In 1898, Sir William Crookes, then President of the British Association stated that: “England and all the civilised nations stand in deadly peril of not having enough to eat”. He was referring to Britain’s reliance on imported wheat and concerns that there was insufficient land to meet global demand when yields were around 1.5 tonnes per hectare.
Crookes was aware of the pioneering work of Sir John Lawes and Sir Henry Gilbert who showed that wheat yields of up to four tonnes per hectare could be produced year after year by application of nitrogen fertilisers. Crookes proposed that the power of Niagara Falls should be harnessed for “oxidating free nitrogen of the air” and thereby enabling “twelve million tons of nitrate of soda to be applied to the global wheat crop”.
Crooke’s idea was fanciful but just 10 years later Fritz Haber in Germany mastered the chemical synthesis of ammonia from gaseous nitrogen and hydrogen which led to the industrial-scale Haber-Bosch production of nitrogen fertilisers.
Today, 55% of reactive nitrogen in the global nitrogen cycle has been fixed chemically and, had it not been for Haber’s discovery, it is estimated that global population would be at least 25% fewer than now (it was 1.7 billion in 1908 and is now 6.5 billion). A world without the Haber-Bosch process would have been one ravaged by even more deprivation, human conflict and misery than has been suffered by so many over the last century and indeed is still experienced by many millions.
Nitrogen availability remains one of the primary drivers of secure global food production, along with sufficient water, essential mineral nutrients, and an ability to restrict losses due to weed competition and attacks by pests and disease. The so-called ‘green revolution’, which resulted in a further doubling of crop yields in many parts of the world, was also founded on the ability of new disease-resistant plant varieties to exploit higher levels of nitrogen fertiliser use.
However, this short history poses some extremely testing questions.
Sufficient reactive nitrogen, in the right place at the right time, is an essential component of being able to produce enough food for a future global population of more than 9 billion. But the chemical and biological fixation of nitrogen is energetically demanding. Furthermore, the natural process of microbe-mediated denitrification yields a greenhouse gas, nitrous oxide, which is 300 times more potent in this context than carbon dioxide. It seems probable that the quantity of nitrous oxide released to the atmosphere will be proportional to the amount of reactive nitrogen in the system – regardless of its source.
Nevertheless, we need to generate more plant biomass for our food and animal feed as well as to increase the size of our terrestrial carbon sinks in soil and vegetation. Increased atmospheric concentrations of carbon dioxide assist this process but only if nitrogen available for plants is not limiting.
The more food we need and the more carbon dioxide we want to fix, the more reactive nitrogen is required and the more nitrous oxide will be released. This strikes at the heart of sustainability and requires resolution.
In due course, the production of ammonia using renewable sources of electricity and hydrogen will undoubtedly become economic and feasible; biological nitrogen fixation in association with non-legume crops may also become a reality. Therefore, it is fair to assume that as we look to the future the availability of appropriate forms of reactive nitrogen should not be a primary constraint on our ability to produce enough food. However, the management of the nitrogen cycle and the ability to restrict emissions of nitrous oxide is a challenge that must be confronted.
Alternatively, can we constrain carbon dioxide and methane emissions from agriculture sufficiently to compensate for our continued need for nitrogen inputs?
This will surely require the establishment of strong synergistic interactions between scientists who understand the complexity of biogeochemical cycles and scientists who are more at home studying the fundamental biology of denitrification with a view to seeking plant-based or chemical inhibition of the process.
Without a major scientific breakthrough somewhere in this complex landscape I am left, like Crookes, to ponder if our ability to produce sufficient food in a truly sustainable way for the global population of 30 years hence will be possible.
About Professor Ian Crute, Chief Scientist, Agriculture and Horticulture Development Board
In 2009, Professor Ian Crute became the Agriculture and Horticulture Development Board’s first Chief Scientist. Prior to this appointment, he was Director of Rothamsted Research and took responsibility for all scientific, operational, commercial and external liaison activities of the institute, a post he held since 1999. He has a First Class Honours degree in botany and a PhD in plant pathology from the University of Newcastle and his committee and board memberships include Chairman of the Sainsbury Laboratory Council and membership of the ‘Future of Food and Farming’ Foresight project.
Professor Ian Crute, Chief Scientist
Agriculture and Horticulture Development Board
Tel: 0247 669 2051