Fermented Botanical Product Modulates Soil Bacterial Communities and Enhances Plant-Growth-Promoting Activity for Sustainable Agriculture

Although biostimulants have attracted attention for sustainable agricultural systems, their efficacy remains poorly understood. In this study, we evaluated the effects of fermented botanical product (FBP) produced by fermenting and aging 41 types of fruits, grains, seaweed, and root vegetables with brown sugar for more than three years. Three crops, tomato, rice, and komatsuna ( Brassica rapa ), were cultivated with the application of 5,000- or 10,000-fold diluted FBP in greenhouses or fields. Application of diluted FBP promoted plant growth, as indicated by increased fresh weights of shoots, leaves, and roots, fruit production in tomato, and rice husk yield. As diluted FBP contained low nutrient levels, an indirect mechanism of plant growth promotion was suggested. Bacterial community structure analysis indicated changes in alpha diversity, beta diversity, and the predominant phyla in FBP-applied soils without plants and in soils cultivated with tomato, rice, and komatsuna. In addition, the abundance of plant-growth-promoting bacteria, such as Arthrobacter , Pseudomonas , Paraburkholderia , and Planifilum , increased in soils treated with diluted FBP. Furthermore, ammonium formation activity was observed in komatsuna cultivation soils treated with diluted FBP, whereas phosphate-solubilizing activity was enhanced in soils from all three crop cultivation systems treated with diluted FBP. These results suggest that diluted FBP influences bacterial communities and promotes crop growth through indirect effects, including increases in plant-growth-promoting bacteria, ammonium production, and phosphate solubilization. Alternatively, FBP may directly stimulate plant growth. Therefore, FBP may be a useful biostimulant for sustainable agricultural systems.