The Dependence of
Agriculture on Nitrogen

Yields for small-holder farmers in sub-saharan Africa are only 15-20% of those in similar climatic regions. Closing the yield gap on these underperforming crop lands could address problems of malnutrition in these regions as well as supporting economic growth.

The relative contribution of political, cultural and environmental factors contributing to the lack of agricultural development in sub-Saharan Africa is a matter of much debate but it has been suggested that the Green revolution strategies of the 1960s were not appropriate where population densities were low, land was cheap and market infrastructure was poor (Pingali, 2012).

Africa does however have abundant arable land and labour, which with the right political support, could be translated into food security for the region (Juma, 2011). It is unsurprising then that African countries and the international community continue to seek a Green revolution in Africa. However, an African Green Revolution that is underpinned by environmental sustainability must address the limited availability of nitrogen, but also address the challenge that faces our agricultural system globally: meeting growing food needs while simultaneously reducing agricultures' environmental harm.

Nitrogen & Agriculture

How do plants normally get nitrogen?

Nitrogen is a common element, but much of it is in the chemically inert form di-nitrogen, a major component of the atmosphere. Plants cannot use molecular di-nitrogen and most species of plants must scavenge reactive forms of nitrogen, such as nitrates, from the soil in which they grow. For most crop plants the availability of reactive forms of nitrogen is the greatest limitation to growth and production. This limitation is generally overcome in agriculture by the application of nitrogenous fertilisers. The development of nitrogenous fertilisers underpinned the Green Revolution of the 1960s and 1970s, which led to massive improvements in agricultural productivity, but heavy fertiliser use has created significant pollution problems, particularly in rivers, lakes and coastal seas.

An Expensive Solution

Why is nitrogen such a problem in small-holder farming systems?

Most small-holder farmers have very limited financial resources and cannot a afford to invest in inorganic fertilisers. Such farmers also have very few domestic animals and so have little access to manures to enrich their soils. It is common for these farmers to repeatedly plant the most desirable crops, such as maize, but such annual cereal crops put heavy demands on the nutrients in the soil and over time these soils become depleted for nutrients. If these soil nutrients are not replenished then crop yields decline. This is a common issue across sub-saharan Africa and unless we address the availability of nitrogen in agriculture, then yields for small-holder farmers will remain low.

Accessing the Inert

What is biological nitrogen fixation?

Molecular di-nitrogen is the commonest form of nitrogen, but the only organisms on the planet that can use this form of nitrogen are bacteria. They have an enzyme, nitrogenase, that can convert di-nitrogen to ammonia, a reactive form of nitrogen then can then be used in biological processes. Bacterial nitrogen fixation is the predominant natural means by which reactive forms of nitrogen enter the environment. The nitrogenase enzyme is extremely sensitive to oxygen and can only function in an environment where oxygen is highly regulated, limiting the situations in which biological nitrogen fixation can occur.

Strategic Symbiosis

How do plants make use of biological nitrogen fixation?

Some species of plants, legumes (peas and beans), have evolved the capability to associate with nitrogen-fixing bacteria. Legumes form specialised organs on the roots, called nodules, that house the nitrogen-fixing bacteria and provide the suitable oxygen-regulated environment for nitrogen fixation to occur. In this association the plant supports bacterial growth by delivering sugars from photosynthesis and the bacteria supports plant growth by delivering the reactive form of nitrogen, ammonia. Hence, this is a mutually beneficial association that is promoted by both the plant and the bacteria.

Harnessing Potential

Can we make better use of biological nitrogen fixation in agriculture?

Legume crops are often used in rotation to naturally enrich the nitrogen content of the soil. However, there is the potential to make much greater use of biological nitrogen fixation to augment agricultural nitrogen or replace the use of inorganic fertilisers. Cereal crops, such as maize, rice and wheat, are highly dependent on inorganic fertilisers for productivity. In the ENSA project we are attempting to transfer the capability of associating with nitrogen-fixing bacteria from legumes to cereals. If we can achieve this then we can create self-fertilising cereals, that can support their own productivity without the need to augment with nitrogenous fertilisers.

Reforming Agriculture

How much nitrogen fixation do we need in cereals?

Legume crops get all of their nitrogen needs from the beneficial association with nitrogen-fixing bacteria. Hence legume crops grow without the need for nitrogenous fertilisers. In contrast, cereal crops such as maize require high levels of nitrogenous fertilisers to achieve maximal growth and productivity. To replace such nitrogenous fertiliser use, we will need to achieve equivalent levels of fixed nitrogen in a cereal as occurs in a legume. However, considerably lower levels of fixed nitrogen in a cereal would significantly increase productivity in small-holder farms.