Molecular identification of nitrogen-fixing bacteria and the effect of inoculation on tropical forest legume species

Abstract

Studies on the diversity of nitrogen-fixing bacteria, particularly in tropical forest species, has been relatively less compared to other ecosystems. The aims of this study were to isolate bacteria from nodules of eight tropical forest leguminous species belonging to two subfamilies of Fabaceae (Caesalpinioideae and Papilionoideae), carry out molecular identification based on 16S rRNA gene sequencing, and evaluate their plant growth-promoting effects in the same hosts. Growth-promoting responses were determined by evaluating plant seedlings with and without inoculation of the bacteria in their respective hosts. Five-month-old seedlings were measured for the following variables: height, stem thickness, number of leaves, vigor, root length, root weight, number of nodules, leaf area, microbial respiration, and nitrogenase activity. We also quantified nutrient content in leaves and roots. The bacterial isolates were identified as Rhizobium miluonense, Rhizobium multihospitium, Bradyrhizobium japonicum, and Bradyrhizobium sp. All morphological variables were higher in inoculated seedlings compared to the uninoculated seedlings (p < 0.05), except for microbial respiration and root length. The Caesalpinioideae subfamily responded better to the inoculant than the Papilionoideae subfamily (p < 0.05). Enterolobium cyclocarpum, Cojoba arborea and Dalbergia retusa had higher nutrient levels at foliar and root levels when inoculated. This study provided insights into the potential of developing inoculants based on nitrogen-fixing bacteria, many of which are yet to be explored, for agriculture and reforestation with leguminous species.