Soil microbial biomass is considered to be an important N pool in soil–plant N dynamics in terrestrial ecosystems. This study was carried out at the Rice Research and TrainingCenter during 2007 and 2008 rice growing seasons to evaluate the potential use of these microorganisms as a natural bionitrogen source under two paddy soils. Three nitrogen sources i.e. rice straw compost, sludge and Urea were used under fresh and autoclaved soils with the rice variety Giza 177 in a pot experiment. Results obtained showed that the pot treated with organic mater recorded the maximum value of total N uptake and microbial biomass nitrogen( MBN) uptake, followed by Urea treated pots. While, pots amended with straw exhibited a lower microbial N forming ability than those amended with sludge under both soils. Nitrogen mineralization (NH₄) rate was significantly affected by nitrogen and microbial N sources. The ammonium concentration increased with straw compost application, which was delaying the utilization of microbial N sources in the rice culture. The results showed that MBN uptake and proportion of plant nitrogen derived from microbial nitrogen sources (P_fix) were under fresh soil treatment found to be higher than autoclaved soil treatment in both soils. A positive correlation was found between the P_fix and the total N in the shoot under both soils for the rice variety under study. The results also showed that microbial biomass N was governed not only by the soil type but also by the type of the nitrogen source. The addition of sludge to fresh soil increased total MBN uptake and consequently could be indirectly beneficial to rice production especially in poor soils. Thus, it can be concluded that soil microbes contribute to plant growth by serving as a N source during the rice growing season.