Role of Some of Mineral Nutrients in Biological Nitrogen Fixation

Atmospheric nitrogen fixation probably contributes at most about 10% of the total annual yield of fixed nitrogen. The most important source of fixed nitrogen derives from the activity of certain soil bacteria that absorb atmospheric N2 gas and convert it into ammonium. The process of biological nitrogen fixation offers an economical attractive and ecological advantage by of reducing external nitrogen input and improving the quality and quantity of internal resources. Mineral nutrients may influence N2 fixation in legumes and nonlegumes at various stages of the symbiotic process: infection and nodule development, nodule function, and host plant growth. Here, review the basic concepts of mineral nutrition, as well as the importance of mineral nutrients specifically for biological nitrogen fixation in the legume-rhizobia symbiosis. For healthy and vigorous growth, intact plants need to take up from the soil: relatively large amounts of some inorganic elements: ions of nitrogen (N), potassium (K), calcium (Ca), phosphorus (P) and sulphur (S); and, small quantities of other elements: iron (Fe), nickel (Ni), chlorine (Cl), manganese (Mn), zinc (Zn), boron (B), copper (Cu), and molybdenum (Mo). The enhancing effect of low levels of combined nitrogen on N2 fixation in legumes is related to the lag phase between root infection and the onset of N2 fixation. Phosphorus (P) is second only to nitrogen as an essential mineral fertilizer for crop production. At any given time, a substantial component of soil P is in the form of poorly soluble mineral phosphates. A high phosphorus supply is needed for nodulation. When legumes dependent on symbiotic nitrogen receive an inadequate supply of phosphorus, they may therefore also suffer from nitrogen deficiency. Potassium and sulphur are not usually limiting nutrients for nodulated legumes, although a K+ supplement for osmoadaptation has to be considered for growth in saline soils. Among mineral nutrients, B and Ca are undoubtedly the nutrients with a major effect on legume symbiosis. Both nodulation and nitrogen fixation depend on B and Ca2+, with calcium more necessary for early symbiotic events and B for nodule maturation.