Take home messages

• Nitrogen fertiliser use efficiency by crops is low, and nitrogen losses to the atmosphere and waterways contributes pollution and climate change.
• Enhanced efficiency fertilisers can reduce the losses associated with nitrogen fertiliser use.
• Knowledge of how the technologies work and when they will be most effective can help growers understand whether to use them or not.
• A new national GRDC project across seven sites in Australia aims to provide multiple years of field data, combined with modelling and economic assessments, to build our knowledge on the role of EEFs in sustainable grains cropping systems.

Background

Nitrogen use efficiency in agriculture

Nitrogen (N) is a major crop nutrient that suffers from inefficiencies in terms of plant utilisation due to loss as gases (the pollutant ammonia (NH₃), the greenhouse gas nitrous oxide (N₂O) or dinitrogen (N₂)), and through leaching of nitrate (NO₃), as well as immobilisation into the soil. The form and magnitude of these losses is driven by climate, soil and land management factors. These losses impact on the nitrogen use efficiency (NUE). NUE can be described by a number of indicators which focus of the production outcomes, environmental outcomes and economic outcomes (Antille and Moody 2021). In Australian dryland cropping systems, the average NUE (reported as percentage of applied N taken up by the plant) is around 35–40% (Chenet al. 2008, Angus and Grace 2017).

Numerous strategies have been developed to improve NUE, including the 4Rs concept and use of alternative N forms (the ‘right source’ from the 4Rs framework). One alternative N fertiliser form approach is what is referred to as ‘enhanced efficiency fertilisers’ (EEFs). These include N fertilisers encapsulated into coatings that control or slow the release of N into the soil and aim to match the release of N with the plants’ needs, and fertilisers mixed or coated with inhibitors. These inhibitors can be urease inhibitors, which slow the conversion of urea to ammonia (NH₄) and thereby reduce NH₃ loss, or nitrification inhibitors, which slow the conversion of NH₄ to NO₃ and therefore reduce the potential for N₂O, N₂ and leaching losses. New products are being developed that include both urease and nitrification inhibitors (dual inhibitors). Details of the loss pathways and the site of action of the different EEFs is indicated in Figure 1.

Figure 1. Simplified N cycle showing N pathways after application of urea and location of impact of the different EEFs: controlled/slow-release fertilisers, urease inhibitors and nitrification inhibitors, and the impacted loss pathways.

How EEFs work

The N transformation processes shown in Figure 1 are biochemical processes and the inhibitor products work by inhibiting the enzymes involved in these processes.

Controlled/slow-release fertilisers

Controlled or slow-release fertiliser products contain coatings that slow the rate of N release from granules, releasing the N slowly in response to the soil conditions (temperature and moisture). They are designed to release N to match the plant needs and therefore, increase plant uptake and reduce the risk of N loss.

Urease inhibitors

The urease inhibitor, of which there is currently one active compound (NBPT) commercially available, inhibits the activity of the urease enzyme which is involved in the hydrolysis of urea (Figure 1), and by doing so, slows down the rate of urea hydrolysis. A slower rate of urea hydrolysis means that the rise in pH that occurs around the hydrolysing urea granule is reduced and less ammonia (NH₃, the gas) and more ammonium (NH₄⁺, in soil solution) forms. This inhibitor has a role in rainfed systems where urea is surface applied. Higher losses of NH₃ will occur in more organic systems (such as pastures) or in cropping systems with high crop residues or high soil organic matter because there is a lot of urease around, urease being associated with organic matter. Higher urease leads to more rapid urea hydrolysis and associated pH increase around the hydrolysing urea granule driving NH₃ loss (Figure 1). Previous research has shown that losses of NH₃ from urea in Australian grains can be around 10% of applied N (Schwenke et al. 2014), and that the urease inhibitor NBPT can reduce this to around 1% (Turneret al. 2010). In pasture systems, the loss of NH₃ can be as high as 30% of the applied N and use of NBPT can reduce this to 10% (Suteret al. 2013).

Nitrification inhibitors

There are a number of nitrification inhibitors on the market, including three commonly used ones: dicyandiamide (DCD), 3,4 dimethyl pyrazole phosphate (DMPP and new formulations), and nitrapyrin (N-Serve^® and new formulations). These inhibitors slow the oxidation of NH₄ to NO₃, a process known as nitrification, and they do this by inhibiting an enzyme called ammonia monooxygenase (AMO) in the nitrifying bacteria (Figure 1). By slowing nitrification, the rate at which NO₃ is produced is slowed, which reduces the risk of NO₃ being lost via multiple loss pathways (leaching and/or denitrification) and means that there is increased likelihood of this NO₃ being available for plant uptake. Positive impacts of nitrification inhibitors on N₂O emissions is often reported, but as with the urease inhibitors, the outcome depends on climate, soil type and management, and have been reported to range from no impact to reductions of 60% or more in N₂O emissions in Australian grains (De Antoni Miglioratiet al. 2016; Wallaceet al. 2018). Under rainfed systems, the recent estimates of N₂O emissions from applied fertiliser N averages 0.56% and ranges from 0.04 % in Western Australia to 0.8 % of applied N in high rainfall zones (>600mm per annum) (Graceet al. 2024). The level of emissions helps to guide where these products may be most beneficial.

Dual inhibitors

Currently, there are limited dual inhibitor products commercially available on the market, and limited research on their efficacy. Because they contain a combination of urease and nitrification inhibitors, they are designed to reduce all loss pathways.

When to use inhibitors

Knowing the best time and situation to use inhibitors can ensure growers are not wasting money. The outcome of their use is influenced by climate, soil type, fertiliser form and fertiliser application methods, and there can be a large variation in outcomes (Lam et al. 2022).

Controlled/slow-release fertilisers

Controlled or slow-release fertilisers can be used in systems where rapid release of N is not required for a crop, and the N release matches the growth pattern of the crop. Currently the adoption of controlled or slow-release fertilisers is limited to high value systems, such as amenity horticulture, due largely to the cost. If the cost of these products can be reduced and/or their effectiveness and therefore value clearly demonstrated in broadacre cropping systems (e.g., improved crop yields, reduced N inputs, return on investment, etc), then they may have a role in the grains industry.

Urease inhibitors

Urease inhibitors are only applied with fertilisers containing urea. The risk of NH₃ volatilisation is greatest when urea is surface applied to highly organic soils and under conditions of low moisture, warm temperatures, and wind (for example, autumn and spring), and this is where the urease inhibitor will have the greatest impact. Ammonia volatilisation is largely eliminated if fertiliser is applied via deep placement, removing the need for a urease inhibitor. Irrigation immediately following urea application likewise reduces the risk of NH₃ loss and, if irrigation or rainfall is guaranteed, the urease inhibitor is not required. However often they are used as a safeguard in case rain doesn’t fall or the irrigator breaks down.

Nitrification inhibitors

Nitrification inhibitors target losses associated with higher soil moisture and rainfall conditions – that is, denitrification and NO₃ leaching. Therefore, their use is best targeted to systems where these loss pathways are relatively large, which includes sites with high soil carbon and high N, as this is a biochemical process, and during periods of high moisture.

Dual inhibitors

These inhibitor products are viewed as applicable for times when both high volatilisation and denitrification losses are expected and are viewed as being useful across an entire crop cycle.

Impact on yields

The agronomic outcomes from the use of the inhibitors are variable, and globally and across agricultural systems, there are conflicting findings regarding the agronomic benefits of EEFs (Abalos et al. 2014; Thapaet al. 2016). In situations where background levels of mineral N are not in excess of plant demand and where the potential N losses are large enough to affect the yield outcomes according to the N response curve, then it is possible to gain equivalent yield at reduced N rates (i.e. the response curve shifts to the left). Generally, it is not possible to increase yield above that achieved with the grower recommended rate that has been developed based on N response curves for the region. Where N losses are not so large, then the impact on yield will be less. However there is emerging evidence that the inhibitors can build the soil N pool via increased immobilisation (Suter et al. 2020).

Method

The new GRDC project (UOM2404-007RTX) aims to understand the benefit of existing commercially available EEFs on grain yield, product quality, N cycling and gaseous emissions, water use efficiency, environmental impacts and ultimately profitability, across the grains’ regions of Australia. Seven sites are included in the experimental trials and 2–3 years of data will be collected from these sites, generating data on the environmental, agronomic and economic performance of EEFs. The N release dynamics in soil, crop N uptake and N losses will be quantified in the year of application as well as residual benefits in subsequent crops using 15N labelled fertilisers. Modelling and economic analysis will utilise the collected data to provide a clearer picture of how these products may fit into grains farming systems and their potential return on investment.

Conclusion

The role of EEFs for improving the sustainability of the Australian grains industry, is the focus of a new GRDC project that is exploring their capacity to maximise NUE and optimise crop production whilst minimising environmental impacts. A greater understanding of how, when and where these technologies work will help growers’ gain confidence with their fertiliser decision-making that result in cost-effective environmental and agronomic outcomes.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. The authors also acknowledge the research partners involved in the project including colleagues in the University of Melbourne and research partners at Department of Primary Industries and Regional Development Western Australia, University of Queensland, Department of Primary Industries and Regional Development NSW, CSIRO, Queensland Department of Environment, Science and Innovation, Queensland University of Technology, La Trobe University, Birchip Cropping Group and Hart Field Site Group. Industry partners include CSBP Limited, Incitec Pivot Fertilisers, NutrienAg Solutions, AgFert Fertilizers Pty Ltd and Fertilizer Australia.

References

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Contact details

Helen Suter
School of Agriculture, Food and Ecosystem Sciences, The Faculty of Science
The University of Melbourne VIC 3010
03 8344 0179
0438 456 602
helenc@unimelb.edu.au