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 wi th legumes are generally the most important in agriculture, although F rankia, cyanobacteria and heterotrophic free-living N-2-fixers may fix significant amounts of nitrogen under specific conditions, The taxono my of N-2-fixing bacteria is undergoing substantial revisions due to t he advent of molecular methods for phylogenetic analysis, and in certa in cases this has proved useful in unravelling ecological relationship s among confusing groups. Molecular methods are also proving useful in studies of biodiversity within populations of rhizobial species. Rhiz obia are surprisingly competent free-living bacteria, although few fix nitrogen in the free-living state, and the major factors that determi ne their population sizes in the absence of legume hosts are environme ntal stresses (such as soil acidity factors), protozoal grazing and so me factors associated with agricultural intensification such as increa ses in salinity or heavy metal pollution of the soil. Rhizobial popula tions generally increase in response to the presence of the host legum e. Due to the high degree of host-specificity between legume hosts and rhizobial species, loss of a single rhizobial species can result in l oss of N-2-fixation by that legume, although many legumes can be nodul ated by several species of rhizobia. However, as only a single, compat ible rhizobial genotype or strain is necessary for establishment of ef fective N-2-fixation (i.e. the basis of the rhizobial inoculant indust ry), it is questionable whether biodiversity within species is necessa ry to ensure function, although this may confer resilience in the face of further environmental stresses. (C) 1997 Elsevier Science B.V.