INSIGHTS from a project tracking nitrogen fertiliser dynamics in soils and plants for up to three consecutive crop seasons will be shared at the 2025 Grains Research and Development Corporation Goondiwindi Update.

Nitrogen losses are being being measured in a national project by applying nitrogen-15-labelled urea to plots as granular urea or, as pictured being applied by Prof Mike Bell, urea in solution. Photo: Prof Mike Bell, UQ
Presented by University of Queensland researcher Professor Mike Bell on day one of the March 4-5 program, the national project looks at 12 sites, including four in the northern region: Gatton; Kingsthorpe; Tamworth, and Wagga Wagga.
As well as grain yield, fertiliser N lost through volatilisation and denitrification is being quantified, and leaching of N below-crop root zones will also be determined.
Laboratory studies of stubble breakdown and N mineralisation and immobilisation are supporting the field program.
Data will also be used to validate or improve the N routines in APSIM, the widely used platform for modelling and simulating cropping systems, to boost confidence in the ability to predict optimal N management strategies and fertiliser N dynamics across the Australian grains industry.
The project aims to provide insights useful in contrasting soil types and cropping systems which can maintain sustainability and economic viability while meeting greenhouse gas-emissions targets.
Following is a summary of aspects of the research from Prof Bell working with: Dr Graeme Schwenke, NSW DPIRD Tamworth; Dr David Lester, Qld DPI, Toowoomba; Prof Peter Grace, QUT, Brisbane; Dr Louise Barton, UWA, Perth; Dr Mark Farrell, CSIRO, Adelaide, and Dr Kirsten Verburg, CSIRO, Canberra.
Low use in year applied
The current project, entitled Predicting nitrogen cycling and losses in Australian cropping systems, began in April 2022 and is due to end in September 2026; it differs from previous studies by looking beyond a single crop season.
The project is looking to quantify N losses in their four forms and pathways:
- nitrate or NO3 and dissolved organic N via leaching,
- ammonia or NH3 gas following volatilisation;
- nitrous oxide or N2O from nitrification and denitrification; and,
- dinitrogen or N2 gas via denitrification.
Behind the project is the acknowledgement that decision-support tools are needed to help calculate fertiliser N rates to fill the gap between soil-mineral N at or near sowing and expected crop N demand.
These currently assume a fertiliser N efficiency factor of 50pc, variously defined relative to either crop fertiliser N recovery in biomass, or removal in harvested grain.
“However, recent studies using N-labelled fertiliser have reported low fertiliser N acquisition by crops in the year of fertiliser N application…suggesting that crops are still heavily reliant on soil-derived N to meet crop N demands,” the report said.
“This raises questions about the efficacy of fine-tuning annual fertiliser N recommendations in response to seasonal forecasts and productivity zones within fields, and suggests that understanding the fate of unused fertiliser N will also be important to system productivity.”
While the extent of loss via N2O has had considerable attention, losses as N2 during denitrification, and via ammonia volatilisation or leaching, have received much less attention.
“Our lack of understanding of the fate of fertiliser N applied to Australian cropping systems has hindered the deployment of effective management strategies for maximising fertiliser N-use efficiency in different regions and cropping systems.”
The field research is using N fertiliser enriched with the naturally occurring stable N isotope to complement traditional N fertiliser rate experiments, enabling the fate of applied N to be tracked for up to three consecutive cropping seasons.
In focus in the north
In the Northern region, research covers summer and winter cropping, with sub-surface banding and spreading of fertiliser N in systems that have varying reliance on in-crop rainfall or water stored in the profile over a fallow.
The summer-cropping sites, Gatton and Kingsthorpe in south-east Queensland, have grain sorghum as the target crop receiving the initial application of urea, while the winter-cropping sites at Tamworth and Wagga Wagga in NSW have wheat as the target fertilised crop.
In all locations, the target crop is grown in rotations typical of local cropping systems, such as sorghum-mungbean-sorghum, and wheat-barley-canola.
The fate of urea applied at sowing is tracked through three consecutive growing seasons from the first application cycle, two successive seasons for the second cycle, and only a single crop season for the third.
“The conduct of these studies over three consecutive growing seasons, each with different rainfall amounts and distribution, combined with access to supplementary irrigation, provides a broad cross-section of conditions that will determine fertiliser N recovery or loss.”
The loss from specific N rates and application times is benchmarked against that from background emissions in unfertilised soil, while in the case of denitrification losses, additional information regarding the form of N2O versus N2 loss is able to be determined by using highly N-enriched urea.
“The latter determination is especially important in heavy clay soils where periodic waterlogging can occur, as the N2O losses that are the focus of greenhouse gas emissions monitoring can be a small fraction of the total denitrification loss given the right combination of recently applied fertiliser N and wet soil.”
Preliminary findings
In the northern region, the second winter crop has just been harvested or the third summer crop commenced at the experimental sites, with complete data sets being assembled and N analysed.
A snapshot of results being generated include insights into fertiliser N removal in harvested grain for crops of summer sorghum in 2022-23 and 2023-24, and for winter wheat grown at Tamworth in 2023.

Figure 1: Fertiliser N removed from the field in harvested grain in the year of initial fertiliser application. Values on the x-axis indicate the rate of applied N in kg N/ha and experiment location, while vertical bars represent standard errors of treatment means for each N rate and site/season. Labels on the chart are the percentage of applied fertiliser N removed in grain. Gatton (2022-23) and Kingsthorpe (2022-23 and 2023-24) sites are sorghum, and Tamworth (2023) is wheat.
The rates of applied N refer to product applied as a band near the crop row at planting for sorghum, and as split-broadcast applications before sowing and in-crop in wheat at Tamworth.
Irrigation was used in a subset of treatments at Gatton and Tamworth to change crop N demand, but in the case of Gatton, wetter soil also increased the denitrification loss risk.
“The efficiency of uptake of fertiliser N by crops ranged from 10-75pc of fertiliser N applied, with this large range primarily a function of seasonal moisture dynamics.
“The best example of this is the contrast between the 2022-23 and 2023-24 seasons at Kingsthorpe, with the second growing season characterised by earlier sowing and much more favourable in- season rainfall compared to the dry 2022-23 season.”
“The partitioning of fertiliser N taken up by the crop into grain N removed at harvest ranged from 60-80pc of the accumulated fertiliser N.
“However, this resulted in fertiliser N removal in grain that was typically less than 30pc of the fertiliser N applied, with the exception being the high-yielding crops in the favourable 2023-24 growing season at Kingsthorpe.”
Irrigation was found to increase fertiliser N removal in grain at Tamworth in 2023 or, as was the case at Gatton, have no real effect.
“These data illustrate that a high proportion of fertiliser N is not removed in grain in the year of application.”
Sampling showed that surplus N has been returned to the field in residues, retained in the soil profile, or lost to the environment, and the fate of that residual fertiliser N will be tracked over subsequent growing seasons.
Crop responses in following year
The full N analyses to quantify recovery of labelled fertiliser N in the second crop season are not yet available, but preliminary assessments of the apparent effects of residual fertiliser N on grain yields have been conducted.
In the winter-crop sites at Wagga Wagga and Tamworth, the only fertiliser N applied in the year two crop of barley was around 7kg N/ha applied to all treatments with either Granulok-Z or MAP at sowing.
At the Gatton sites, a common rate of 28kg N/ha was applied to all maize plots as broadcast Green Urea NV prior to sowing.

Figure 2. Fertiliser N mass balance after the 2022-23 sorghum crop at Gatton. Two rates of fertiliser N were applied in sites that were rainfed, or that received supplementary irrigation. Enriched fertiliser allowed the fate of applied N in the soil and plant components to be quantified through the tracking of isotopes.
“At Tamworth, the relatively poor recovery and subsequent removal of fertiliser N in the dryland wheat the previous season, only 13-15pc fertiliser N removed in grain, was consistent with a large yield response to residual fertiliser N in the second crop season, with residual fertiliser able to increase yield by up to 3t/ha.
“Interestingly, responses were tightly correlated with the original N rate throughout the data set.
“Responses at Wagga were similar, although the magnitude of the grain yield increase with increasing N rate was reduced to 1.2 t/ha.
The responses to residual N at Gatton were quite different to those in the winter-crop sites.
“The dryland site showed minimal maize yield increase as prior season application rates increased to 150kg N/ha, and averaging 3kg grain/kg applied N, but residual benefits increased sharply at rates of 20-30kg grain/kg applied N beyond that.
“This suggested larger quantities of residual N at these higher rates.
“In contrast, there was no evidence of any residual N response across all rates in the irrigated trial, which was consistent with the suggestion of greater in-season losses, especially at the higher N rates arising from the mid-season irrigation in 2022-23.
“Once the N mass balance data is complete, we will be able to quantify fertiliser losses and recovery efficiency over multiple crop seasons.
“When this is combined with field measurements of denitrification and volatilisation losses in the contrasting cropping systems, we should be able to identify the specific loss pathway by which these losses occurred.
“These analyses will then allow a more targeted focus on N management strategies and fertiliser products that can minimise loss risk and increase N use efficiency across diverse soils and farming systems.”
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