Eco-functional deployment of indigenous nitrogen-fixing microbes to enable temperate crop cultivation in tropical climates

Climate change and increasing food demand pose major challenges to agricultural productivity, particularly for temperate crops cultivated under tropical conditions. This study investigated whether indigenous bacterial isolates obtained from cooled tropical soils exhibit putative diazotrophic traits and promote the growth of lettuce, a representative temperate crop, under tropical greenhouse conditions without soil cooling. Seven bacterial isolates were initially screened using preliminary assays indicative of putative diazotrophic traits, including nitrogen-free semisolid medium, bromothymol blue assays, and PCR detection of nif-related genes. Two strains, Agromyces sp. C10 and Bacillus sp. C21, were selected for further evaluation. Inoculation with these strains enhanced lettuce biomass in the presence and absence of fertilizer and was associated with increased total soil nitrogen after harvest. Although these results do not provide direct functional evidence of biological nitrogen fixation, they indicate that indigenous bacteria from cooled soils may serve as promising plant growth-promoting candidates for temperate crop cultivation in tropical environments.