There’s a harmless addiction that many people share: knuckle-cracking. That ‘pop’, better than the one from squeezing bubble wrap, results from the synovial fluid between your joints releasing a small bubble of nitrogen when you twist or compress your fingers or toes. It’s also a reminder of the centrality of nitrogen to life. The fifth-most abundant gas in the universe, the most pervasive on earth, there is roughly three times more nitrogen in the air than oxygen. While it only constitutes a mere 3% of our body weight, life, without it, would be impossible.
NO, or nitric oxide, inside your body mediates the efficient transmission of messages among the nerves. It dilates arteries easing the flow of blood and improves immunity. Adenosine triphosphate (ATP), the fundamental currency of energy for cells, is part-constituted of nitrogen. Finally, the building blocks of DNA (deoxyribonuclease), the blueprint of life, are made up of four nitrogenous bases.
Symbiosis at play
We can breathe in nitrogen but ironically cannot directly use a smidgen of it. The lungs – masters at extracting every molecule of oxygen from inhaled air to keep our cells alive – are flummoxed by nitrogen.
This is one of the reasons humanity and the rest of the animal kingdom depend on plants, who by the way are also inept at extracting nitrogen from the air. Via what could be described as The First Great Outsourcing, some plants were smart enough to form an alliance with certain ancient bacteria and archaea called ‘diazotrophs’.
Nearly 20,000 species of plants of the family Leguminosae – which include beans, chickpeas, lentils, soybeans, and peanuts – exist because of this symbiotic relationship between diazotrophs and them. The barter is that the former live on their nodules and break down nitrogen in the air into ammonia and ammonium, and the latter provide them a free supply of sugars. Nitrification is the process by which bacteria – that usually attach themselves to the root or live in the soil – turn the ammonia and ammonium into nitrites and nitrate.
That is the form a plant finally identifies as ‘useful’ and then goes on to make proteins, chlorophyll, their own proteins, and most other stuff necessary to their lives. These make their way into other animals and us. Plants that are non-leguminous – rice, wheat, maize for instance – must depend on diazotrophs in the soil.
Not all of the available nitrogen and the manufactured nitrate can be used and a large ecosystem of microbes exists to convert these nitrates back to nitrogen, called denitrification, and the cycle continues.
Need for nitrogen
The other way to keep nitrogen in the ground is fertiliser. Keeping this cycle going in a sustained manner is what organic agriculture has been about for the last 11,000-odd years.
Composting, manuring, crop rotation were the strategies employed to keep nitrification and denitrification going on and feeding people. Populations waxed and waned. Drought and disease kept populations in check but given that in the last millennium, human population has generally been on the rise, nitrate increasingly started to become short on supply.
Animal and bird excrement, crushed animal bones were the main external sources of fertiliser, being rich sources of nitrate. Called ‘saltpetre’, this was probably the first ‘dual-use’ technology – a label that is now often applied to products and applications that have the potential for war and civilian use – as it was a vital ingredient for both explosives and farming. To hark back, the ‘pop’ in the crack of knuckles is similar to the explosiveness in gunpowder.
King Charles I of medieval England ordered subjects to collect their urine for creating nitrate beds to aid small-scale saltpetre mining. The mines in Chile, that manufactured saltpetre and deposits of guano – the nitrogen-rich excrement of birds and bats – in Peru became key sources of fertiliser and gunpowder for Europe. The Guano War fought in the 1860s between Spain and its former colonies Peru and Chile revolved around control of nitrate reserves. English raiders pillaged the tombs of pharaohs – not for their treasures – but for the nitrogen in the bones of thousands of slaves buried along with the mummified pharaohs. “More than three million human skeletons,” the novelist Benjamin Labatut writes, “along with the bones of thousands of dead horses that those soldiers had ridden in the battles of Austerlitz, Leipzig and Waterloo were sent to the port of Hull in north of England, where they were ground in the bone-mills of Yorkshire to make nitrate and fertilise fields.”
While only nitrifying bacteria have over aeons mastered the art of cleaving apart these nitrogen bonds, lightning strikes are the other way that atmospheric nitrogen can be broken down to nitric acid – another natural source of nitrogen and the reason some soils are naturally rich in nitrogen and fertile.
Industrial scale
Like the energy in lightning, it takes an enormous amount of pressure and heat to rip apart the intimate narcissism of nitrogen atoms. Only in 1907 did the German chemist, Fritz Haber, figure out the degrees of heat and pressure needed to combine nitrogen from the air with hydrogen and synthesise ammonia. While this produced very little ammonia, it was after the know-how was transferred to the chemical company, BASF, did their engineer Carl Bosch figure out – most vitally the addition of an iron catalyst – a way to manufacture ammonia on an industrial scale. For the first time, humanity via the Haber-Bosch process was able to, as newspaper headlines of the day put it, “pull bread out of the air”. Being the early 20th century, this revolutionary way of making ammonia was almost exclusively put to making explosives. It would be a few years before the impact of the Haber-Bosch process would truly be felt in agriculture. In 1900, on the cusp of World War 1, the global population was around 1.6 billion with Malthusian concerns about agricultural productivity being unable to match birth rates, widely prevalent. It had taken the globe roughly 350 years to double its population to 1.6 billion. The next doubling took only 64 years – largely credited to the widespread availability of nitrogenised fertiliser.
Just a few years before he was conferred the Nobel Prize for his discovery, in 1918, Haber – ethnically Jewish but a proud German – had used his talents to create chlorine gas, mustard gas and phosgene, some of which were used against the French Allied troops, during World War 1, initiating the era of chemical warfare. Though outlawed by various conventions, humanity today has never entirely eschewed its use. Just one example being the use of tear gas, first used in 1914, by Delhi police personnel on protesting farmers. As provider and destroyer, the two-faced nature of ‘reactive nitrogen’ – as nitrogen freed from its natural atmosphere state is called – first revealed itself in war. The dark face of nitrogen still shows up in its wide application even in modern-day explosives – think trinitrotoluene and nitroglycerine. Its benign avatar manifests in firecrackers and the thousands of bags of fertiliser that douse farms across the world.
However, in the last half a century, science is reconciling with the evidence that fertilisers may have become too much of a good thing.
Way too much
Nitrogen in the air is placid, stable and keeps to itself. It forms a kind of blanket around the earth that keeps the much more flammable oxygen in check. An atom of nitrogen will prefer pairing up with another nitrogen via three strong bonds but once these bonds are broken by bacteria or industrial methods, it becomes extremely reactive.
In its quest for triple-bonded stability, nitrogen often pairs up with multiple atoms of H and O. Nitrogen paired up with three atoms of hydrogen makes the prized ammonia. The industrial fixing of nitrogen to produce ammonia via the Haber-Bosch process relies on burning up fossil fuel to produce high heat and temperature in confined spaces such as boilers and internal combustion engines. When this ammonia is fixed by bacteria or during nitrification, reactive avatar and oxygen form a melange of compounds: Nitric oxide (NO), Nitrogen dioxide (NO2), nitrous oxide (N2O), nitrate (NO3). Before the era of industrialisation, reactive nitrogen was not a problem. Only a small portion of the nitrogen available to bacteria is fixed into compounds such as ammonium and used by plants as nitrate. Other bacteria then work on ‘denitrifying’ the unused nitrate into ammonia and converting it back into nitrogen, which returns to the atmosphere, thus completing the cycle.
However, in a reversal of the historical trend, there is now too much ammonia and ammonium nitrate fed into the soil. Being extremely soluble, ammonium nitrates are washed away during rains and enter into canals and streams, and stimulate algal blooms. This process, called eutrophication, ravages biodiversity. First noted in the Gulf of Mexico, there are large patches of ‘dead oceans’ where algal blooms resulting from a surfeit of nitrates rapidly multiply, decompose and use up vast quantities of ocean inside the lakes and seas. These make them unavailable for other organisms that live in the ocean or freshwater reserves.
Then there is NO and NO2 collectively called Nox that results from burning fuel in engines, forming smog and triggering a host of respiratory ailments. Nox can also constitute acid rain if mixed with water vapour, turning into nitric acid. Nitrogen dioxide will break apart in sunlight and the free oxygen atoms latch onto oxygen molecules forming dangerous ground-level ozone. Nitrous oxide (N2O), otherwise useful as a rocket propellant and as ‘laughing gas’ – once the intoxicant of choice for Victorian England’s thrill seekers – now used in dental clinics as an anaesthetic is now the third largest greenhouse gas. This is not due to dentists but a consequence of industrial agriculture.
N2O is the third most abundant of the greenhouse gases emitted as a result of human activity. It accentuates the greenhouse effect in the same way as carbon dioxide does by capturing re-radiated infrared radiation from the Earth’s surface and subsequently warming the troposphere (lower atmosphere). It is chemically inert in the troposphere and stays in the troposphere for about 120 years before moving into the stratosphere where it ultimately leads to the destruction of stratospheric ozone. A 2024 assessment from the United Nations, called the Global Nitrous Oxide Assessment, warns that nitrous oxide (N2O), emissions are rising faster than expected, and that immediate action is required to curb the environmental and health impacts of this super pollutant.
The findings from the assessment are clear: urgent action on N2O is critical to achieving climate goals, and without a serious reduction in emissions, there is no viable path to limiting warming to 1.5°C in the context of sustainable development as outlined in the Paris Agreement.
“Abating N2O emissions could avoid up to 235 billion tonnes of CO2-equivalent emissions by 2100,” said David Kanter, associate professor of Environmental Studies at New York University and co-chair of the assessment. “This is equivalent to six years’ worth of current global carbon dioxide emissions from fossil fuels.”
Nitrous oxide is believed to be responsible for approximately 10% of net global warming since the Industrial Revolution.
This assessment identifies “practical, cross-sectoral abatement strategies” that could cut N2O emissions by more than 40% from current levels. By transforming food production systems and rethinking societal approaches to nitrogen management, even deeper reductions could be achieved, offering a critical opportunity to move the world closer to its climate, environmental, and health goals.
It also shows that N2O emissions from the chemical industry can be quickly and cost-effectively abated; agricultural and industrial practices impact the natural nitrogen cycle, leading to increased N2O emissions.
“A sustainable nitrogen management approach not only reduces nitrous oxide emissions but also prevents the release of other harmful nitrogen compounds,” said A.R. Ravishankara, chemist and atmospheric scientist, Colorado State University, and co-chair of the assessment. “This could improve air and water quality, protect ecosystems, and safeguard human health, all while maintaining food security.”
Humanity’s singular focus on CO2 emissions has blinded us to the dangers of excessive N2O, and that keeps Nandula Raghuram, Professor of biotechnology at Indraprastha University, New Delhi, awake at night.
Cause for concern
In the 1990s and early 2000s, when the research around the impact of anthropogenic greenhouse gases on climate was beginning to dribble outside academia into mainstream discourse, it appeared – contrary to the current perception – that increasing CO2 levels may be beneficial. More CO2 implied that more of it would be available for plants to imbibe and via increased photosynthesis gain more biomass and bigger seeds.
“Rising yields in Indian agriculture can partly be attributed to rising CO2 levels,” said Raghuram, “However, the proportion of nitrogen that can be absorbed through root systems in plants is limited and multiple factors – genetic and environmental – can influence this. This opens up an entirely new dimension regarding how we regard nitrogenous fertilisers.”
The current estimates from the United Nations Environmental Programme suggest that about 200 million tonnes of reactive nitrogen applied as fertiliser, or about 80%, is lost to the environment leaching into soil, rivers and lakes and emitted into the air. According to 2019 estimates, this works out to an annual loss of about $200 billion. Pretty much the same proportion applies to India. The last and only systematic survey of the sources of reactive-nitrogen emissions by sector in India was conducted in 2010 by the Indian Nitrogen Initiative, the name reflective of the fact that it was completely undertaken in a spirit of scientific altruism and not backed by governmental support. They reported that 70% of N2O emissions were from agricultural soils, 12% from wastewater, 6% from residential and commercial units, 3% from electricity generation and 2% from crop residue burning.
Chemical fertiliser (over 82% of it is urea) accounts for over 77% of agricultural N2O emissions in India, with natural sources such as manure and composting making up the rest. The overwhelming quantity of fertilisers goes into cereals, such as rice and wheat, which account for most of the N2O emissions from India.
To Raghuram, this represents a major paradox. India’s Green Revolution dramatically altered agricultural production by giving farmers a massive incentive to grow rice and wheat. Over time, however, the cultivated area under rice and wheat dramatically ate into the area allocated to leguminous plants – or the natural fixers of nitrogen. “Before the Green Revolution it was about 60: 40 (cereals: legumes) and now it is close to 90:10 with most of our legumes being imported. Most of the subsidised fertiliser that farmers get merely adds to pollution of the water, lakes and, with crop burning, the air,” he told The Hindu.
“The next battle will be for nitrous oxide,” said a scientist, formerly with the Union Environment Ministry and who’s been part of several negotiations at the annual Conference of Parties negotiations. While CO2 is the undoubted elephant in the room – for every unit of nitrous oxide emitted there are 1,241 as many CO2 atoms in the atmosphere — two major factors make N2O pressing. One, every unit of the latter warms the earth 300 times more than a similar quantity of carbon dioxide and secondly, the various forms of nitrous oxide can cause a range of blights from soil acidification to respiratory diseases, thus directly burdening health systems.
Simultaneously reducing nitrogen oxide emissions and ammonia would also significantly improve air quality, the UNEP report notes, potentially avoiding up to 20 million premature deaths globally by 2050. Abatement measures would also enhance water quality, improve soil health, and protect ecosystems from the impacts of nitrogen runoff.
“Addressing nitrous oxide emissions is essential for ensuring sustainable, inclusive and resilient agriculture that simultaneously helps countries achieve their climate and food security goals. As the assessment clearly shows, there are ways to produce more with less, by improving the efficiency of nitrogen use in agriculture and reducing excessive nitrogen application,” said Kaveh Zahedi, Director of FAO’s Office of Climate Change, Biodiversity and Environment.
With several cities in India regularly making global lists of the most polluted cities in the world, as well as the significant levels of NO2 emissions from agriculture, the health harms from nitrous oxide in India are potentially staggering though the specific linkage from nitrous oxide alone has not been teased apart. Outdoor air pollution from all sources accounts for 2.18 million deaths per year in India, second only to China, according to a 2023 modelling study published in The British Medical Journal.
The Intergovernmental Panel on Climate Change, whose reports over the years, have alerted the world to the risks of carbon dioxide concentrations and climate change has neglected the global warming impact of nitrous oxide, according to Raghuram.
Moreover, the link to nitrous oxide emissions and its role in agriculture means that it could open a new can of worms on which countries ought to be taking greater responsibility in cutting emissions. China is the world’s largest emitter, followed by India and then the United States. As far as carbon dioxide emissions go, it’s the same three countries except for India and the United States switching places. However, the principles of fairness and historical responsibility mean that per capita, the United States and several other countries in the developed world have much higher emissions than India, putting a greater onus on taking deeper emission cuts.
India’s commitment to achieving net-zero, meaning becoming carbon neutral by 2070, opens up opportunities to tackle nitrous oxide emissions. “While cutting carbon dioxide emissions is a complicated debate, India can quite easily halve its urea production – the biggest source of nitrous oxide – by improving nitrogen use efficiency,” said Raghuram.
Internationally, India’s efforts at insisting on utilising neem-coated urea, the use of nano-urea fertiliser, and organic farming are viewed as important steps to improve such nitrogen use efficiency. However, whether these are enough to make a meaningful dent in nitrous oxide emissions remains an open question.
Published – April 20, 2025 11:09 pm IST