Valorization of Ulva prolifera Biomass into Biostimulants: Effects of Extraction Processes on Cherry Radish Growth, Nutritional Quality and Soil Fertility

Frequent green tides of Ulva prolifera in China’s Yellow Sea generate massive algal biomass that pollutes coastal environments and disrupts aquaculture. Valorizing this waste into biofertilizers offers a sustainable strategy for marine ecological remediation and agricultural production. This study compared the effects of U. prolifera extracts prepared via enzymatic hydrolysis (UE) and enzyme-coupled microbial fermentation (UEF) on cherry radish growth, nutritional quality, and soil fertility across six concentrations (5–30%). Both extracts exhibited a typical biphasic “low-promotion, high-inhibition” dose–response pattern. UEF at 5–10% significantly promoted root development, with maximum root weight (5.1 g/plant) at 10% and diameter (1.7 cm) at 5%. UE at 15–20% achieved root weight comparable to the Hoagland solution (HO), but with lower peak values than UEF. At equivalent concentrations, UEF generally outperformed UE in enhancing nutritional quality—maximizing anthocyanins (4.8 mg/g FW), vitamin C (5.2 mg/g FW), soluble proteins (13.5 mg BSA/g DW), and soluble sugars (17.0 mg glucose/g DW)—as well as soil fertility (N/P/K contents). Both extracts lowered soil pH by 0.5–0.6 units (converging to ~7.1 after 40 days), suggesting their potential for saline–alkali soil improvement. These findings suggest that U. prolifera extracts, particularly those prepared via enzyme-coupled fermentation, may serve as promising dual-function biostimulants and soil conditioners. The optimal concentrations (5–10% for UEF; 15–20% for UE) provide practical guidance for field application, offering a “waste-to-value” solution that simultaneously mitigates green tide hazards and supplies sustainable agricultural inputs, thereby contributing to both marine ecological restoration and sustainable crop production.