Agricultural intensification, soil biodiversity and ecosystem function in the tropics: The role of nitrogen-fixing bacteria

Among the nitrogen (N-2)-fixing bacteria, the rhizobia in symbiosis with legumes are generally the most important in agriculture, although Frankia, cyanobacteria and heterotrophic free-living N-2-fixers may fix significant amounts of nitrogen under specific conditions, The taxonomy of N-2-fixing bacteria is undergoing substantial revisions due to the advent of molecular methods for phylogenetic analysis, and in certain cases this has proved useful in unravelling ecological relationships among confusing groups. Molecular methods are also proving useful in studies of biodiversity within populations of rhizobial species. Rhizobia are surprisingly competent free-living bacteria, although few fix nitrogen in the free-living state, and the major factors that determine their population sizes in the absence of legume hosts are environmental stresses (such as soil acidity factors), protozoal grazing and some factors associated with agricultural intensification such as increases in salinity or heavy metal pollution of the soil. Rhizobial populations generally increase in response to the presence of the host legume. Due to the high degree of host-specificity between legume hosts and rhizobial species, loss of a single rhizobial species can result in loss of N-2-fixation by that legume, although many legumes can be nodulated by several species of rhizobia. However, as only a single, compatible rhizobial genotype or strain is necessary for establishment of effective N-2-fixation (i.e. the basis of the rhizobial inoculant industry), it is questionable whether biodiversity within species is necessary to ensure function, although this may confer resilience in the face of further environmental stresses. (C) 1997 Elsevier Science B.V.