British and Indian researchers on a new way to reduce farmers' great dependence on nitrogenous fertilisers. Kam Patel reports
After ten years of research by British and Indian scientists, a ground-breaking agricultural experiment is about to begin in India that promises to help farmers reduce dramatically their use of polluting nitrogenous fertilisers for crops such as maize, wheat, rice and sorghum.
Collaborators on the project, which is backed by the World Bank and the British Council, include Tamil Nadu Agricultural University in Coimbatore and the Indian Council for Agricultural Research in New Dehli. Leading the collaboration in Britain is Edward Cocking, director of Nottingham University's Centre for Crop Nitrogen Fixation and a world-leading authority on biological nitrogen fixation in "non-legume" crops such as maize, rice and sorghum.
Professor Cocking explains that at present all non-legume crops (except sugar cane) are unable to make use of, or "fix", atmospheric nitrogen for their nutrient needs directly because they cannot process it. In contrast, legume crops such as beans and peas are able to do so. This is possible because they possess "nodules" (usually around the root system) within which special nitrogen-fixing bacteria have established themselves. The bacteria fix the nitrogen to form ammonia, which the plant takes up and converts to amino acids, which are used in turn to produce proteins.
Non-legume crops are wholly dependent on securing their nitrogen needs in the form of nitrates dissolved in the water that is taken up by the plant.
A key reason for high yields from modern rice crops is the use of nitrogenous fertiliser in the form of ammonium nitrate and potassium nitrate. Its use, however, results in pollution: "Nitrates are highly soluble in water. Although some nitrate is taken up by the plant, much of it runs off into the groundwater system, polluting it. It is an extremely serious problem that is getting worse and worse," says Professor Cocking. The polluted water ends up in rivers, lakes, estuaries and oceans causing, for instance, vast blooms of algae that can kill fish. Scientists are also concerned that valuable nutrients in soils worldwide are being displaced by nitrogen compounds and threatening biodiversity.
But it is a vicious circle. Abdul Kareem, vice-chancellor of TNAU, points out that rice is the staple diet for more than 40 per cent of the world's population, making it the most important food crop. For India alone, official figures show that rice and wheat make up 75.6 per cent of the country's total food grain production. During 1996-97 the country produced 81 million tonnes of rice and 70 million tonnes of wheat. Demand is estimated to rise to about 124 million tonnes and 110 million tonnes respectively by 2020.
Dr Kareem says that much rice is grown in countries with rapidly growing populations, small land areas and scarce resources. High yields are essential to avoid food shortages in such countries. Nitrogenous fertilisers produce large yields, and vast quantities are used by farmers. "If half of the nitrogen fertiliser applied to the 120 million hectares of lowland rice in the world could be replaced with biologically fixed nitrogen, the equivalent of about 7.5 million tonnes of oil could be conserved annually," Dr Kareem says. By 2020, twice as much fixed nitrogen will be needed for cereal crop production worldwide to meet the food requirements of increasing populations, he says.
The researchers believe the novel case of a variety of sugar cane growing in the natural soil of Brazil offers pointers to cutting reliance on nitrogenous fertilisers. Unlike other non-legume crops, sugar cane is propagated through cuttings, not by seed. In the Brazilian variety, which has been under intensive study for ten years, this characteristic has helped imbue the cane with a distinctive property, which the Indian programme is trying to extend to other non-legume crops: it does not need nitrogenous fertiliser. This is because, over the course of many generations, special nitrogen-fixing bacteria have entered the cane plant and established themselves along the plant's water "pipework".
Professor Cocking says: "In crops such as maize, rice, sorghum and wheat there is no such opportunity because the crop is gotten rid of at the end of the season. What we have been trying to do is find a nitrogen-fixing bacterium that is able to get inside and establish itself in the pipework of these other crops, to try to imitate for seeding crops what we know is already taking place in sugar cane in Brazil." The researchers believe that a nitrogen-fixing bacterium called Azorhizobium could be the answer to their problem. "It is the best candidate for non-legume crops," Professor Cocking says.
At his Nottingham University laboratories, Professor Cocking has already shown that Azorhizobium can establish itself in the water "pipework" of the rice plant. The challenge now is to extend this to a range of Indian rice varieties and to investigate nitrogen-fixing benefits to the plant. The work will eventually cover the world's other major cereal crops.
Although the Indian programme is focused primarily on cereal crops, Professor Cocking says that non-legume trees are also on the agenda because they are important not only for their fruit but also as sources of fuel and other non-food products.
The experiments will be carried out across a giant network of Indian agricultural research centres specialising in particular crops. Rice is TNAU's main concern as part of the programme; sugar cane is the remit of the Lucknow-based Sugar Cane Research Centre in North India.
"It will take time to perfect the technique but all the pointers suggest we are on the right track," Professor Cocking says. "If successful, the results will benefit many countries including China and Bangladesh. Even farmers here, in East Anglia, where they are looking to reduce their use of nitrogen fertiliser, could benefit."