Biological nitrogen fixation in the tropics: Social and economic contributions

Brazil has become the world leader in replacing N fertilisers by biological N-2 fixation (BNF). Even though agriculture in Brazil is one of the main export-producing activities, it has become the country which uses the lowest N applications (mean 10 kg ha(-1)). Soybeans selected with zero N applications have become the country's largest export product. Cereals, also selected with N supplies much below their need, unknowingly were selected for associations with diazotrophic bacteria and can obtain up to 30% of their N from BNF when fertilised with ample PK and minor elements. The largest effect in this group was obtained with sugar cane, which can obtain up to 150 kgN ha(-1) from BNF. This has become the key to the success of the Brazilian Bio-fuel programme where because of increases in the energy balance when the cane is planted without any N fertiliser and without burning the leaves, the energy produced is more than 5 times that required to produce it. Recently possibilities of expanding this biofuel programme to diesel oil are arising from the isolation of probably new diazotrophs which colonise the stems of oil palms, of the Brazilian palm genotypes which usually are not fertilised with N. (C) 1997 Elsevier Science Ltd.