The nitrogen cycle: a story of food, war and bacteria.

16/02/2022

Nitrogen is crucial to life because it is a building block of amino acids, proteins and DNA. But it turns out that half of the nitrogen in our tissues does not come from nature, but rather from factories, because humanity is interfering with the natural nitrogen cycle. To make crops grow faster, we produce nitrogenous fertilizers on an industrial scale. And now we are in a situation where humanity can no longer do without those huge factories. It all started with a major scientific breakthrough more than a century ago. In this article, we learn how the nitrogen cycle works, what prompted humans to kick it into high gear, how big the consequences are, ... and how to do things differently.

Author: Kathelijne Bonne. 

What is nitrogen?

Most crops grow due to added nitrogen fertilizers.
Most crops grow due to added nitrogen fertilizers.

A quick chemistry class: nitrogen is a chemical element, the atom with symbol N, which is ranked seventh in the famous Periodic Table of Elements. Alongside carbon, hydrogen and oxygen, nitrogen is the most important and common element in living things. Like most atoms, N doesn't like to be alone, and makes bonds with other atoms, that is how molecules or compounds are formed. To get the nitrogen into our tissues, we have to absorb it in the form of certain compounds. On the figure below you can see some common ones. Ammonia is emitted by agricultural activities; it reacts chemically in the soil, and it can be taken up by plant roots. Nitrogen oxides are important particles in smoke and soot. Atmospheric nitrogen gas (N₂) accounts for 78% of all gases in the air and is the largest source of nitrogen on Earth.

The great mill of life

And what is the nitrogen cycle? It is one of the biogeochemical cycles on Earth that sustain life and provide a continuous supply and removal of substances necessary for life. The nitrogen cycle, in turn, provides a continuous exchange between the various forms of nitrogen. There are many nitrogen molecules, but actually they can all be divided into two major groups: one group can be taken up (assimilated, as biologists say) by living things, like ammonia, this is the reactive "bioavailable nitrogen". The other group cannot be taken up by life, like nitrogen gas from the atmosphere. (By comparison, you can't eat rocks, even though they contain carbon, oxygen, hydrogen and nitrogen; meat or vegetables, on the other hand, can be eaten.) The fascinating thing about such a cycle is that through biological, chemical and geological processes one form can be converted to another. Elements such as carbon, phosphorus, iron, oxygen and hydrogen also travel through biogeochemical cycles. And this way all the elements needed for life are cycled through the great mill of Earth's processes.

But all of Earth's resources are finite, and the cycles that make essentiel nutrients available proceed at a leisurely rate. Therefore, there is a natural limit to how many mouths nature can feed. It was already known in the nineteenth century that the growth of crops depended strongly on the amount of nitrogen in the soil, and that this amount is exhaustable. Scientists therefore found ways to push back the natural limits by accelerating the nitrogen cycle. Let's look at what happened in Europe over a century ago.

Prosperous Europe, more mouths to feed

In the nineteenth century, Europe had become prosperous thanks to many great inventions and technological advances. The population had also been expanding since the Industrial Revolution; there were more - and bigger - mouths to feed. More crops had to be grown to ensure food security. Nitrogen, the fertilizer par excellence, played a crucial role. Until then, people used natural sources of fertilizer, such as manure from cattle, foliage of nitrogen-fixing plants (see further), saltpetre deposits and guano, imported from faraway countries such as Chile and Peru (*). But because of the political tensions in the early 20th Century, Germany was denied access to some resources. This led the Germans to experiment. They wanted to make bioavailable nitrogen from atmospheric nitrogen gas. It is important to know that the latter is not bioavailable. Even though it is the most common form of nitrogen, we (and most other life forms), cannot process it.

Meanwhile, Europe was on the brink of collapse, war was impending. And it just so happened that nitrogen was also needed in explosives. So there was an urgent need for nitrogen products.

Breakthrough: Fritz Haber and Carl Bosch

Fritz Haber.
Fritz Haber.

At that pivotal moment, German chemist Fritz Haber (1868-1934) was busy conducting experiments in his lab. One summer day in 1909, he managed to collect ammonia (a bioavailable nitrogen molecule), drop by drop, from his instrument in a test tube. BASF, the huge German chemical concern, bought the patent and recruited Carl Bosch (1874-1940) to make the process feasible on a large scale. They succeeded. Both men received the Nobel Prize after World War I. Their way of producing ammonia would go down in history as the Haber-Bosch process.

Carl Bosch.
Carl Bosch.

And in fact, the Haber-Bosch process is the most impactful invention there has ever been. Ammonia and similar molecules are widely applied as fertilizers for crops. Agricultural productivity increased dramatically (especially after World War II). Numbers of livestock also grew exponentially. The human world population quadrupled in a century, from 1.6 billion in 1900 to nearly 7.9 billion by the end of 2021. In fact, there are now twice as many people eating than nature can handle. Hence, half of the nitrogen in our bodies is produced through the Haber-Bosch process.

The nitrogen cycle disrupted

The result is that the natural nitrogen cycle is completely disrupted. There is a huge excess of bioavailable nitrogen that nature must take in. Pollution comes from two sources: 

  • Nitrogen deposition is the precipitation of nitrogen oxides from the air. These are created by burning fossil fuels in factories, cars, and heating installations. 
  • Nitrate leaching results from the overfertilization of fields with ammonia products as described above, in order to keep agriculture and livestock running at (over)full capacity. On lawns, nitrogen granules create a beautiful green grass carpet and repress "weeds."

What are the consequences of nitrogen pollution?

The main impacts of nitrogen deposition and nitrate leaching are:

  • Some plants grow much faster, and repress others, which reduces biodiversity (insects including pollinators die, which further increases the decline in biodiversity).
  • Soils become acid. In acidic soils, toxins become more mobile and cause damage to crops and nature. Most crops don't tolerate acidity.
  • In surface, ground and sea water, eutrophication occurs: some algae multiply unnaturally fast (algae bloom), causing the water to become oxygen-poor and other aquatic life to die, thus forming a dead zone, in rivers, lakes, deltas and oceans. Check out this map from NASA showing dead zones in the sea.
  • Amplification of global warming because nitrogen oxide is a greenhouse gas.
  • Explosive increase in number of livestock (because "thanks" to fertilization, there is a lot of livestock feed - an area the size of North and South America combined is cultivated just to feed livestock), and associated deforestation, and ammonia and methane emissions.
  • High concentrations of nitrogen in drinking water and in the air are bad for human health. Nitrogen is a smog particle.
  • The production of fertilizer in chemical plants requires natural gas, i.e. fossil fuels, to make hydrogen gas, needed for the production of ammonia. The energy to run the plant is also usually obtained from fossil fuels. 

It is unfortunate that because of a lack of efficiency (and low fertilizer prices) about half of all produced nitrogen is lost. We produce twice as much of it than is needed to sustain today's society. In Asian rice fields, for example, the loss can be close to 80%.

Let's now look at how nature provides nitrogen to plants and animals. After all, the mill of life was running smoothly before Haber and Bosch interfered.

The natural nitrogen cycle: bacteria in action

Certain types of bacteria that live in the soil can process atmospheric nitrogen gas. These are the nitrogen-fixing bacteria. They breathe nitrogen gas as an energy source, in much the same way that we breathe oxygen. And just as we exhale carbon dioxide, they emit bioavailable nitrogen molecules that can be taken up by plant roots. Other bacteria perform the opposite process, the denitrifying bacteria, returning nitrogen gas back to the atmosphere. Bacteria are thus the actual engine of the nitrogen cycle (and similar processes occur in the oceans).

Where the magic happens: nitrogen root nodules

On the land, nitrogen-fixing bacteria don't live freely in the soil. They need a cozy place to live. There is a family of plants that provides clever solutions for this need. The legumes (or Fabaceae), which include acacias, mimosas, clover, lupine and vetches, have root nodules that are found to be pleasant for nitrogen fixers. It is hence not an unnecessary luxury to have leguminous trees, plants, herbs or weeds in gardens or fields. Thanks to the symbiosis between leguminous plants and bacteria, there is enough nitrogen in the soil biosphere for plants to continue growing. And animals eat plants. And while eating - and only in this way - humans also receive nitrogen.

Nitrogen Cycle.
Nitrogen Cycle.
Mimosa, a leguminous tree (photo: K. Bonne)
Mimosa, a leguminous tree (photo: K. Bonne)

The show must go on

Presently, humanity is addicted to - and cannot live without - the industrial production of ammonia. Ammonia-producing factories are megalomaniac installations, towering reactors, coolers, compressors, heaters and catalysts rise above the horizon, and their skyline is as apocalyptic as that of nuclear power plants. It is to the benefit of all that the nitrogen cycle is eased down, even though production cannot stop entirely if we want to feed everybody.

BASF factory, Germany.
BASF factory, Germany.

But if industry can be made more efficient overall to curb losses, and if meat production declines, then ammonia production can also be greatly reduced. On a citizen level, a switch to a sustainable lifestyle will also have an impact (less meat, less motorized travel, less consumption (of imported and mass-produced products). At community level, recycling household and urban waste into compost, which is the most natural source of nitrogen, will reduce the need of artificial fertilizers. Green manure, i.e. the planting of leguminous plants in fields between harvests, was already done in the past, and should be applied again.

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(*) There was a war between Chile, Peru and Bolivia (1879-1884), the Saltpetre War, in which rights and access to nitrogen-rich deposits (saltpetre and guano) were fought.

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Sources: 

Bernhard et al., 2010, The Nature Education Knowledge Project: The Nitrogen Cycle: Processes, Players, and Human Impact. https://www.nature.com/scitable/knowledge/library/the-nitrogen-cycle-processes-players-and-human-15644632/

Smil, Vaclav, Distinguished Professor Emeritus at the University of Manitoba, 2011, Nitrogen cycle and world food production. World Agriculture. https://www.vaclavsmil.com/wp-content/uploads/docs/smil-article-worldagriculture.pdf

Smil, V. 1999. Detonator of the population explosion. Nature 400:415. https://www.vaclavsmil.com/wp-content/uploads/docs/smil-article-worldagriculture.pdf

Wikipedia and references therein: The Nitrogen Cycle: https://en.wikipedia.org/wiki/Nitrogen_cycle

Images:

BASF factory: de.wikipedia.org/wiki/BASF#/media/Datei:BASF1.jpg

Nitrogen cycle: https://en.wikipedia.org/wiki/Nitrogen_cycle#/media/File:Nitrogen_Cycle_2.svg

keywords: nitrogen cycle disrupted, nitrogen population explosion, nitrogen meat consumption, nitrogen food security, natural nitrogen cycle, haber bosch nitrogen, bioavailable nitrogen, what is nitrogen