The contribution of N-2-fixing tree legumes to land reclamation and sustainability in the tropics

Biodiversity, nutrient cycling and energy flux are the bases for sustainability of any system. Among the essential nutrients for plant growth, N is the most expensive and energy-consuming, and potentially is an environmental pollutant. Mixed crops with N-2-fixing trees (NFTs) have been thought to maintain biodiversity and sustainability of systems in the tropics. However, most of the empirical knowledge and scientific research done with NFTs have taken for granted that all legume species nodulate and fix N under field conditions without inoculation or without supplementing phosphorus, the most common limiting essential nutrient for legumes in the tropics. In the last few years, surveys on nodulation of legume tree species and selection of highly efficient rhizobial strains for legume trees have received more attention. More than 600 species have been observed for their nodulation status in Brazil and from their nodules approximately 2400 rhizobial strains have been isolated. From 44 legume species with potential uses in the different agrosystems, efficient N-2-fixing rhizobia have been selected and are available for inoculant production. New species of rhizobia or bradyrhizobia have been described and large collections of isolates are being developed. Useful legume tree species may contribute around 12 tons of dry litter and 190 kg of N ha(-1) y(-1) to renovate degraded soil. These studies have prompted use of nodulated and mycorrhizal legume trees to revegetate poor or depleted soils with the goal of restoring their fertility. Experiments with native and introduced legume tree species have been successful in revegetating exposed subsoil, open mining areas and acidic residues from bauxite mining without addition of organic soil. However, supplements of rock phosphate, gypsum, micronutrients and potassium are required. (C) 1997 Elsevier Science Ltd.