Agricultural nitrogen pollution is global threat, but circular solutions await

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Agricultural nitrogen pollution is global threat, but circular solutions await

by Claire Asher on 16 January 2024

• Nitrogen is an essential element for living organisms, needed to build DNA, proteins and chlorophyll. Although nitrogen makes up nearly 80% of the air we breathe, it’s availability to plants and animals is extremely limited. As a result, nitrogen has been a limiting factor in crop growth since the dawn of agriculture,
• Humanity shattered those limits with the Haber-Bosch process to make ammonia and synthetic fertilizers, driven by fossil fuels, and now used in vast amounts on crops. But that nitrogen influx has disrupted Earth’s natural nitrogen cycle. Today, nitrogen pollution is causing overshoot of several planetary boundaries.
• Nitrates pollute waterways, causing eutrophication. Nitrous oxide is a powerful greenhouse gas and an ozone-depleting substance. Ammonia is a cause of air pollution, with severe health impacts. Nitrogen is also used to produce potentially long-lived synthetic substances that themselves can become pollutants.
• Better agricultural management and technology could cut a third or more of nitrogen pollution. Circular economy solutions include better fertilizer efficiency, enhanced natural nitrogen fixation, and recovery and reuse of wasted nitrogen. Societal changes are also needed, including a shift in human diet away from meat.

 

As the world grapples with escalating climate change, policymakers remain laser-focused on CO2, with humanity striving to decarbonize energy systems, capture carbon, issue carbon credits, and plant millions of trees to absorb emissions.

But carbon dioxide is just one of several powerful greenhouse gases destabilizing the global climate, and just one of the human-produced pollutants severely impacting the natural world and threatening to push Earth out of its current habitable state.

Another substance that rarely enters the limelight, but arguably has an even greater impact on our planet’s life support systems, is nitrogen.

“Nitrogen is everywhere and invisible,” and its influences are many, says Mark Sutton, an environmental physicist at the UK Centre for Ecology and Hydrology. Massive imbalances in Earth’s natural nitrogen cycle, brought about by humanity’s agri-food, industrial and transport systems, have caused scientists to sound multiple alarms — especially over the last two decades – warnings that have gone largely unheeded. But analysts say circular economy solutions await, though they’ll require sweeping agricultural and societal changes.

Haber-Bosch: A double-edged sword

Nitrogen is essential. “Without nitrogen, there is no life. It’s the building blocks of DNA, amino acids, and proteins. It’s in chlorophyll that fuels photosynthesis,” explains David Kanter, an environmental scientist at New York University.

But although this element makes up nearly 80% of the air we breathe, and is a vital stimulus for plant growth, nitrogen’s availability to plants and animals is extremely restricted. As a result, nitrogen has been a limiting factor on crop yields since the dawn of agriculture. Historically, farmers had just two tools to deliver more: planting legumes to fix nitrogen into the soil, and applying livestock manure to recycle nitrogen waste.

That changed dramatically in the early 20th century, when German chemists Fritz Haber and Carl Bosch developed an industrial process to extract nitrogen from the atmosphere and turn it into ammonia, using high temperatures and pressures. Their invention was first harnessed to meet the demand for nitrate to make explosives during World War I. After the war, the chemical industry pivoted to produce large amounts of synthetic nitrogen fertilizer.

Between 1900 and 2000, the global population increased from 1.6 billion to 6 billion people, but agricultural land only expanded by 30%. This gap was largely filled by synthetic nitrogen fertilizers created via the Haber-Bosch process, enabling astronomical increases in crop yield.But by capturing nitrogen from the atmosphere and applying it to agricultural lands, Haber-Bosch also doubled the scale of the global nitrogen cycle.

“It’s the most consequential invention in human history,” Kanter says.

Since 1961, synthetic fertilizer use in agriculture has increased more than tenfold, but much of that nitrogen ends up not in the food on our plates, but polluting the air, land and water.

The problem arises because nitrogen applied to agricultural land doesn’t stay there. “The system in all its components is leaky,” explains Rasmus Einarsson, a researcher at the Swedish University of Agricultural Sciences.

Atmospheric nitrogen in the form N2 is extremely unreactive because the two N molecules are joined by a strong triple bond. But when atmospheric N2 is fixed — by lightning, by plants via nitrogen-fixing microbes in their roots, or by humans through the Haber-Bosch process — that triple bond is broken and nitrogen is converted into more reactive forms, such as ammonia, nitrate, nitrogen oxides or nitrous oxide.

Reactive nitrogen easily converts from one form to another, and this chemical dexterity allows it to rapidly cascade through the environment. “It can go on this journey of environmental destruction, where it starts off perhaps as ammonia, contributes to air pollution, and gets transformed into nitrate, contributes towards water pollution, then gets lost to the atmosphere as nitrous oxide,” exacerbating climate change and ozone depletion, explains Kanter.

It may in part be the mind-boggling complexity of this chemical reactivity — and the difficulty of communicating these largely invisible processes to the public — that has contributed to humanity’s slow response to the escalating nitrogen pollution crisis.

Nitrogen impacts on multiple planetary boundaries

As nitrogen cascades through its reactive forms, it contributes to the dangerous overshoot of many of the nine planetary boundaries that scientists say must be respected if we’re to keep Earth habitable.

Today, our massive alterations of the global nitrogen and phosphorous biogeochemical cycles are so severe they’ve been defined as a planetary boundary in their own right — the first of the nine to be dangerously transgressed.

Nitrates seep from agricultural lands into groundwater and are washed into rivers, lakes and estuaries, where they can cause eutrophication and oxygen-depleted dead zones, suffocating aquatic life and impacting both the freshwater and biosphere integrity planetary boundaries.

Agriculture also releases around 5 million metric tons of nitrous oxide into the atmosphere annually, impacting the climate change planetary boundary. Nitrous oxide is a potent greenhouse gas, with a life span of more than 100 years and around 275 times the climate-warming potential of carbon dioxide.

Nitrous oxide also contributes to the ozone-depletion planetary boundary. The huge reduction in chlorofluorocarbon (CFC) production achieved by the 1987 Montreal Protocol has now made nitrous oxide the dominant ozone-depleting substance emitted by humans.

A 2022 report by the World Meteorological Organization revealed that the atmospheric concentration of nitrous oxide is at the highest level ever recorded, up 125% over pre-industrial levels.

An additional bane: nitrogen oxides and ammonia, both reactive forms of nitrogen, are responsible for 15-30% of particulate air pollution, seriously affecting human health.

Another source of impacts: Nitrogen is used to make novel substances, including TNT, nylon and other synthetic products that have the potential to be long-lived environmental pollutants, linking the element to yet another transgressed planetary boundary.

On top of all this, escalating climate change is likely worsening nitrogen pollution. Extreme rainfall and flooding increase nitrate runoff, while a warmer climate enhances nitrous oxide and ammonia emissions from agriculture.

A regional problem with global consequences

The first estimate of the safe planetary boundary for human nitrogen use was set in 2009 at around 35 million metric tons per year — 25% of current levels — although experts admit this was a very rough first guess. Six years later, Will Steffan of Australian National University and the Stockholm Resilience Centre, along with his colleagues, revised this estimate to 73 million metric tons per year, roughly 50% of current levels.

One reason it’s proved so difficult to quantify a safe planetary boundary for nitrogen is that while nitrogen pollution has global consequences, both its use and most direct effects are local.

“It’s multi-scale and multi-impact,” says Sutton.

These local effects aren’t uniformly distributed across the planet. “It’s a regional problem,” says Wim de Vries, an environmental systems analyst at Wageningen University & Research in the Netherlands. “It’s more a matter of being too much here and too low there,” he explains.

In 2022, de Vries and colleagues estimated regional boundaries for agricultural nitrogen losses to protect freshwater and biodiversity, which added up to a budget of 57 million metric tons of nitrogen per year, or around 48% of current levels.

Local and regional boundaries, all taken together, “add up to a planetary boundary,” de Vries explains, but “that total number assumes optimal distribution of the nitrogen,” which is not currently the case. In some areas, crop growth is stunted by lack of nitrogen; elsewhere, fertilizer overuse is causing severe air, land and water pollution.

A stark example is China. Home to nearly 20% of the world’s population but just 7% of global farmland, China uses almost a third of all nitrogen fertilizer produced. The resulting overuse caused China’s nitrogen pollution to increase by 60% each year between 1980 and 2010, with serious impacts on ecosystems and human health.

One estimate suggests that in India, Pakistan and eastern China, reductions in fertilizer use of more than 80 kilograms per hectare per year, or about 70 pounds per acre, are needed to return these regions to the planetary boundary safe zone.

But the problem is the opposite in sub-Saharan Africa, where farmers are unable to afford or access synthetic fertilizer. Underuse of fertilizer there results in insufficient nitrogen in soils for optimum plant growth, severely limiting crop yields and leading to malnutrition. Intensive farming with limited fertilizer also means that farmers are inadvertently depleting soil nutrient reserves; in the past 30 years, soils in sub-Saharan Africa have lost an average of 22 kg per hectare (20 lbs/acre) of nitrogen.

In many parts of Europe and the United States, fertilizer use is more moderate, but nitrogen pollution is still a pressing issue. In the Netherlands, which has the highest livestock density in the world, excessive nitrogen pollution is devastating ecosystems. A 2011 analysis estimated that nitrogen pollution cost the EU between 70 billion and 320 billion euros annually ($97 billion and $445 billion at the time) — more than double the estimated value that fertilizers add to EU farm income.

Overuse worsens the leaky nitrogen cycle

The ease with which nitrogen flows from agricultural land into the environment has trapped humanity in a vicious cycle: Farmers must apply fertilizer to provide the nitrogen needed by crops, but much of that nitrogen is lost, becoming a harmful pollutant.

And “Everything that is lost or removed from the agricultural system has to be replaced,” Einarsson explains, by planting nitrogen-fixing crops, or applying synthetic fertilizer.

As little as 20% of the nitrogen applied to agricultural land as fertilizer makes it into our food, a metric known as nitrogen-use efficiency. “That’s 80% being lost back to the environment, and [contributing to] a very non-circular economy,” says Sutton. That’s hugely inefficient and harmful.

Nitrogen-use efficiency varies greatly depending on crop variety, soil type and local climate, but there’s one universal: It plummets when fertilizer is overused. For example, once crop nitrogen requirements are met and overuse ensues, nitrous oxide emissions from soil increase rapidly

In Europe, where environmental regulations are stricter and fertilizer use more targeted, nitrogen-use efficiency is generally higher than in China, where fertilizer is often applied excessively. Sub-Saharan African soils are so depleted, and crops so starved of nitrogen, that as much as 80% of the little fertilizer that is applied is taken up by crops.

This nonlinear relationship between fertilizer application and nitrogen pollution means farmers in sub-Saharan Africa could greatly increase their fertilizer use with a relatively small impact on climate change-causing nitrogen emissions. In contrast, reducing fertilizer use in countries like China could offer a disproportionally large benefit for curbing climate change by cutting agricultural nitrous oxide emissions.

In regions where synthetic fertilizer is overused, farmers making drastic use cuts “would not only be able to maintain their yields, but could potentially even increase their yields.” That’s because a reduction in nitrogen inputs would mitigate the local impacts of nitrogen pollution, Kanter says.

Unfortunately, simply cutting fertilizer use without technological and societal changes isn’t an option. We can’t achieve agreed-upon environmental target boundaries and still feed a growing global population, now topping 8 billion people, without an increase in nitrogen-use efficiency.

Circular economy solutions

Human activities have now driven us past the safe zone for six of the nine planetary boundaries, so addressing poor nitrogen management habits has never been more urgent. “Returning to the [safe] planetary boundary for nitrogen would also make it much easier to return to [safety for] essentially every other planetary boundary,” says Kanter.

That statement deserves to be underlined: Successfully addressing agricultural nitrogen overuse could positively impact many of humanity’s worst environmental conundrums. Thankfully, there are solutions at hand — provided we have the political will to embrace them.

Research suggests at least one-third of lost nitrogen could be avoided through better management and better technology. These solutions fall into three categories: improving nitrogen fertilizer use efficiency so more of the applied nutrients end up in our food; enhancing natural nitrogen fixation; and recovering and reusing nitrogen that is currently wasted.

Solution 1: Improving nitrogen-use efficiency

Precision agriculture techniques can increase nitrogen-use efficiency by inserting fertilizer directly into soil, and applying the right amount at the right growth stages to maximize crop uptake. Techniques already exist to meet these requirements.

Around 200 million metric tons of nitrogen are currently lost to the environment each year from human activities, equivalent to between $200 billion and $600 billion in synthetic fertilizer. So improving nitrogen-use efficiency wouldn’t only reduce pollution, it could save farmers vast sums of money because they’d need to buy much less fertilizer.

“Increasing nitrogen-use efficiency is extremely important to further reduce [nitrogen] losses,” says de Vries.

Solution 2: Enhancing natural nitrogen fixation

Increasing the use of nitrogen-fixing cover crops, such as clover and vetch, could bring more nitrogen into the agricultural system naturally and reduce the need for synthetic fertilizer.

Unfortunately, this traditional farming practice gave way over the past century to intensive cultivation by industrial agribusiness, which uses synthetic fertilizers to grow profitable high-yield monocrops.

New biotechnology solutions could also help solve the fixation problem, by genetically engineering crops to enhance their existing nitrogen-fixing capabilities, or by introducing that capability to crops currently lacking it.

Solution 3: Recovering lost nitrogen

Wasted nitrogen could be recovered and reused at every step of the food system, reducing the need for synthetic fertilizer and slashing pollution.

On farms, crop residues could be recycled to produce organic fertilizer. Stored manure could also be covered to reduce ammonia emissions to the atmosphere, allowing more of the contained nitrogen to return to the land when the manure is spread.

Likewise, reconnecting crop and livestock farming would make it easier for farmers to make use of manure and rely less on synthetic fertilizer. Over the past century, “synthetic fertilizers have allowed us to separate livestock farming from crop farming,” Einarsson notes, a development that was economically profitable for farmers but had harmful environmental results.

Where once there were mixed farms, with nitrogen-rich livestock manure used to fertilize crops, today the most productive land is used for specialized crop production, largely fertilized with synthetic nitrogen, while livestock is pushed to the less-productive edges.

“When you industrialize, you specialize,” Kanter explains. By decoupling crop and livestock production, we’ve made it harder to implement circular solutions.

Cutting nitrogen waste in the food supply chain

Nitrogen impacts could also be slashed by reducing food waste. An estimated 30% of food produced globally is wasted during cultivation, processing and consumption (though some of that includes vegetable and fruit peels, animal bones, and other inedible organic matter).

Minimizing this waste could reduce demand on the agricultural system, thereby reducing the need for synthetic fertilizers. One study found that reducing food waste by 50% could lower total agricultural nitrous oxide emissions by 10-20%, helping curb climate change.

In our towns and cities, agricultural nitrates in the water supply could be turned into a fertilizer source. Currently, to avoid eutrophication and drinking water contamination, municipal wastewater treatment plants remove nitrates, then release this nitrogen back to the atmosphere as unreactive N2 through a process called denitrification.

One analysis found that around $40 billion worth of nitrogen produced each year by the expensive Haber-Bosch process doesn’t cultivate our food, but ends up being removed from water by treatment plants and expelled skyward.

“That is a waste of very expensive resources,” says Sutton.

We could “redesign our wastewater treatment plants for the future, to not [waste] that nitrogen through denitrification, but to recover it and put it in a fertilizer bag,” says Sutton. Innovative technology now in development could turn the wastewater treatment facilities of today into the “fertilizer distribution plants of the future.”

However, major investments in revamping wastewater infrastructure would be needed to implement these technologies at scale, Sutton says. In the long-term, these investments could pay for themselves, by supplying recovered nitrogen as an alternative fertilizer, reducing reliance on the financially and environmentally costly Haber-Bosch process.

The residual solid waste from wastewater treatment plants, known as ‘sewage sludge’, is widely used as an agricultural fertilizer. But by replacing denitrification with nitrogen recovery technologies, “a much bigger amount of nitrogen could be returned as fertilizer”, explains Sutton.

Environmentalists have raised concerns over contamination of sewage sludge fertilizers with a cocktail of toxic chemicals from industrial wastewater, including polychlorinated biphenyls, dioxins, phthalates and microplastics. Keeping domestic and industrial wastewater streams separate would alleviate these concerns and allow for much more efficient processing of sewage sludge into fertilizer, Sutton says.

In addition, novel technologies could remove many contaminants from domestic wastewater and recover more plant-accessible nitrogen from manure, sewage sludge and other biological waste, with the potential to replace about 10% of synthetic nitrogen fertilizer. “[C]ircular reuse, recycling, and upcycling of what is today seen as a waste [is] an obvious opportunity to increase overall system efficiency and reduce waste,” says Einarsson. However, “circularity has to be seen in the system perspective, reducing the need for new inputs,” he warns.

If we learn to efficiently reuse nitrogen rather than waste it, then we can produce and use less synthetic fertilizer. If, however, we apply solutions to reduce nitrogen loss without cutting fertilizer use, we risk making nitrogen pollution worse.

Dietary changes for a healthy planet

The circular solutions just described can play an important role in reducing nitrogen’s environmental footprint. But on their own they won’t be enough to bring us back into the safe zone of the nitrogen planetary boundary. Drastic cuts to nitrogen fertilizer use in Europe, for example, would only reduce nitrogen losses there by 30%.

But combining the technological and management solutions discussed so far with societal changes could achieve far more significant reductions.

A key nitrogen solution hinges on dietary change. At present, about two-thirds of agricultural land is used to graze livestock to feed humanity’s growing appetite for meat and dairy products. But plant protein offers a far better use of nitrogen.

“If you are eating a plant-based diet, it’s more efficient,” says de Vries.

Reducing the proportion of animal protein in the human diet, particularly in Europe and North America where meat consumption has soared in the past half-century, could substantially reduce reliance on synthetic fertilizer. But this societal change is more challenging to achieve than technological solutions, relying on the individual choices of billions of people.

Fragmented science and policy

Another barrier to rapid change: The intricacies of nitrogen pollution suffuse many scientific fields and policy sectors, so efforts to understand and address the problem have been fragmented across scientific disciplines and policy frameworks.

As a result, isolated actions can cause unforeseen trade-offs, with a policy designed to reduce one source of pollution inadvertently increasing another, and potential synergies missed.

In 2020, Kanter and colleagues assembled a database of global nitrogen policies. Their analysis revealed a severe lack of integration between environmental sectors. Kanter was surprised to discover that “the bulk of policies in the agricultural sector either incentivize or facilitate nitrogen use.”

National and regional policies have focused mostly on reducing nitrogen pollution of air and water, but nitrogen has been largely ignored in climate policies. For example, despite nitrous oxide’s significant contribution to both ozone depletion and climate change, it is not regulated by the Montreal Protocol on ozone, nor included in most countries’ nationally determined contributions to reduce greenhouse gas emissions under the Paris climate agreement.

But this situation is starting to improve, with more scientists and policymakers working toward systemic solutions. The International Nitrogen Initiative (INI), for example, established in 2003, brings scientists together to work on integrated solutions to nitrogen pollution.

“If you clump it all together, you get a bigger, stronger picture for taking action, and less reasons for objecting,” says Sutton, who chaired the INI from 2012 to 2018.

In 2016, the U.N. Environment Programme (UNEP) and INI launched the International Nitrogen Management System, gathering evidence to support international policies for better nitrogen management. INI has also developed a nitrogen footprint tool called N-print, which allows individuals and institutions to calculate their nitrogen emissions.

International organizations are taking steps to address some of the most serious sources of nitrogen emissions. The European Commission’s 2020 Farm to Fork strategy set targets for reducing nitrogen pollution by 2030, including a 50% reduction in nutrient losses from food systems.

This is an “extremely ambitious goal,” says Einarsson. In 2022, the U.N. Convention on Biological Diversity followed suit, setting a target to halve nutrient loss by 2030 as part of the Kunming-Montreal Global Biodiversity Framework.

The Farm to Fork strategy says it will require at least a 20% reduction in synthetic nitrogen fertilizer use to achieve its target. However, research published this year suggests the policy’s recommended measures will be insufficient to meet the goal of halving nutrient losses.

Balancing environmental goals with livelihoods

These international nitrogen goals need to be translated into national policy with care, experts say. Lives and livelihoods depend on it.

At present, most policies to address nitrogen pollution mistakenly focus on changing farmer behavior, says Kanter “They’re based on the assumption that farmers have a lot of autonomy and a lot of decision-making power, when in fact, they don’t.” He adds, “There are market constraints, regulatory constraints, social constraints, knowledge constraints, [and] land constraints … often being imposed by actors and forces beyond the farm.”

Exemplifying this problem are recent volatile disputes between farmers, environmentalists and policymakers in the Netherlands. The Dutch government’s attempts to address the nitrogen crisis by reducing the numbers of cows, pigs and chickens raised were met by angry protests from farmers who saw their livelihoods being attacked.

Kanter believes that alternative policy approaches, targeting producers of synthetic fertilizer rather than consumers, could be more effective drivers of change. He suggests that performance standards set by governments for fertilizer manufacturers could spur innovation in a sector little changed since the Haber-Bosch process was invented more than a century ago. But that might be politically challenging to achieve, since nitrogen fertilizer production is tightly bound to the powerful fossil fuel industry.

The global community has had past success in tackling planetary boundary transgressions; the Montreal Protocol’s effectiveness in tackling ozone-depleting CFCs is a prime example. But reducing nitrous oxide emissions will not be so straightforward: Whereas CFCs were produced by a small number of industries, reducing nitrogen pollution requires fundamental changes across the entire food system.

“You have many diverse actors making it much harder to mobilize change,” Sutton cautions.

Experts who spoke to Mongabay also mused about future practices in developing nations. Will they follow the path wealthy OECD countries took in the past century —increasing synthetic fertilizer use and meat consumption as poverty decreases — or will they be able to sidestep the vicious cycle of nitrogen waste, and make wiser use of this critical element to boost food production while respecting planetary boundaries?

Some worry that nitrogen pollution may prove even more challenging to address than climate change because of the essential role this element plays for life on Earth. “You could imagine a world without CFCs … you can even imagine an energy system without carbon, [but] you can’t imagine a food system without nitrogen,” says Kanter.

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