As noted in section 1 of this lecture, the process known as biological nitrogen fixation (BNF) takes place in nodules located on the roots of the plants. Thus, before BNF can begin nodules must be formed. In general terms, nodules are formed as a result of infection of the roots by soil bacteria. In the case of forage crops, the bacteria most often infecting the roots are bacteria in the genus Rhizobium. The complex process by which plant roots are infected by rhizobia is known as infection. The complex process in which nodules are formed is known as nodulation. Although the processes have different names, they are very closely interrelated and in one sense might be considered simply two parts of one process. Both processes have been studied in great detail, although neither is completely understood. The following sections give an overview of the process. For greater detail, consult the references section associated with this topic.
The process of infection in many forage legumes begins when Rhizobium bacteria come in contact with the root hairs of the host plant. Rhizobia bacteria are known to be free-living bacteria, that is they are able to live in the soil even when forage legumes are not present. Thus, when an appropriate host forage crop is planted in the soil, the rhizobia may already be present and able to form the symbiosis that leads to BNF. On the other hand, it is important to note that not all bacteria in the genus Rhizobium will form a symbiotic relationship with all forage crops. In fact, there is a certain specificity to the process.
Among the genus of bacteria known popularly as rhizobia are a number of different species and among those species are different subgroups known as biovars. As it is explained in section 6, scientists have discovered that specific rhizobia species and biovars are apparently designed to infect only certain legume species. For further discussion on this point, see section 6.
Assuming the correct rhizobia species and forage species are brought together, the presence of the forage roots in the soil apparently stimulates the bacteria to reproduce itself and thus larger and larger numbers of bacteria are produced. Research suggests that the presence of rhizobia stimulates the forage crop roots to produce more root hairs. The bacteria gradually form an infection thread which allows bacteria to enter root cells of the plant via the root hairs. Bacteria in the root cells gradually grow and develop into structures known as bacteroids. The bacteroids eventually develop the biochemical 'machinery' which is able to absorb atmospheric N2 and convert it to N in the form of ammonia (NH3).
During the infection process, the bacteria stimulate cell division in the root cells. This results in the eventual formation of the structures known as root nodules. Nodules vary in size and shape as shown in the figures below. The process that is known as BNF takes place in bacteriods within these nodules.