The following points highlight the four factors controlling the biological nitrogen fixation. The factors are: 1. Environmental Factor 2. Stage of Plant Growth 3. Fertilizers in the Soil and 4. Genetic Factors.
Factor # 1. Environmental Factor:
Usually, the factors which favour photosynthesis such as adequate moisture, warm temperatures, bright sunlight and higher CO2 levels, also enhance biological nitrogen fixation. Rate of biological nitrogen fixation is usually maximum in early afternoon when carbohydrates (mainly as sucrose) are rapidly trans-located from shoots to root nodules through phloem. At this time, transpiration is also rapid which facilitates translocation of fixed nitrogen from root nodules and roots to shoots through xylem.
Factor # 2. Stage of Plant Growth:
In leguminous crop plants such as soybean, pigeon pea and peanut, maximum nitrogen fixation occurs after flowering when demand for nitrogen increases for developing fruits and seeds. These legumes have especially rich protein content in their seeds. In soybean, the seeds may contain up to 40% protein which is highest known protein content in any plant. In such legumes, about 90% of nitrogen fixation occurs during reproductive stage and only about 10% of it occurs in first two months of vegetative stage.
Rate of nitrogen fixation is higher in perennial legumes such as alfalfa in comparison to annual legumes such as soybeans, peas and beans. It is because in annual legumes, root nodules are to be formed a fresh every year while in perennial legumes, the nodules are formed in the very first year of their growth which persist in subsequent years and therefore, they fix more N2.
Factor # 3. Fertilizers in the Soil:
Increased amounts of NO3‑ and NH4+ in the soil suppress formation of root nodules and cause more rapid senescence of already established nodules. Consequently, nitrogen fixation is markedly inhibited under such conditions.
Factor # 4. Genetic Factors:
Many genetic factors control nitrogen fixation and yields of leguminous crop plants:
i. As has been mentioned earlier, there are host plant specific nodulin or Nod genes and rhizobial nodulation or nod genes that are involved in the formation of root nodules. Scientists are attempting to increase the efficiency of nodules formation by altering the rhizobial genes and selecting more compatible varieties of host plants.
ii. As has been discussed earlier, reduction of N2 to two molecules of NH3 by nitrogen fixing organisms is coupled with the production of one H2 molecule also causing wastage of energy. Active hydrogenase enzyme found in some rhizobia and free living bacteria can split this H2 to electrons and protons (H2 <=> 2H+ + 2e–). Efficiency of such organisms in nitrogen fixation is obviously more than those organisms which lack this enzyme. Through genetic engineering techniques, bacterial strains can be developed with active hydrogenae enzyme and consequently, rate of nitrogen fixation can be enhanced.
iii. Studies with free living bacterium Klebsiella pneumoniae and certain free living species of Rhizobium have revealed that the process of nitrogen fixation is controlled by a set of genes which are named as Nif genes. In these organisms, such genes are repressed while in rhizobia associated with root nodules, they are usually de-repressed even in the presence of NH4+. Significance of this is quite obvious for mankind. The scientists are engaged in research work all over the world, to introduce (Nif) genes into the higher plants which are normally incapable of fixing nitrogen.