residues and balanced crop rotations, will preserve organic matter (Armstrong et al. 2019) and if all limiting nutrients are appropriately supplemented beyond crop removal, soil organic matter can increase (Kirkegaard et al. 2023).

Acknowledgements

Much of this report is based on a report supported by Fertilizer Australia and resources developed by the International Plant Nutrition Institute (see for further reading).

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Contact details

Robert Norton
robnorton001@gmail.com