-1 Relationships between rhizobial diversity and host legume nodulation and nitrogen fixation in tropical ecosystems Abdullahi Bala 1, * and Ken E. Giller 2 1 Department of Soil Science, Federal University of Technology, Minna, PMB 65, Minna, Nigeria; 2 Department of Plant Sciences, Plant Production Systems, Wageningen University, P.O. Box 430, 6700 AK, Wageningen, The Netherlands; *Author for correspondence (fax: +234-9-4130973; e-mail: abdullahi_ bala@yahoo.com) Received 1 September 2004; Accepted in revised form 4 August 2005 Key words: Ecological adaptation, N 2 -fixation, Nodulation, Rhizobial diversity Abstract With recent advances in rhizobial phylogeny, questions are being asked as to how an ecological framework can be developed so that rhizobial classification and diversity could have greater practical applications in enhancing agricultural productivity in tropical ecosystems. Using the results of studies on tropical rhizobia which nodulate agroforestry tree legumes, three ecological aspects of rhizobial biodiversity were used to illustrate how its potential can be exploited. The results showed that legumes nodulate with diverse rhi- zobial types, thus contributing to the success of legumes in colonising a wide range of environments. There was an apparent shift in the relative dominance of rhizobia populations by different rhizobial types as soil pH changed. The Rhizobium tropici-type rhizobia were predominant under acidic conditions, Mesorhizo- bium spp. at intermediate pH and Sinorhizobium spp. under alkaline conditions. The R. tropici-type rhi- zobia were the most effective symbiotic group on all the host legumes. However, strains of Sinorhizobium spp. were as effective as the R. tropici types in N 2 -fixation on Gliricidia sepium, Calliandra calothyrsus and Leucaena leucocephala; while Mesorhizobium strains were equally as effective as the R. tropici types on Sesbania sesban. Classification of rhizobia based on phenotypic properties showed a broad correlation with groupings based on 16S rRNA sequence analysis, although a few variant strains nested with the dominant groups in most of the clusters. Some of the phenotypic characters that differentiated different rhizobial groups are highlighted and a case is made for the need to standardise this method. Introduction The family Leguminosae is one of the most suc- cessful families of angiosperms, with about 650 genera and 20,000 species (Sprent 1995) and a cosmopolitan distribution (Raven and Polhill 1981). The success of legumes can in part be attributed to their ability to colonise environments with low soil nitrogen because of their symbiotic association with N 2 -fixing rhizobia (Sprent 1994). The current rhizobial taxonomy has 6 genera and 29 species, most of which were described in the last decade using rhizobia isolated from tropical legume species (Giller 2001). In spite of this rela- tively high turnover of rhizobial groups, it is likely that we are still orders of magnitude away from a true assessment of the diversity of tropical rhizobia (Giller 2001). This has led to questions being asked as to how this can be explored to enhance agri- cultural productivity in the tropics. This requires Nutrient Cycling in Agroecosystems (2006) 76:319–330 Ó Springer 2006 DOI: 10.1007/s10705-005-2003-y