Nitrogen (N) is an essential element for all life forms. It acts as the building blocks of biomolecules including proteins and DNA – as well as chlorophyll – the green pigment that allows plants to capture light energy from the sun.
In excess though, reactive nitrogen (known as Nr and including all chemical compounds of nitrogen except N₂) adversely affects the natural environment, ecosystems and human health.
Emissions in the form of ammonia (NH₃) and nitrogen oxide (NOⅹ) contribute to particulate matter and acid rain while nitrous oxide (N₂O) is a potent greenhouse gas, over 300 times more effective at trapping heat in the atmosphere than carbon dioxide (CO₂).
The adverse effects of Nr loss are most prevalent from nitrogen used in agriculture because more than 50 per cent of the nitrogen fertiliser applied to plants and more than 70 per cent of the nitrogen fed to livestock is lost to the natural environment, causing air and water pollution, growth of algae, and most famously, damaging the Great Barrier Reef.
Managing nitrogen in agricultural production is key to achieving global food security with minimal degradation of ecosystems and impacts on human health. The challenge of managing excess nitrogen extends beyond farms and requires concerted efforts from a range of stakeholders.
While varied metrics have been developed to inform different aspects of nitrogen management, few efforts have tried to combine nitrogen metrics to derive a coherent set of actions for each party that is involved and affected by the use and emissions of nitrogen.
Our research proposes the ‘5 Ps’: Production, People, Planet, Policy and Partnerships as priorities to shape guidelines for sustainable management of nitrogen using multi-dimensional nitrogen metrics.
1: Production - nitrogen use efficiency (NUE) in food production
The technical efficiency of nitrogen use (NUE) is measured as the percentage of nitrogen added to soil that becomes available to be taken up and used by plants. The NUE is an effective, widely-used indicator used for assessing practices of food production and potential Nr losses.
For example, about half of Australia’s nitrogen fertiliser use is for growing wheat, with an average annual application rate of 45 kilograms of nitrogen per hectare and yields around 2.5 tonnes² hectare, every year.
In this case, the nitrogen used by the wheat plant, the NUE, is 44 per cent ± 14 per cent. This is a measure of technical efficiency, indicating the percentage of the nitrogen applied as fertiliser that is recovered in yield when the above-ground wheat plant is harvested.
To reduce nitrogen pollution and maximise the amount of nitrogen that enters the plant the aim is for NUE to be as close to 100 as possible, without leaching too much nitrogen from the soil.
2: People - nitrogen footprint and nitrogen neutrality
The nitrogen footprint of an individual or population is another technical measure to quantify Nr losses to the environment.
The N-footprint concept is wider than the NUE measure and encompasses Nr losses from the processes of food production, food consumption, fossil fuel combustion for housing and transportation, as well as the provision of goods and services.
Estimates made using the N-Calculator model suggest the N footprint of Australia is large compared with most other countries. High levels of coal and beef consumption in Australia contribute to the size of this measure of Nr losses.
The concept of N-neutrality extends the concept of carbon (C) neutrality (or net-zero CO₂ emissions) to nitrogen.
The objective of N-neutrality is to prompt people to consider implications of their activities on nitrogen losses to the environment, focusing attention on zero net Nr loss through using less nitrogen and offsetting unavoidable N losses.
For instance, researchers have proposed ‘The Cercedilla Manifesto’ to indicate how to organise scientific meetings that are N-neutral. As a large institution with lots of staff and students, the University of Melbourne has also taken steps to cut its nitrogen footprint.
3: Planet - spatial intensity of reactive nitrogen (NrSI) and nitrogen boundary
The Nr spatial intensity (NrSI) concept is used to estimate the intensity of Nr losses on a per area basis and to map the intensity and hotspots of Nr losses for geographic locations.
The NrSI measure is different from the nitrogen footprint because it depends on area of land, population density and proportions of food supply imported and exported.
For instance, the N footprint of the Australian continent is large but the NrSI is low, while the Netherlands has a small nitrogen footprint but high NrSI.
The notion of ‘planetary boundaries for nitrogen’ is a benchmark used to assess the sustainability of nitrogen management. If we look at food security, agricultural systems generally need nitrogen inputs to meet the nitrogen requirements of crops and animals; ‘too little’ nitrogen use leads to ‘soil nitrogen mining’ where plants remove too much nitrogen from the soil and at the same time deliver lower crop and pasture yields and negatively affecting soil health.
‘Soil nitrogen mining’ not only leads to low production and profit, but also to soil degradation, through declines in soil organic matter and soil erosion which in turn reduce drought and climate resilience of agricultural businesses.
Meanwhile, the planetary nitrogen boundary sets the maximum NrSI for NH₃, NOⅹ, N₂O, NO₃ – leaching and runoff to water bodies. These are critical thresholds for environmental protection and to prevent nitrogen runoff-related issues like algal blooms.
4. Policy - nitrogen cost and price
The social cost of nitrogen is the total cost of damage to the environment, ecosystems and human health from Nr losses. Just as the social cost and price of carbon is the value of the reward for reducing carbon pollution, so too with the social cost and price of nitrogen.
As with the difference in resource use and CO₂ emissions among developed and developing nations, inequality in the use of nitrogen resources is a global dilemma that requires a commensurate national and international focus that, to date, has been lacking.
Like CO₂, consistent methods to establish inter-governmental markets in nitrogen emissions and determining a price of nitrogen that reflects the social cost of nitrogen would be a major step towards solving the nitrogen externality problem worldwide.
The focus on ‘Production’, ‘People’, ‘Planet’, and ‘Policy’ provides guidance for developing nitrogen management measures on multiple fronts and facilitates ‘Partnerships’ amongst countries and stakeholders.
A multidisciplinary Australian Research Council (ARC) Research Hub for Smart Fertilisers at the University of Melbourne has been established to develop a new class of sustainable ‘smart fertilisers’ to improve the efficiency of nitrogen use and reduce the adverse environmental effects of productive agriculture.
This Hub will facilitate ‘Partnerships’ between farmers, fertiliser suppliers, processing industries, consumers, researchers, the public sector and policymakers to provide technology and knowledge of significant value to the Australian agricultural and agribusiness sector.
Improving nitrogen management at private and public levels in society requires measures and indicators, tools, guidelines and incentives – as well as education, training and demonstration, and ultimately, effective policy.
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