African Journal of Agricultural Research Vol. 7(12), pp. 1782-1788, 26 March, 2012 Available online at http://www.academicjournals.org/AJAR DOI: 10.5897/AJARX11.034 ISSN 1991-637X © 2012 Academic Journals Review Effective factors on biological nitrogen fixation Khosro Mohammadi 1 *, Yousef Sohrabi 2 , Gholamreza Heidari 2 , Shiva Khalesro 2 and Mohammad Majidi 3 1 Department of Agronomy, Islamic Azad University, Sanandaj Branch, Sanandaj, Iran. 2 Department of Agronomy, University of Kurdistan, Sanandaj, Iran. 3 Islamic Azad University, Sanandaj Branch, Sanandaj, Iran. Accepted 17 January, 2012 Although relationships among plant, biological N 2 fixation, and response to soil and environmental conditions have received considerable coverage in the scientific literature, a comprehensive summary and interpretation of these interactions with specific emphasis are lacking. Fluctuations in pH, nutrient availability, temperature, and water status, among other factors, greatly influence the growth, survival, and metabolic activity of nitrogen fixation bacteria. The subsequent inhibition of nitrogenase would result in O 2 accumulation in the infected zones, inducing the decrease in nodule permeability. Poor nodulation of legumes in arid soils is likely due to decreases in population levels of rhizobia during the dry season. Fixation, therefore, also tends to decrease with legume age, mainly because of the concomitant increase in soil N. Calcium deficiency, with or without the confounding influence of low pH also affects attachment of rhizobia to root hairs. Rhizobia may have different tolerances to soil acidity factors than the host plant. Relatively, high-root temperature has also been shown to influence infection, N 2 - fixation ability, and legume growth. Also, root nodulation by the bacteria can be dependent on the formation of mycorrhiza. Key words: Legume, nitrogen fixation, rhizobia, root, stress. INTRODUCTION Biological Nitrogen Fixation (BNF) is an efficient source of fixed N 2 that plays an important role in land remediation. Interest in BNF has focused on the symbiotic systems of leguminous plants and rhizobia because these associations have the greatest quantitative impact on the nitrogen cycle. Deficiency in mineral N often limits plant growth and as such, symbiotic relationships have evolved between plants and a variety of N 2 -fixing organisms. The symbiotically fixed N 2 by the association between Rhizobium species and the legumes represents a renewable source of N for agriculture. Values estimated for various legume crops and pasture species are often impressive, commonly falling in the range of 200 to 300 kg N ha -1 per year. This underlines *Corresponding author. E-mail: khosromohammadi60@yahoo.com. Tel: +98 918 8732916. Fax: +98 871 6387100. the significance of Rhizobium and legume symbioses as a major contributor to BNF. Nitrogen fixation, along with photosynthesis as the energy supplier, is the basis of the soil environment under a constant state of change and, as such, can be relatively stressful for both macro- and micro-organisms. Fluctuations in pH, nutrient availability, temperature, and water status, among other factors greatly influence the growth, survival, and metabolic activity of nitrogen fixation bacteria and plants, and their ability to enter into symbiotic interactions (Werner and Newton, 2005). Despite this situation, soils represent one of earth’s most productive ecospheres, accounting for a majority of primary and successional productivity. Consequently, microbes, plants, and other soil inhabitants have evolved to adapt to the ever changing and often inhospitable soil environment. Stress factors in soils influence symbiotic nitrogen fixation. However, the reader should be aware that, while some stress factors simultaneously affect both symbiotic partners, water stress, others may differentially influence each partner to