Nitrogen fixation and growth of non-crop legume species in diverse environments

Of the 643 legume genera, few have been exploited in agriculture and 40% have not even been evaluated for their ability to nodulate and fix nitrogen. Most of these are in tropical/subtropical regions, with habitats ranging from extremely dry to flooded. Recent work in some of these areas shows that plants can nodulate under conditions previously thought to be disadvantageous. The accepted dogma that nitrogen fixing legumes have a high demand for P is challenged and examples of how legumes can extract P from soils with low available P described. Species tolerant to shading and high soil Al are cited, although the mechanisms of adaptation are not yet clear. Some tropical soils have high nitrate levels and, contrary to perceived wisdom, there are legumes which can nodulate under such conditions. Many tropical tree legumes prefer ammonium to nitrate and are able to fix nitrogen and assimilate ammonium at the same time. In all these cases, there are genotypic differences, both within and among species. Large areas of tropical fresh water, such as the Brazilian Pantanal and the Orinoco floodplain have nodulated legumes predominant in their flora. The ecological potential of these has not been evaluated. One of the sites of nodule evolution is likely to have been in such areas. Modes of infection of legumes by rhizobia vary with taxonomic tribe and may represent evolution for survival in different environments. As more legumes, from more ecosystems are studied, a wider range of adaptations is likely to be found. Work is urgently needed to study these, especially in areas being cleared for agriculture or by logging.