Rasapirose Somwatcharajit1, Suchonma Sookruksawong1, Ruengsak Klinchan2, Janpen Prakamhang1*
1Department of Applied Biology, Faculty of Sciences and Liberal Arts, Rajamangala University of Technology Isan, Muang, Nakhon Ratchasima, 30000, Thailand
2Forest Resource Management Office No.8 Nakhon Ratchasima, Muang, Nakhon Ratchasima, 30000, Thailand
*Corresponding author’s email: janpen.pr@rmuti.ac.th
Received: 17 January 2026 / Revised: 20 April 2026 / Accepted: 22 April 2026 / Published Online: 09 May 2026
Abstract
Native rhizobia are essential for biological nitrogen fixation and the establishment of leguminous trees in both reforested and natural forest ecosystems. We examined the diversity, phylogeny, and symbiotic performance of rhizobia associated with legumes in the Nongteng–Chakkarat National Reserved Forest in northeastern Thailand. Samples were collected from 10 forest plots representing different restoration histories. A total of 45 legume species and 156 rhizobial isolates were obtained. BOX-PCR fingerprinting revealed 55 distinct genetic profiles, indicating high genetic diversity. Phylogenetic analysis based on partial 16S rRNA gene sequences categorized effective isolates into the genera Bradyrhizobium, Rhizobium, and Mesorhizobium. Similar genotypes were present across various forest types and host species without clear clustering. Diversity indices were higher in long-term reforested deciduous dipterocarp forests. However, neither plot-level differences nor PERMANOVA indicated significant variation between natural and reforested forests. These patterns suggest that rhizobial assemblages were not strongly differentiated by forest type under the present sampling design and were more likely associated with host availability and local ecological context. Cross-inoculation assays showed that 21 isolates were capable of nodulating Dalbergia cochinchinensis. Native isolates significantly increased nodulation, nitrogenase activity (up to 1,326 nmol C2H4 h-1 g-1 nodule dry weight), chlorophyll content, and seedling biomass (p < 0.05). Total chlorophyll content was positively correlated with nodule number, nodule dry weight, nitrogenase activity, and plant biomass. This suggests a close association between symbiotic effectiveness and plant biomass. Among the tested isolates, 10Es1 and 9Pm1 consistently exhibited enhanced symbiotic performance. Native rhizobia displayed considerable genetic diversity and were widely distributed across forest types. However, only a limited number of isolates formed effective symbioses with D. cochinchinensis, suggesting functional filtering rather than broad symbiotic compatibility. This pattern highlights the importance of host compatibility and identifies several native strains as candidates for further evaluation as bioinoculants in legume-based forest restoration.
Keywords: Rhizobial diversity, Forest restoration, Host compatibility, Leguminous plants, Symbiotic effectiveness, Dalbergia cochinchinensis
