Recycling Fruit and Vegetable Wastes into Fermentation Broths and Effects of Their Application on Soil Properties and Crop Growth

Fruit and vegetable wastes can be recycled into value-added fermentation broths (FBs) through anaerobic fermentation, but the characteristics of FBs and the effects on soil ecological processes remain insufficiently understood. This study analyzed FBs produced from 14 wastes and selected five FBs (garlic, tomato, sweet potato, apple, and lettuce) for pot experiments. The results showed significant differences among the FBs in nutrients, enzymes, and microbial communities. Garlic FB had the highest concentrations of ammonium nitrogen (309.81 mg/L), total phosphorus (327.73 mg/L), total potassium (1365.8 mg/L), and organic matter (28.99 g/L), along with the highest activity of acid phosphatase, urease, protease, and catalase (p < 0.05). FB application improved soil nutrient availability and enzyme activities, with garlic FB showing the strongest effects, increasing catalase, urease, acid phosphatase, and β-glucosidase by 83.34%, 180.72%, 112.34%, and 21.95%, respectively. Metagenomic analysis revealed that the soil treated with garlic FB contained beneficial taxa, including Saprospiraceae, Chitinophagaceae, Azotobacter, and Sphingomonas, which are associated with enzyme production and organic matter decomposition. Furthermore, the application of FBs reduced the incidence of downy mildew and leaf spot and promoted the growth of Brassica chinensis. Though chemical fertilizer produced the highest biomass due to immediate nutrient availability, the FB treatments, especially garlic FB, showed advantages concerning soil health, disease suppression, and sustainability, highlighting their potential for organic waste recycling and sustainable agriculture.