Environmental factors affecting N2 fixation in grain legumes in the tropics, with an emphasis on Brazil

Biological nitrogen (N2) fixation is key to sustainable agricultural systems in tropical soils, which are frequently deficient in N. However, high temperature, drought and soil acidity constrain legume root-nodule formation and function in the tropics. In most cases, the microsymbiont is the more affected partner, with plants growing on mineral N usually less sensitive to these stresses. High temperature and moisture deficiency are major causes of nodulation failure, affecting all stages of the symbiosis and limiting rhizobial growth and survival in soil. They may also contribute to undesirable changes in rhizobia, including plasmid deletions, genomic rearrangements and reduced diversity. Acidity affects several steps in the development of the symbiosis, including the exchange of molecular signals between the legume and the microsymbiont. Liming is effective in overcoming soil acidity and aluminium toxicity, but in Brazil few soils are limed to near neutral pH. Selection of rhizobial inoculant strains that are genetically stable under the often harsh soil conditions of this region is essential, but that task is impaired by a lack of knowledge of tolerance in the microsymbiont. However, good results have been obtained by selecting naturally occurring rhizobia from acid tropical soils affected by water stress and high temperatures. In Brazil, increases in grain yields of common bean and soybean have resulted from inoculation with such stress-tolerant strains. Appropriate soil management practices, e.g. no-till, are other approaches that may decrease soil temperatures and preserve moisture, thereby increasing N2 fixation.