With a keen interest in sustainability innovations, Lillian Hearn’s environmental chemistry skills have found applications within a national nitrogen accounting project supported by a GRDC PhD scholarship

How nitrogen fertiliser moves through the environment once it is applied to crops continues to be a topic of keen interest for growers and researchers alike.

To close knowledge gaps, GRDC has invested in a national consortium that aims to improve the current understanding of nitrogen cycling and loss pathways in Australian cropping systems.

Taking part is GRDC Research Scholar Lillian Hearn, who is enrolled at the University of Melbourne. She is based at Agriculture Victoria’s Horsham SmartFarm where Professor Roger Armstrong leads the Victorian arm of the consortium.

Ms Hearn works on one of the three main nitrogen-loss mechanisms – denitrification. In particular, she quantifies the denitrification pathway in Victorian cropping systems.

Denitrification is a reaction pathway driven by soil microbes scavenging oxygen from nitrate (NO_3-). This occurs when soil oxygen levels are depleted during waterlogging events. This pathway removes plant-available nitrate nitrogen (NO_3-) from cropping systems by transforming it into gaseous products, nitrous oxide (N₂O) and dinitrogen (N₂).

Ms Hearn’s mission is to trace missing nitrogen – the nitrogen that is not accounted for in the grain, crop residue and soil at the end of the season.

By quantifying denitrification losses – given a range of soil types, seasonal conditions and nitrogen rates – we seek to understand what is driving variability in nitrogen fertiliser use efficiency.

Gas samples taken from the semi-automated emission chambers are helping to quantify nitrogen loss through denitrification. Photo: Agriculture Victoria

Chemistry background

With a background in environmental chemistry and a keen interest in sustainability, Ms Hearn finds the project both captivating and challenging.

“I’m working in an industry that is concurrently addressing increasing grain production demand and trying to minimise the environmental impact of nitrogen fertiliser use,” she says.

It is challenging but I am mentored by national collaborators, and I get to work with cutting-edge technologies.

Previous studies in Victorian cropping systems have suggested that denitrification could be the dominant mechanism driving nitrogen loss in seasons that experience intermittent or prolonged periods of waterlogging.

Which N is which?

Direct measurements of denitrification N₂ losses from fertiliser are sparse. This is due to the technical challenge of detecting denitrification product N₂ in the atmosphere, which is 78 per cent N₂.

Specialised equipment is being used to better understand how nitrogen applied as fertiliser is lost through the denitrification pathway. Photo: Agriculture Victoria

Ms Hearn explains that through collaboration between engineers and chemists the consortium is successfully collecting direct measurements of denitrification gaseous loss products N₂O and N₂ throughout the cropping season.

While N₂ is environmentally inert, N₂O is the third-most important human-made greenhouse gas.

It is much less abundant in the atmosphere than carbon dioxide or methane.

However, its global warming potential is nearly 300 times that of carbon dioxide over a 100-year timescale. It is also a strong ozone-depleting substance.

Over 40 years to 2020, nitrous oxide emissions have grown by 40 per cent.

Ms Hearn explains that distinguishing fertiliser-derived N₂ losses from background atmospheric N₂ has become possible with the use of semi-automated emission chambers, coupled with highly enriched 15N isotope labelled fertiliser.

She operated the sampling chambers at Victorian field sites in Horsham and Hamilton across the 2024 cropping season and, with support, completed complementary laboratory and glasshouse studies.

The GRDC Research Scholarship has allowed me to become familiar with the grains sector and its quest for sustainable and resilient productivity gains.

“The scholarship also supported me to travel to Denmark to take part in wider international conversations on nitrogen fertiliser tracing techniques and fertiliser use efficiency at the 2024 nitrogen workshop conference,” she says.

Ms Hearn’s PhD is supervised by Professor Roger Armstrong (Agriculture Victoria), Professor Helen Suter (University of Melbourne) and Professor Peter Grace (Queensland University of Technology).

More information: Lillian Hearn, lillian.hearn@agriculture.vic.gov.au

GRDC Research Scholarship

Since GRDC began its GRDC Research Scholarship (GRS) program, it has supported more than 120 students.

A GRS supports PhD candidates in delivering innovative research that addresses constraints or builds opportunities for Australian grain growers.

The three-year scholarship, worth more than $35,000 a year, provides high-achieving students with the financial and industry support to complete their PhD in an area aligned with GRDC’s Research, Development & Extension Plan.

Some of the more recent research topics range from investigating fertiliser toxicity and tackling herbicide resistance to enhancing the understanding of mungbeans’ flowering pathway, supporting barley maltsters to develop improved malts and how growers manage risk.

GRDC manager capacity and extension Katelyn Lubcke says the program is an important investment in developing researcher capacity and ability.

It is key to ensuring we have the best researchers with the skills and expertise working on problems or opportunities relevant to the grains industry.

More information: Katelyn Lubcke, katelyn.lubcke@grdc.com.au

Resources: GRDC Research Scholarship.