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Give Peas a Chance

Thursday 26th Jun 2014, 04.45pm

What do peas, antlers and explosives have in common? Nitrogen; it's a very interesting element. It's crucial to plant growth and therefore global food suppl. In this video we explore how science is revisiting an old relationship that involves a family of plants including peas.

Nitrogen

Nitrogen could be one of the most interesting elements in the periodic table.  It’s vital to life, but humans have gone to war over it.  In its elemental form it’s incredibly stable, and yet in compounds it’s so reactive it is used to make explosives.  Its availability as nitrate and ammonia limits the growth of plants, the global food supply and hence the growth of human civilisation. Then there’s the chemist who used it to make poisons, which were later used on his own family.

Humans have gone to war over it

Inert gas

As a pure element, nitrogen is a colourless, odourless gas.  It makes up nearly 80% of the air that we breathe and is found in all organisms – mostly in amino acids, and so proteins, but also in DNA and RNA.  Take us humans apart and you’ll find that around 3% of our mass is nitrogen – that’s the fourth most abundant element in our bodies after oxygen, carbon, and hydrogen.

In molecular nitrogen (N2), the two molecules are held together by a very strong triple bond which makes it so very stable.

Not so inert compounds

Once nitrogen is combined with other elements to make compounds, it becomes extremely reactive.  This instability comes about as nitrogen returns to the ultra-stable N2 form.  Such compounds are so unstable that they can be used to create explosives.  In “Give Peas a Chance”, we show nitrogen triiodide.  This is a contact explosive, which means that even a light touch will make it explode.

In the 1860s, members of the Nobel family, now famous for the Nobel Prizes, were researching the safe handling of another unstable nitrogen explosive, nitroglycerin.  A huge explosion at the plant at Heleneborg, Sweden, in 1864 killed the youngest brother, Emil Nobel.  His brother, Alfred, went on to produce a stable, and therefore safer, version - dynamite.

Once nitrogen is combined with other elements to make compounds, it becomes extremely reactive... Such compounds are so unstable that they can be used to create explosives.

Nitrogen wars

As well as dynamite, nitrogen is core to other potentially deadly agents.  Combine it with carbon and you make a chemical group known as the cyano group – central to cyanides.  Then there are fertiliser bombs and, as you’ll know from “Give Peas a Chance”, fertiliser contains nitrogen.  Nitrogen could also be potentially used in chemical warfare, as ‘nitrogen mustards’ are cytotoxic chemotherapy agents similar to mustard gas.

But as well as nations going to war with nitrogen, they also go to war over nitrogen.

Animal hair and fur is a particularly rich source of nitrogen.  Although, like humans, nitrogen makes up 3% of total mass, this figure is as high as 15% in hair.  This is due to the amino acid composition of keratin, the protein which is the main structural component of hair, nails and antlers.  But keratin takes a long time to decompose.  Manure, however, gives up its nitrogen to plants very quickly, making it an excellent fertiliser.

The Chincha Islands sit off the southwest coast of Peru.  From the 1840s to 1870s, Peru enjoyed an economic boom during what was known as the Guano Age.  Guano (bird poo) was a valued fertiliser that is rich in nitrogen and phosphate, and the Chincha Islands were home to a long established colony of seabirds.  With populations growing, the value of the islands increased and in 1864, Spain and Peru went to war over them. When the guano ran out it was replaced by mined nitrate again largely controlled by Peru. However, in The War of the Pacific (1879-1883) Chile fought Bolivia and Peru for control of the nitrate mines in the Atacama Desert region. Chile won, taking control of nitrate production and a significant part of the world’s food production. Nitrate was the oil of its day and a huge source of wealth.

Haber-Bosch

Since The War of the Pacific, the population has grown several-fold, but we no longer go to war over poo.  This is mainly due to the Haber-Bosch process, a method of directly synthesising ammonia from hydrogen and nitrogen.  Ammonia is the key constituent of fertiliser that is so important for plant growth.  In peas, beans and other members of the legume family of plants, the bacteria in the root nodules, called rhizobia, give a helping hand.  The rhizobia take atmospheric nitrogen and combine it with hydrogen to form ammonia – a process called nitrogen fixation, because it fixes inert N2 into a biologically usable form.  They do this in return for a carbon source from the plant, which is ultimately derived from photosynthesis in their leaves. The use of nitrogen-fixing legumes had been essential to agricultural productivity for centuries and only in the 19th century did guano and then mined nitrate start to supplant legumes as the main source of fixed nitrogen in agriculture. However, by the beginning of the 20th century an increasing global population meant that even mined nitrate could not keep up with mankind’s insatiable appetite for feeding fixed nitrogen to its crops to drive up grain yields.  Mankind faced mass starvation.

The huge breakthrough for chemists Haber and Bosch was creating a method of producing ammonia on an industrial scale, which is then easily converted to nitrate and urea to be fed to plants as fertiliser.

After working on the problem of artificial nitrogen fixation, Fritz Haber went on to develop chemical weapons in the First World War.  His wife Clara, also a chemist, was so distraught by her husband’s choices, that it contributed to her taking her own life while her husband was at the Western Front deploying chlorine gas.

Following the war, Haber’s research moved to developing pesticides based on cyanide.  This ‘Zyklon process’ was used by the Nazis in their death camps.  Millions were killed with Zyklon B, including Haber’s own extended family.

The modern approach

An ongoing problem with the Haber-Bosch process is the release of excess nitrate into the environment.  As well as soil run-off from fertilisers causing dead zones in waterways, bacteria can also use the excess nitrate to produce nitrous oxide – a greenhouse gas.

But feeding the ballooning World population continues to be a huge problem, so now we are turning to biology for solutions.

Feeding the ballooning World population continues to be a huge problem, so now we are turning to biology for solutions.

Legumes could be the answer

Before, the use of guano, mined nitrate and then chemically synthesized Haber-Bosch ammonia it was nitrogen-fixation by legumes that supplied the biggest nitrogen input into global agriculture. However, although legumes are the third largest plant family, only a handful of species have been cultivated for human consumption.  These crops include peas, beans, soybeans and peanuts.  The variety can be extended through selective breeding for productive crops, which can replace non-legume crops that require artificial fertilisers.

After they have grown, died and rotted down, legumes provide nitrogen to other plants, such as cereals (eg wheat and maize), that are grown in the same soil in rotation.  Historically, this process has been exploited by farmers to improve or maintain productive soil.  But with the advent of cheap fertilisers to increase yields, in recent decades, farmers have preferred to reduce inefficiency by simplifying planting and harvesting.

Yet another approach is for humans to become matchmakers between legume’s root nodules and the bacteria that colonise them.  Plants have no way of knowing which bacteria are the most efficient at fixing nitrogen, but the balance can be artificially adjusted so the more efficient bacteria are taken up. It is also possible to get some nitrogen-fixing bacteria to colonise cereal roots.  While these are not nitrogen-fixing powerhouses like legume nodules, we may be able to get significant fixed nitrogen from them.

This relationship is symbiotic, with benefits to both plant and bacteria.  The rhizobia fix atmospheric nitrogen into ammonia and the plant provides organic acids as a food source to the bacteria.  This mutual benefit is the clue to how the relationship started – it is thought that the environment in the root nodules provided an advantage in allowing the rhizobia to fix nitrogen more efficiently than they could if living freely in the soil.

To avoid pollution, global warming, and continue to feed an expanding population, we really should “Give Peas a Chance”!

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