Summary:

Biological nitrogen fixation (BNF) is the process that provides organic ni-trogenous compounds to the plants by using molecular nitrogen in atmosphere. Higher plants are not capable to use molecular nitrogen in atmosphere as a nitrogen source to generate essential proteins. Therefore plants either should be fertilized by adequate nitrogenous fertilizers or the microorganisms which are capable to produce nitrogenase should provide nitrogen to the plants by BNF. From among a number of factors affecting BNF, soil moisture content and ambient temperatures are considerably effective on the fixation rate. Therefore the global warming would be dramatically defective on BNF, thus effects of soil moisture as well as soil and ambient temperatures on BNF should evaluate prior rising temperature. A pot experiment was carried out to determine the effects of soil water contents on BNF. Four different soil water contents (%25, %50, %75 and %100 of water holding capacity) were adjusted either every 3 days or just after plants indicate wilting point. Non-inoculated pots were added to experiment as a control. The re-sults revealed that BNF is affected by different level of soil water content. The mechanism of this effect would not be the direct effect of water, but the side effect of water on soil oxygen content; therefore, an aeration capability.