**2.6 Nitrogen fixation**

Phosphorus assists in enhancing rhizobial bacterial population in soil. It is considered as a principal plant nutrient that is strongly involved in biological nitrogen fixation and nodule performance in legumes. *Rhizobium* bacteria require energy for growth, reproduction, and functioning, which is provided by ATP that is transformed into ADP with concurrent liberation of inorganic phosphate. For each molecule of N2 reduction for fixation, at least 16 molecules of ATP are hydrolyzed, showing that N-fixation is an energy-demanding process. Nodule formation and development are largely inhibited in P deficit condition, and as a result, nitrogen fixation is greatly impaired. Nitrogenase enzyme activity is also positively correlated with plants P nutritional status. Researchers found higher root nodule biomass in pea plants with higher supply of P [31] and lower concentration of ATP and energy charge in soybean nodules with a low P supply [32].

### **2.7 Seed germination**

The embryo in plant seeds requires P for growth and development until the formation of a root system. For seed germination and seedling vigor, the seed P-content is a vital factor because seed P is the only P available to plants during germination that aids in nutrition and faster establishment of young seedlings. With the rise of P-concentration in seed, dry matter production of seedlings of annual plants increases. P quantity in seeds of plants differs from about 0.15 to 0.60% in seeds of wheat, oats, barley, and lupins and from 0.30 to 1.10% in those of subterranean clover and annual medics [33]. Conversely, according to some recent studies [34, 35], a lower concentration of seed P exerted no difference in seedling vigor, plant biomass, and yield in comparison with a high concentration of seed P, suggesting that an optimum concentration of seed P is adequate for seed germination whereas, higher P-concentration in seeds might be useless.
