Enhancing Soil Health, Growth, and Bioactive Compound Accumulation in Sunflower Sprouts Using Agricultural Byproduct-Based Soil Amendments

This study investigated the effects of organic soil amendments derived from agricultural byproducts—specifically cow manure (CM) at 0% and 1% w/w, and rice husk biochar (RHB) at 0%, 1%, 3%, and 5% w/w—on soil health, plant growth, and the accumulation of bioactive compounds in sunflower sprouts. The application of 1% CM significantly improved the soil properties—enhancing macroaggregates (MaAs) by 54.5%, mesoaggregates (MeAs) by 16.7%, and soil organic carbon (SOC) by 27.2%. It also increased the shoot and root biomass by 22.3% and 25.8%, respectively, and boosted soil respiration by 67.0%, while reducing the nitrate (NO₃⁻) content by 33.7%. However, the CM also decreased the total phenolic content (TPC) by 21% and chlorophyll by 44.7%. The RHB, particularly at rates of 1–3% w/w, increased the MaAs by 62%, microaggregates (MiAs) by 3%, leaf area by up to 43.9%, root-to-shoot ratio by 26.5%, SOC by 13.1%, and DPPH antioxidant activity by 42.8%, while lowering the MeAs by 9% and NO₃⁻ content by up to 56.1%. In contrast, excessive RHB application (5% w/w) negatively impacted root development. The interaction effects revealed that the combination of 1% w/w CM with 1% w/w RHB maximized the MaAs by 12%, increased the root dry biomass by 101.9%, and also increased the TPC by 40.1% compared to the manure-only treatment. The Principal Component Analysis (PCA) indicated that CM primarily promoted plant growth and respiration, while RHB contributed to organic matter retention and nutrient availability. Applying 1% w/w CM and 1% w/w RHB showed promising effects and is recommended for short-cycle crop production.