Ammonia volatilization, denitrification and leaching of nitrate ions result in increasing economical and bioenvironmental problems during
flooding of paddy soils. Therefore, using biological nitrogen fixing systems to replace chemical fertilizers become more urgent. In recent years, berseem clover - rice rotation has been practiced in Mazandaran province. Rhizobium trifolii which lives in symbioses with clover, is an aerobic nitrogen fixing
bacteria and the harmful effects of nooding on its survival and consequently decrease in N2 - fixation could be expected. To determine flooding effects on population of R.trifolii, six paddy soils in areas where berseem clover-rice
rotation was practiced, were sampled in monthly intervals. Population densities of R.trifolii and some important physical and chemical characteristics of the soil samples were measured in every interval. Adequate soil samples
were also prepared at the begining of clover cultivation to determine the nitrogen fixing potential of native rhizobia and the necessity for rhizobial inoculation. A completely randomized design factorial experiment with 3 rhizobial and 2 nitrogen treatments and 4 replicates was conducted in greenhouse conditions on soil samples. The results indicate that during
flooding period, the population density of R.trifolii decreased gradually and was lowest at the end of this period. The average reduction was 2 log units
and was statisticaly significant at 5% level. Regional indigenous bacteria were classified as effective and highly effective, and the effect of 2 superstrains of them were equated to 180 kg Nha-l. The results of inoculation experiment indicate that in spite of sever reduction in bacterial population during flooding
period, the Rhizobial population increased after this period and in some soils the former damage has been repaired. In soils containing more than 1000
native R. Trifolii per gram of soil, response of plant to inoculation was not significant. In soils, with lower number of indigenous Rhizobia (100-300 g-l soil), however better results were obtained from inoculation and some
inoculated treatments had significant differences with non - inoculated control. On the basis of these results the use of Clover - Rhizobium symbioses
in rotation with rice and also, inoculation with highly effective Rhizobial strains in the case of bacterial limitation will be recommended.
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