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Helping legumes become Africa's nitrogen factories

Bacteria living in nodules on the roots of legumes deliver nitrogen directly to the plant (Ken Giller)
Bacteria living in nodules on the roots of legumes deliver nitrogen directly to the plant
Ken Giller

The natural ability to fix nitrogen makes grain legumes an important crop for smallholders and a key to integrated soil fertility management (ISFM). In Africa, a major new project is getting to the heart of the complex interaction that makes nitrogen fixation work, and finding ways to make it work better.

All plants need nitrogen, and it often runs short in depleted soils. Legumes such as soyabean, cowpea, beans and groundnut provide their own solution by fostering a symbiotic relationship with bacteria living in nodules on their roots. Collectively called rhizobia, these bacteria are able to capture dinitrogen (N2) from the air and deliver 'fixed' nitrogen directly to the plant. Biological nitrogen fixation (BNF) is chemically similar to the industrial recipe for manufacturing fertiliser, but costs the farmer and the environment nothing.

This symbiosis has made legumes essential to many farming systems and diets, contributing more than 20 million tons of nitrogen to agriculture each year. But the partnership isn't perfect: many legume crops will only form root nodules for specific rhizobial strains, which may not exist in every field, and some strains are better partners than others. Farmers can help by introducing the appropriate rhizobia through inoculation, or they can plant more 'promiscuous' varieties of crops, ones more open to forming associations with different bacteria. These technologies are not new, but remain limited in their reach.

N2 for Africa

This year, however, marked the start of N2Africa, a new four year project to enhance the inputs from BNF to African smallholders. The project is funded by the Bill & Melinda Gates Foundation through a grant to Plant Production Systems at Wageningen University, which has joined forces in turn with the International Center for Tropical Agriculture's Tropical Biology and Fertility Institute (CIAT-TSBF), the International Institute of Tropical Agriculture (IITA) and other partners in eight African countries.

Such action is needed in Africa, where nitrogen-depleted soils are common, smallholder farmers have little access to fertilisers, and imported legumes often react poorly with native rhizobia. While N2Africa researchers have recorded nitrogen fixation of more than 250 kg per hectare per year in some smallholder soybean plots, the ideal conditions necessary for this are rare, and rates are often less than 8 kg per hectare across an entire farm.

N2Africa are testing rhizobial strains (Ken Giller)
N2Africa are testing rhizobial strains
Ken Giller

As Ken Giller, Chairman of the N2Africa steering committee, explains, "This is a four way interaction, requiring good vigorous legume varieties; compatible and efficient Rhizobium strains; that this pairing is well matched to the agroecological environment and the needs of farmers; and that the agronomy is handled well." The full picture contains many of the elements of ISFM, a model at the heart of the N2Africa approach. "You could say that ISFM is central for legume nitrogen fixation, and legume nitrogen fixation is a core component of ISFM," says Giller.

The N2Africa project will tackle all sides of this equation. Existing inoculants and promiscuous varieties can bring immediate improvements, and in many regions the researchers expect that available nitrogen can be boosted further within another four years if farmers can increase the area planted under these legumes and work the residues back into the soil. In the next 3-5 years even more efficient rhizobial strains will be identified and field tested in Africa, and the next 10-15 years may see new legume varieties developed based on a better understanding of how the plants and bacteria adapt to different environments.

The last mile

In this first growing season, the project is trying out the first of the new technologies with 6,000 households in eight countries. "Responses to inoculation in non-promiscuous varieties of soybean can be massive," Giller reports, "from almost no yield to 2.5 tons per hectare or more." Current inoculants offer lesser benefits to bean, cowpea and groundnut, more naturally promiscuous crops. These will depend more on the other three sides of the equation, Giller believes. "Improvements in fixation and production that we see with these crops are due to new varieties, coupled with supplemental phosphorous fertilizers and good agronomic management."

Yields of inoculated plants (right) are higher than uninoculated plants (left) (Ken Giller)
Yields of inoculated plants (right) are higher than uninoculated plants (left)
Ken Giller

Meanwhile the project is already working towards self-sustaining inoculant production in several countries. "We are ramping up production and quality of inoculants in Zimbabwe through a semi-autonomous production plant run by the government research service. In Kenya it's being produced by a private company that deals mainly in mineral fertilizers. In West Africa we will be producing locally through research labs in due course and will look for private partners once demand has been demonstrated."

"Soybean inoculants have been used in Zimbabwe and South Africa since the 1960s, but at a small scale. And promiscuous varieties of different grain legumes have been promoted very successfully in Nigeria, Ghana and Rwanda," Giller points out. "The Bill & Melinda Gates Foundation likes to talk of projects that address 'the last mile' - to take technologies that have been developed and work well but have never been delivered at scale to farmers." In the next four years, N2Africa aims to do just this, carrying legumes the last mile to a place of prominence in a more fertile Africa.

Written by: T. Paul Cox

Date published: November 2010


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