A comprehensive field research program aims to plug gaps in our understanding of nitrogen cycling and losses across Australian grain growing regions.

Key points

• The current knowledge of nitrogen cycling pathways is limited to specific locations and cropping systems
• GRDC has invested in a national program to develop a comprehensive understanding of nitrogen cycling and loss in Australian grain growing regions
• The data will be used to improve nitrogen forecast models that underpin adviser recommendations and predict greenhouse gas emissions

Growers spend about $1.1 billion on nitrogen fertiliser each year. In most cases nitrogen losses are not substantial; however, as much as 25 to 70 per cent of applied nitrogen can be lost from the system under particular climatic scenarios and in certain soil types.

Nitrogen cycling through the soil, air and water is complex and there are many ways that nitrogen – in different chemical forms – can enter and leave the system.

The main pathways for loss are the gaseous pathways of volatilisation and denitrification, while leaching occurs where water flushes the nitrogen below the crop root zone. Nitrogen can also be temporarily immobilised in the soil, but this can become available to the crop in the future via mineralisation.

Soil properties, climate and plants all combine to influence the chemical and biological processes that drive nitrogen cycling.

Previous studies have investigated nitrogen pathways in specific cropping systems and locations and this data has been used in nitrogen modelling that underpins adviser recommendations. However, the amount of nitrogen that remains unaccounted for is highly variable and depends on many factors including the soil type, cropping system, location and seasonal conditions.

This means that models are still not able to accurately estimate losses in a specific situation or to determine if, and when, immobilised nitrogen might again become available to plants.

Nitrogen losses will be measured at 12 field sites across the country by applying nitrogen-15-labelled urea to the trial plots as either granular urea or as urea in solution. In this image, University of Queensland’s Professor Mike Bell demonstrates in-soil banding of urea in solution. Photo: Professor Mike Bell, University of Queensland

Plugging the gaps

A national $11.9 million GRDC investment aims to plug these gaps in our understanding of nitrogen cycling and loss pathways. The four-year project, led by the University of Queensland, will be the first to take a systematic approach to quantifying nitrogen cycling and losses across the diverse range of soil and environmental conditions where grains are grown.

Partner agencies include Queensland University of Technology (QUT), CSIRO, University of Western Australia, Murdoch University, New South Wales Department of Primary Industries, Queensland Department of Agriculture and Fisheries, Western Australian Department of Primary Industries and Regional Development, the South Australian Research and Development Institute and Agriculture Victoria.

The research will focus on the fate of ‘missing’ nitrogen, which is not accounted for in the soil, the grain or the crop residue at the end of the season. The aim is to help growers and advisers understand and minimise nitrogen losses, while maximising fertiliser use efficiency in crops.

The work will build on previous research to deliver a comprehensive national dataset that will be used to improve the Agricultural Production Systems sIMulator Next Generation model.

The modelling, led by Dr Kirsten Verburg and Dr Peter Thorburn from CSIRO Agriculture and Food, will use the data collected from the field trials to test whether the improved models can more accurately predict nitrogen cycling and losses in the field.

Nationwide trials

Twelve field trials – four in each GRDC region – will target different soil types, crop sequences and rainfall zones. Duplicate irrigation treatments, at one site in each region, will improve the understanding of fertiliser nitrogen dynamics under wet versus dry seasonal conditions. The field sites will be managed by the relevant state agencies.

The project is unique in that the same research techniques will be used throughout the country and will track the fate of fertiliser nitrogen for up to three consecutive seasons, rather
than just looking at the first season after application.

This consistency will enable the comparison of different rainfall environments and production systems, including mixed-farming systems and summer cropping. The sites will cover a broad range of background fertility and soil types.

Portable mass spectrometer and gas emission monitoring equipment will measure dinitrogen gas (N₂) and nitrous oxide (N₂O) emissions from denitrification in the lab to supplement field experiments. Photo: Professor Peter Grace, Queensland University of Technology

Pathway focus

Each three-year trial will track the fate of nitrogen fertiliser using fertiliser enriched with the naturally occurring nitrogen-15 isotope . Denitrification and volatilisation will be evaluated in detail at selected sites augmented with laboratory studies.

The denitrification work, led by Professor Peter Grace from QUT, seeks to better quantify the impact of different soil types and seasonal conditions. For instance, we know that the amount of nitrous oxide (N₂O) produced is similar in clay and sandy soils; however, the losses of di-nitrogen gas (N₂) are substantially higher in clay soils due to the greater frequency of waterlogging and low oxygen availability.

Volatilisation research will be led by Dr Graeme Schwenke from NSW DPI. Volatilisation is highly dependent on climatic conditions after fertiliser nitrogen application and has so far been little studied in the field under Australian conditions. This loss pathway could become increasingly important as more growers broadcast their nitrogen onto the soil surface, rather than tilling it into the soil.

Not all processes lead to nitrogen being permanently lost from the system. CSIRO Agriculture and Food’s Dr Mark Farrell and Dr Gupta Vadakattu will evaluate the impact of soil microbes on nitrogen mineralisation and immobilisation and residue decomposition to better understand how nitrogen moves in and out of the soil organic matter pool. The impact of stubble retention and decomposition rates are an important factor in this research.

The project also provides an opportunity to develop future capabilities with PhD students based at the University of Queensland, QUT and Agriculture Victoria.

More information: Professor Mike Bell, 0429 600 730, m.bell4@uq.edu.au