Meeting the world's growing food and energy needs while minimising nitrogen's negative impacts is one of the greatest challenges we face in the 21st century.

Over the past century, the world's population has increased four-fold. We all need to eat and more than half of us eat food grown with synthetic nitrogen fertilisers.

There's also nitrogen produced as a byproduct of the burning of fossil fuels and other industrial processes.

And tackling the issue of nitrogen pollution is where our research comes in.

So why is nitrogen a problem?

Nitrogen is an essential component of the nucleic acids and enzyme proteins found in the living cells of plants, animals and humans. Nature and people can only exist because of the availability of reactive nitrogen (Nr), which includes all forms of nitrogen except for the unreactive N2 gas that makes up 80 per cent of the air we breathe.

However, only a tiny fraction of the nitrogen we produce goes into building up our muscles. Instead, most of it gets released into the environment. And the world's production of reactive nitrogen has accelerated and continues to gather pace as our population grows and our consumption of fossil fuels and meat-intensive diets increases.

While reactive nitrogen is essential to keep pace with rising global demand for food and energy, its unintended consequence is nitrogen pollution. This takes a toll on the environment and on human health and welfare.

It escapes into the environment - it cascades through atmospheric, terrestrial, aquatic and marine pools. In the atmosphere, reactive nitrogen leads to smog, acid rain, intensifying the greenhouse effect and stratospheric ozone depletion. In terrestrial ecosystems, reactive nitrogen leads to soil acidification, forest dieback, and biodiversity loss.

In marine and freshwater ecosystems, reactive nitrogen contributes to freshwater acidification, groundwater pollution, ocean acidification and eutrophication – a build-up of nutrients in water that causes dense algae growth.

High levels of reactive nitrogen in water and air have been directly and indirectly connected with human diseases and allergies like increased incidence of asthma and colon cancer.

So, while we're all becoming more aware of our carbon footprint - one way to understand Australia's contribution to nitrogen pollution is to look at our nitrogen footprint; this is the amount of reactive nitrogen released to the environment from our food, housing, transportation, goods and services. It's a measurement that can help inform our daily choices with the resulting nitrogen pollution.

But the nitrogen footprint doesn't tell the whole story.

This is why we've proposed a new indicator that combines the nitrogen footprint with the area over which the nitrogen is released, called Nr spatial intensity (NrSI). It aims to improve how we predict the environmental impacts of nitrogen and could help identify reactive nitrogen emission hotspots, highlight potential environmental impacts, and lead to recommendations to better manage nitrogen's impacts.

Australia vs The World

The European Union estimates the cost of reactive nitrogen-related damage to be €70 billion to €320 billion per year in 2000.

Here in Australia, our research, which was released in 2016, shows we have a large nitrogen footprint – 47 kg of nitrogen per person each year. This is way ahead of Austria, Germany, Japan, the Netherlands, the United Kingdom and the United States of America (see our figure above).

Our diet, which is high in animal-protein and our reliance on coal-dependent power appear to be just two of the key drivers behind Australia's high nitrogen footprint. The consumption of animal products accounts for 82 per cent of Australia's food nitrogen footprint, with beef making up 33 per cent and dairy products another 16 per cent.

However, per capita, nitrogen footprints do not directly correlate to environmental impacts.

The nitrogen footprint reports the nitrogen pollution released from our choices, but it doesn't connect to a particular region or environmental effect. For instance, despite its large nitrogen footprint, Australia is often considered a 'green and clean' country with little environmental pollution.

This discrepancy raises questions about how our per person nitrogen footprints are spreading throughout the environment.

Getting the measure of nitrogen

Our new research demonstrates that a nation with a high per-capita nitrogen footprint could have a low per-area NrSI and vice versa. Of the seven countries we sampled, Australia had the lowest NrSI (6 kg N ha-1 yr-1) while the Netherlands had the highest NrSI (217 kg N ha-1 yr-1).

Having a large land area available for food production is associated with low NrSI in Australia and the USA. In contrast, the small land areas of the Netherlands and Japan means their NrSI is higher than other countries, despite their lower per capita nitrogen footprint.

There are factors that affect these comparisons - like land area, population density, as well as the import and export of food.

The discrepancy isn't really surprising because the NrSI and nitrogen footprint present two different concepts. The NrSI is a per-unit area indicator reflecting the potential for environmental damage in a region. The nitrogen footprint is a per-capita indicator that represents consumer behavior contributing to environmental threats.

The nitrogen footprint is a powerful tool that increases understanding of what contributes to reactive nitrogen loss.

The NrSI connects reactive nitrogen loss with the land area over which it is lost and it can better identify reactive nitrogen emission hotspots - indicating the potential for environmental impacts and leading to recommendations to help manage those impacts.

Reducing total nitrogen footprints is important but must also address the connection between the actions that create the footprint and the environment.

The United Nations Sustainable Development Goals calls on us to improve nutrition and energy access, while also improving our natural environment.

Given the pressures from our growing global populations and the world's increasing food and energy consumption, research like this is critical if it means we can identify and ultimately combat our nitrogen pollution hotspots.

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