We rely on high yields to feed a population of 7 billion, and these are made possible by our ability to create an ideal environment for the crop. Ironically, the methods used to do this can create conditions which are increasingly unsuitable for growing crops in.
To combat drought we add water, causing problems such as water shortages and soil salinisation. To ensure that a crop’s nutritional needs are met we add fertilisers containing nitrogen, potassium and phosphorus.
Fertilisers have direct impacts on the local environment, with nitrogen leeching leading to a loss of wildlife. In addition to nitrogen which is lost to the environment, a small percentage is converted to nitrous oxide (N2O), a potent greenhouse gas with a global warming potential about 298 times that of carbon dioxide.
The contribution of nitrogen fertilisers to global warming doesn’t end there: the Haber-Bosch process used to create them is energetically expensive, and is a major source of carbon dioxide emissions from agriculture. The same is true of the farm machinery used to spread fertiliser on the field.
Nitrogen use has risen dramatically in recent decades, but this can’t continue. Estimates suggest that without the Haber-Bosch process global food production might only be able to support about half of today’s population, yet we need a more sustainable approach to providing food for a growing population in an increasingly uncertain climate.
There is a growing interest in using agroecology to reduce the need for synthetic fertilisers and other inputs, along with a focus on creating plants which are resilient to stresses such as nitrogen shortages.
The complexity of creating a plant which uses nitrogen more efficiently means that many scientists have turned to genetic engineering to attempt this. Trials have taken place in a variety of crops using different genes, and there have been many experimental successes over recent years. In 2007 field trials, for example, GM canola plants maintained yields even with 40% less nitrogen than is normally applied.
In 2016, Professor Mechthild Tegeder created soybeans which fixed twice as much nitrogen as their conventional counterparts. Soybean is a legume, so bacteria in its roots turn nitrogen from the air into a form which is available to the plant. By engineering soybeans to increase the flow of nitrogen from the bacteria into the seed-producing organs, Professor Tegeder and her colleagues increased yields in their glasshouse experiments.
These are just two examples of successes in experiments designed to increase nitrogen use efficiency. However, one problem faced by many of the promising studies is replication. Nitrogen pathways in plants are complex and not fully understood, so it is perhaps unsurprising that results can be unreliable in field conditions. Whilst these studies are encouraging, it will be a long road to commercialisation, and the road towards nitrogen efficient crops which benefit poorer farmers might be even longer.
New genome editing techniques have allowed some research groups to set their sights even higher, creating cereals such as rice or wheat which can use nitrogen from the air so it doesn’t need to be added to the soil. This could either be done by modifying the cereal plant to encourage bacterial symbiosis, as happens in legumes, or by adding a nitrogen fixing enzyme from bacteria into the plant itself.
Such crops could theoretically bring great environmental benefits where nitrogen fertiliser is currently applied, and yield benefits in parts of the world where access to fertiliser is limited. For now, however, there are many technical, regulatory and commercial barriers to be overcome.