Kentaro Hayashi began his career as a natural scientist out of an interest in living things.

But these days, he’s more focused on the environments they inhabit and how the global cycling of chemicals — particularly nitrogen — affects them.

Hayashi, a professor at the Research Institute for Humanity and Nature, now leads research on the nitrogen cycle and sustainable nitrogen management — a crucial area of study as the planet struggles to keep warming in check.

The invention in the early 20th century of the Haber-Bosch process, which made the industrial synthesis of ammonia from nitrogen gas and hydrogen gas possible, has transformed how we grow food. As a primary source of chemical fertilizers, ammonia has supported an explosive growth in food production alongside a fivefold growth in the world’s population since 1900.

While nitrogen gas is harmless and makes up 78% of the air we breathe, human activities have dramatically increased the production and release into the environment of various forms of reactive nitrogen — chemically active compounds with myriad negative impacts on human health.

Excess reactive nitrogen is a cause of air and water pollution, acid rain and eutrophication — a term that refers to a body of water that is overly enriched with minerals and nutrients.

Starting on Monday, Hayashi will chair the 10th International Nitrogen Conference in Kyoto. The forum is held roughly every three years by researchers to share their findings and get them reflected in policymaking.

The Japan Times recently caught up with Hayashi to ask why nitrogen matters and what can be done about the issues caused by its overabundance. The interview has been edited for clarity and brevity. What drew you to the study of nitrogen?

I love living things, so I started out as a natural scientist. But I realized the environment surrounding them is also crucial. If that environment deteriorates, it naturally causes harm to those creatures. And human activity is now having an incredibly large impact on that deterioration; global warming is definitely a prime example.

I joined Japan’s Antarctic research expedition about 10 years ago, but even before that, I had conducted research on an island in the Arctic. As expected, the sea ice is melting at an incredible rate, and the permafrost on land is thawing as well. As a result, the creatures are struggling to survive amid these rapid changes in the environment, and I’ve been thinking there must be something I can do to help.

When it comes to global warming, the discussion usually centers on rising temperatures, but that also involves the cycle of carbon, which makes up living organisms, and carbon dioxide and nitrogen. Nitrogen is essential for producing proteins. Living organisms need protein to survive, and they also need DNA to store genetic information. None of these things can be produced without nitrogen.

We learn in elementary or middle school science that roughly 80% of the air is made up of nitrogen. However, the nitrogen in the air is bound in a very tight, stable bond, with two atoms stuck together so firmly that they can’t be separated. To put it simply, even though we can breathe in as much nitrogen from the air as we want, once it goes in, it just goes right back out. So instead we eat other living animals and plants, and recycle them inside our bodies and break down the proteins and the amino acids they contain, and then reassemble them.

That’s how all living animals, including humans, function. We’re part of that cycle. So I realized just how incredibly important nitrogen is. I started wondering how, if human activities change, the flow of nitrogen will change, and how that would affect the ecosystem. But there are not that many researchers approaching the subject from this perspective. How has the Haber-Bosch nitrogen production process changed our lives?

Before it was developed around 1900, the global population was about 1.6 billion. Today, 125 years later, it is 8 billion, a fivefold increase. We need five times more food, and that is supported by chemical fertilizers. Approximately half of the nitrogen in the food consumed by humanity today comes from fertilizers made in the Haber-Bosch process. Without it, the world could only support about half of the current population. So what are the downsides?

Nitrogen is a classic case of “too much of a good thing.“We are now able to produce a lot of food — so much so that we can even produce food for livestock. Cows used to feed on grass, but now we give them food that humans can eat, like corn, and we grow crops for them.

Crops only absorb about half of the nitrogen fertilizers applied to agricultural fields, and the rest leaks into the air and water. Furthermore, raising livestock requires large amounts of feed crops. About 50% of the nitrogen is converted into crops. But with beef, only 10% of nitrogen is converted into meat.

As a result, meat consumption leads to significantly higher nitrogen emissions, and not many are aware of this downside. Why does nitrogen pose a problem?

Broadly speaking, there are two forms of nitrogen: stable nitrogen and reactive nitrogen. Most nitrogen in the atmosphere is largely non-reactive. A small portion is converted into reactive nitrogen compounds, including inorganic substances such as ammonia and nitric acid, and organic substances such as amino acids and proteins.

Microbes in nature create nitrogen compounds, too, and they do so at room temperature and normal pressure. The industrial Haber-Bosch process, meanwhile, requires extreme conditions like 400 degrees Celsius and 100 atmospheres of pressure, and it also consumes a tremendous amount of energy.

Plus, to produce ammonia, we need hydrogen. In the Haber-Bosche process, we extract hydrogen from natural gas, which is a fossil fuel, and then bind it with nitrogen through heat to produce ammonia. So we should still use nitrogen, but use it wisely?

Yes. When reactive nitrogen leaks into the environment, it causes global warming, air pollution, water pollution and eutrophication. If you raise livestock, the amount that gets leaked inevitably grows through feces. Ideally, we want to recycle the feces through composting, and then return them to the farmland as organic fertilizers.

Food loss is also a major issue because producing food requires nitrogen, including through fertilizers, and when that food is wasted, the nitrogen used to produce it is wasted, too. Overeating is also a problem, however. Humans are animals, so what’s beyond our consumption limit passes through our body as waste. So I would recommend giving the food you can’t consume to others, or leave it for your next meal. How does nitrogen pollution affect human health?

The biggest problem is air pollution. Nitrogen oxides themselves are air pollutants. They also contribute to the formation of photochemical oxidants, including ground-level ozone, which can cause respiratory illnesses.

In Japan, nitrogen contained in compounds such as ammonium and nitrate accounts for 10-20% of the total mass of PM2.5, expressed in terms of nitrogen. These fine particles can penetrate deep into the lungs, contributing to respiratory and cardiovascular diseases and millions of premature deaths globally each year.

Economically, nitrogen pollution caused an estimated $1.2 trillion in global damages in 2010.

Japan’s air and water are cleaner today due to strict environmental controls established after historical pollution crises during the high economic development period (from the mid-1950s through the mid-1970s). As a result, so many people in Japan actually don’t get a sense of how nitrogen-derived air pollution kills people.

But at the same time, Japan’s clean air and water have been made possible through its heavy reliance on food imports. Importing food effectively offloads nitrogen pollution to producer countries. In the future, there’s a possibility that food-exporting countries will raise the price of products based on the environmental burdens of food production, like an environmental tax, which would put Japan in a bind. So I strongly support raising Japan’s food self-sufficiency rate from an energy and food security point of view.

Beyond air quality, nitrous oxide is the third largest contributor to human-driven global warming after carbon dioxide and methane. Tell us about international efforts to tackle the nitrogen problem.

The International Nitrogen Conference began in 1998, initiated by researchers studying acid rain and transboundary air pollution. They recognized that nitrogen issues extend beyond local sites to regional and global scales.

The problem with nitrogen, however, is that its role is hard to see and understand. Research on global warming started in the 1970s, but it took until the 1990s for it to attract wide attention. The depletion of the stratospheric ozone layer became a huge issue because it increased the amount of ultraviolet rays that reach the Earth and increased the chance of developing skin cancer and cataracts.

In contrast, the issues with nitrogen are not widely understood and many only see the benefits with chemical fertilizers. Only a small number of people have raised issues with overproduction. What are you trying to achieve at the Kyoto conference?

The central theme is “Sustainable Nitrogen Management for Future Generations.”Ultimately, the actions we take and the decisions we make today can have a decisive impact on future generations. So, whenever we do something, we should think about whether people in the future will thank us for it or complain about it, rather than just considering our own interests. That’s the basic idea behind this concept.

It aims to bridge science and policy for nitrogen management, so we can maintain the benefits of nitrogen use while reducing threats such as air and water pollution. Is it difficult to create a single international nitrogen treaty?

Nitrogen touches such diverse issues that it overlaps with existing treaties, like climate change (UNFCCC), biodiversity (CBD), transboundary air pollution treaties and local water management regulations. So instead of creating a new treaty just on nitrogen, we would need an interconvention coordination mechanism.

For example, there’s currently a push to move away from fossil fuels and use ammonia as a fuel. Japan is actually among the world leaders in ammonia combustion technology. Since ammonia is made up of nitrogen and hydrogen, burning it doesn’t release carbon dioxide. That can certainly help decarbonization. However, producing ammonia can have environmental impacts, and ammonia itself is toxic, so its leaks into the environment could pose problems. In other words, the shift could potentially create other problems of its own.

If UNEP (United Nations Environment Programme) could be given a little more authority, it could help resolve conflicts and trade-offs between the existing treaties, and ideally create positive synergies. UNEP would develop the framework, but it would also need to ask scientists to work with it. I think establishing such a mechanism could help advance science-policy engagement, though it’s quite an ambitious dream.

In 2024, Japan released its national action plan on sustainable nitrogen management. What is your view on this?

Japan is actually the first and only country to have a national nitrogen management action plan. But unfortunately, there are no clear (numerical) targets for Japan’s plan. I hope targets will be set when the action plan is reviewed. What can individuals do to address the nitrogen problem in daily life?

The single most effective action individuals can take is to value food and eliminate food waste. Thinking about where food comes from and avoiding overeating is key. Staying away from meat would help, but it is difficult to change food preferences drastically.

For myself, I try to reduce my nitrogen footprint through my diet, but I also believe in enjoying things when the occasion calls for it. In Japanese terms, it’s called hare and ke: special occasions and everyday life. I eat simply most of the time, but when I feel like having meat, I enjoy it with gratitude and savor it. That way, the meat I eat occasionally tastes all the better.