Mapping
nitrogen scarcity

Research led by IIASA and Beijing Forestry University shows that about 80 percent of African countries are confronted with nitrogen stress or scarcity, which, along with poverty, causes food insecurity and malnutrition. Published in the Proceedings of the National Academy of Sciences (PNAS) on 12 April 2010, the study also indicates that globally, two-fifths of nitrogen used in agriculture is lost to ecosystems, with harmful environmental effects.

The research makes a comprehensive assessment of global nitrogen flows in cropland for the year 2000. By quantifying the nitrogen flows into and out of cropland, the researchers were able to identify the areas of the globe suffering from both under and over use of nitrogen fertilizer (see top set of maps).

The doubling of world food production in the past four decades could only have been achieved with an almost sevenfold increase in nitrogen fertilization. The new study confirmed the huge increase in the use of nitrogen fertilizer, showing there was no nitrogen scarcity for the world as a whole in 2000. At a national level, however, nitrogen scarcity did occur, mostly in African countries (see bottom map). Among 50 African countries, 29 had nitrogen scarcity, meaning the average nitrogen input per capita to the countries’ cropland was less than the minimum nitrogen level needed to support a healthy human being.

“Poor infrastructure in rural areas of Africa leads to nitrogen scarcity by driving up the farm-gate price of fertilizer to between two and six times that in the rest of the world,” says Professor Junguo Liu, lead author of the study. “And the benefits from fertilizer application are also reduced because of the lack of regional transport and storage facilities. Higher crop yields mean local markets are flooded with a crop that cannot be cheaply transported elsewhere. This lowers the local market price of the crop and in turn the incentive for farmers to invest in fertilizer.”

Such disincentives saw a fall in the application of nitrogen fertilizer in Africa between 1990 and 2000 (from 3.4 to 3.1 kg/cap/yr) and also contributed to a decline in cereal production (from 149 to 141 kg/cap/yr). While developed countries that suffer from nitrogen scarcity can afford to heavily import food to meet domestic needs, Africa has far more limited financial resources. Consequently the falling crop yields have played a role in the lack of progress by Africa to reduce malnutrition.

However, excessive use of nitrogenous fertilizers also poses serious negative consequences. The research calculated that globally 41 percent of the nitrogen applied to cropland was lost into ecosystems. This was particularly high in Western Europe, the eastern United States, and southern China where wet and moist soils encourage nitrate leaching into lakes, reservoirs, estuaries, and coastal seas. Nitrogen leaching damages the environment by causing eutrophication, air and water pollution, soil acidification, and emission of the greenhouse and ozone-depleting gas nitrous oxide, among others.

So how can the environmental consequences be minimized without compromising food production? The study showed growing leguminous crops or carefully selecting crops for a given agro-ecological zone can reduce the amount of nitrogen fertilizer required. It also demonstrated that leaving more crop residues in cropland or reducing tillage can also reduce nitrogen losses, although this is not always possible, as crop residues are widely used in many parts of Asia and Africa to help meet other needs such as fuel. The researchers concluded that more effective management of nitrogen is essential to minimize the harmful environmental consequences.

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Last edited: 11 August 2012