Studies on cowpea Rhizobium population in soil of revived forests at the Royal suggestions and plant germplasm forest project in Nakhon Ratchasima province, that included grassland regrown forest at Kornburi district and, decidous mix with dipterocarp forest at Nongrawieng subdistrict were carried out. Soil sampling was conducted 4 times between June 1998 to Mar 1999. Results showing the averages of Rhizobium population were significantly different among all area. There are a lot of Rhizobium population in two grassland plots, which were grassland checkdam planting (GLDP) and grassland planting (GLP). No correlation were found between the mean of Rhizobium population and physical properties of soil. Antibiotic resistance and plasmid profiles were determined for 32 fast-growing Rhizobia isolates recovered from nodules of two varieties of Vigna radiata (L.) Wilczek grown on soil of grassland checkdam planting (GLPD) and decidous mix with dipterocarp forest (NT1R2). A total of 11 antibiotic profile groups and 9 plasmid profile groups were indentified, with a plasmid size in range 164-600 Mdal. Nearly 70% of all isolates were grouped similarly by the plasmid profiles and antibiotic profiles. No correlation were found between plasmid profiles and origin of them. Genotypic properties of all isolates were identified by RAPD analysis of 5 primers which provided the varies profile of RAPD in range 0.29-3.0 kilobase. A dendrogram based on similarity of RAPD profiles obtained by neighbour-joining method were divided into 5 clusters. The result indicated that the different of soil types were the principle factor causing genetic variation rather than variety of plant. It was found that results of RAPD correlated with plasmid profiles except in some case of plasmid profile showing high power of discrimination which was disadvantage of this technique, when used discrimination alone. However, RAPD analysis in combination with plasmid profiles was not only discriminate to enable grouping of closely related isolates but also provided more detailed picture of diversity within same plasmid groups. These techniques could be confirmed with antibiotic profiles, increased the details relationship and efficiency of isolate discrimination. Nitrogen fixation efficiency were tested on mungbean kampeangsan 2. It was found that enzyme nitrogenase activities from nodules as measured by acetylene of 32 isolates were not significantly different. Most isolates showed relationship between plant dry weight, ARA and nodule dry weight. The characteristics of nodulation were found, nodulation were scattered along tap roots. Second and third, nodulations were formed along lateral roots and tap roots include lateral roots, respectively.
