According to some scientists, the "probably largest geoengineering experiment of all time" began in 1909: in a laboratory in Karlsruhe, the chemist Fritz Haber and his team managed to convert molecular nitrogen (N2) out of the air into ammonia (NH3). Carl Bosch from BASF brought Haber's process from the laboratory to the factory hall in record time. After a few years in Oppau am Rhein, the first nitrogen factory started operating, which produced ammonia on an industrial scale under high pressure and high temperatures. Because ammonia was and is a coveted raw material for the production of fertilizers and explosives.
Since then, the number of ammonia facilities has grown continuously. According to sulfuric acid, ammonia is the most produced chemical worldwide. According to estimates, the Haber Bosch process for the production of "bread from air" supplies half of humanity with vital nitrogen.
"The problem with the Haber-Bosch process is that it has made us almost gods about the nitrogen cycle on Earth," says Earth system scientist and head of the Potsdam Institute for Climate Impact Research, Johan Rockström. "We are circulating more reactive nitrogen on Earth, on land and in the oceans than all natural processes combined.«
A shift from scarcity to abundance
Reactive forms of nitrogen such as ammonia (NH3), nitrate (NO3), nitrogen dioxide (NO2) or amino acids under living beings were sought-after in short supply over large parts of the earth history. They are a biological currency, and until recently it was the nutritional element nitrogen in many ecosystems on the planet that limited the growth of plants.
But from agriculture, transport, industry and power plants, a lot of biologically available nitrogen ends up in the environment all over the place via the air and water. In addition to the synthesis of ammonia, the combustion of fossil fuels at high temperatures also contributes to this abundance, for example in diesel vehicles, from the exhaust of which nitrogen oxides then flow. Due to the entries, fast-growing, competitive plant species have an advantage, they are increasingly displacing frugal species in European forests, among others.
A number of studies indicate a loss of biodiversity through nitrogen overgrown, both for plants and insects. In the world's oceans, the entry of reactive nitrogen near coasts favors algae flowers and thereby the spread of oxygen -free death zones. According to World Ocean Assessment, their number has increased from 10 to more than 700 since 1960. It is estimated that around 80 percent of the seas are affected on the coasts of eutrophication, a nitrogen oversupply.
A comparison with the carbon cycle reveals how drastic the intervention in the nitrogen cycle is. Due to human activities, the content of the greenhouse gas carbon dioxide (CO2) in the air is currently as high as it has not been for three million years. However, with regard to nitrogen, it is not enough to go back millions of years. "The main reaction pathways of the modern nitrogen cycle developed about 2.5 billion years ago. Since then, probably nothing has had as great an effect on this material cycle as humans," says biogeochemist Donald E. Canfield from the Southern Danish University in Odense.
In front of Haber and Bosch is the nitrogen cycle.
It is true that the Earth's atmosphere has been composed of large parts of molecular nitrogen for billions of years. However, in contrast to virtually all other nitrogen compounds, N2 is chemically extremely stable due to a triple bond between the two N atoms, correspondingly unreactive and therefore not readily usable biologically.
Lightning manages to convert air nitrogen into nitrogen oxides and thus make it usable for living beings. But until the 20th century, the replenishment of reactive nitrogen was primarily a biological process: Probably around 3.2 billion years ago, the first microorganisms had a special enzyme, nitrogenase. Provided that it is supplied with enough energy, it converts N2 into ammonia and makes it usable - without any high pressure and high temperatures. This nitrogen fixation is probably the most important biological process together with the photosynthesis. One secures supplies with carbon for living beings, the other with nitrogen.
But it is possible that the two vital processes in Earth's history have gotten in each other's way over about two billion years. This is the assumption of biochemist John Allen from University College London. Together with a colleague from Düsseldorf, he published a theory in 2019. The starting point: The enzyme nitrogenase reacts very sensitively to molecular oxygen (O2) in its environment and then no longer functions. However, it is produced as a waste material during photosynthesis. "Through their photosynthesis, the microorganisms virtually cut off the nitrogen supply themselves with the help of nitrogenase," explains John Allen. Laboratory tests have shown that it becomes problematic when there is more than around two percent O2 in the ambient air. This corresponds to the oxygen content of the Earth's atmosphere over about two billion years.
The dilemma was solved only when the first plants conquered the country more than 500 million years ago. They performed photosynthesis in their leaves, spatially separated from nitrogen fixation. The microorganisms took over in the soil or at the roots of the plants. There they were better protected from the rising oxygen levels in the air. In communities, both plants and microbes benefited from the exchange of sugar and nitrogen compounds, the oxygen content in the atmosphere could continue to rise to today's almost 21 percent, and the development of new species accelerated.
The earth cooled like a fern.
This kind of symbiosis between CO2 fixing plants and nitrogen fixing microbes also controls the Earth's climate. For example, researchers suspect that azolla water ferns, which live in symbiosis with nitrogen-fixing cyanobacteria of the genus Nostoc, multiplied en masse around 50 million years ago.
In a low -salt, separated sea area in the Arctic, they found optimal growth conditions where other plants were missing the nitrogen - and thanks to their on -board nitrogen suppliers, formed a huge carpet. Her enormous biomass of large amounts of CO2 pulled out of the air for over 800,000 years. The result: The climate became cooler, the previous warm period went into the ice age, which officially continues to this day.
The human intervention in today's nitrogen cycle also has an effect on the climate – on the one hand cooling, on the other warming. Among other things, aerosols from nitrogen oxides and ammonia, which are harmful to health, contribute to a temporary cooling. In addition, to a limited extent, forests in certain regions of the world grow better due to human nitrogen inputs and thus bind a little more CO2
On the other hand, laughing gas (N2O) has a global warming. The greenhouse gas is 271 times as effective as carbon dioxide. It is created by natural microbial processes in floors and waters, including the so -called denitrification. Microbes change nitrate back into molecular air nitrogen. This process helps to close the circulation between unreactive N2 in the atmosphere and reactive nitrogen in the biosphere. Without denitrification and similar processes, much more nitrogen in reactive form would possibly accumulate in reactive form. From a human perspective, the denitrification reduces the planetary over -fertilization, but at the same time contributes to climate change.
The nitrogen trend points upwards
From a global point of view, the microbial-induced nitrous oxide generation is mainly promoted by excess nitrogen fertilizers from agriculture. According to experts, this is also the decisive factor for the climate effect of the excess of nitrogen from human sources. "Because the nitrous oxide emissions are linked to food production and because nitrous oxide stays in the atmosphere for an average of 109 years, the bottom line is that the warming effect outweighs the long-term effect," explains Sönke Zaehle, director at the Max Planck Institute for Biogeochemistry in Jena.
More nitrous gas in the atmosphere not only warms up the climate, it also damages the protective ozone layer in the stratosphere and has developed into the number 1 ozone killer in the 21st century.
In the future, humanity may tap even more heavily into the nitrogen supply in the air. Because ammonia is not only still used for fertilizer production, but is also traded as a promising, climate-friendly fuel, among other things for the propulsion of ships. Prototype plants for the production of "green" ammonia are being built worldwide, for example in Porsgrunn, in southern Norway.
There the company uses Yara hydropower to produce ammonia from water and air nitrogen. Whether ammonia will cause similarly drastic environmental changes as a fuel as its previous use as a raw material for fertilizer production, which will decisively depend on how effectively new engines burn ammonia. The less nitrogen oxides and laughing gas, the better.
The planetary nitrogen guide's outer limits
What does the human interference in the nitrogen cycle mean for the preservation of human livelihood? The earth system scientist Johan Rockström, with an international research team, first tried to give an answer with an international research team in 2009. At that time, the team presented the concept of planetary load limits. At this point it was already clear that humanity also brings more reactive nitrogen into circulation than is good for the stability of the earth as a habitat.
According to Johan Rockström, it is not a sensible option for humanity to ignore the exceeded nitrogen loading limit: "If we do not tackle the nitrogen problem, then we will also fail with all other planetary borders." Among other things, climate change, among other things The loss of biodiversity, the damage to the ozone layer and air pollution.
The human interference in the nitrogen cycle is consequence. In order to alleviate the damage, it is crucial to manage the nitrogen flows better. This includes, among other things, the significantly more targeted use of nitrogen fertilizers, which in the global average lands only about half in the fertilized plants. The rest loads air, water and the rest of the environment.
According to researchers, it would be desirable to halve the environmental impact of reactive nitrogen compounds from human sources worldwide by 2030. It is unclear whether this will happen. In March 2022, at the United Nations Environment Assembly in Nairobi, 193 countries at least agreed to "significantly reduce the burden of excess reactive nitrogen on the Earth by 2030".
