Isolation of Sinorhizobium meliloti Bacteria in Soils of Karaj Alfalfa Fields and Their Effect on Alfalfa Yield

One of the most important legume forages of Iran is alfalfa، which annually covers more than 600 thousand hectares of arable land. This plant is capable of fixing significant amounts of molecular nitrogen، if Sinorhizobium meliloti bacteria exist in the soil and suitable climatic and soil conditions prevail. In order to isolate and study the effect of different strains of S. meliloti on alfalfa yield، 20 soil samples were taken from different fields in Karaj and were planted to alfalfa in the green house. Later، 32 strains related to S. meliloti were isolated and 20 of which were confirmed، using the contamination test. To study the effect of these 20 different strains on alfalfa yield and to compare with applied nitrogen fertilizer in the presence of S. meliloti bacteria، an experiment was done using factorial treatments with completely randomized design in three replications (total of 72 treatments). Treatments were (inoculation، no-inoculation، and nitrogen fertilizer). Results of soil analysis showed that the amount of P، B، Fe، and Zn in most of alfalfa fields was less than critical level. Deficiency of these elements was one of the limiting factors of yield and nitrogen fixation in the field. The greenhouse experiment results showed that the effects of inoculation and nitrogen fertilizer on yield were significant (P<0. 05). Also، the effect of nine isolates on height of plant and fresh and dry weight of roots were significant compared to the control، while 10 isolates had significant effects on the nodules weight. Application of nitrogen fertilizer decreased nodulation significantly (P<0. 05) compared to the control and the inoculated treatments. The S11 treatment، which had the highest mean of shoot dry matter (1. 38g)، was in the same statistical group with S9 (with the average of 1. 36 grams) and both treatments showed a significant increase compared to the control. The S17 had the lowest average dry weight (0. 77 g).