Relationships between rhizobial diversity and host legume nodulation and nitrogen fixation in tropical ecosystems

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 rhizobial 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, Mesorhizobium spp. at intermediate pH and Sinorhizobium spp. under alkaline conditions. The R. tropici-type rhizobia 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.