Sludge-based liquid fertilizer enhances tomato yield and quality in greenhouses by boosting soil active organic carbon content and microbial activity

Over 40% of protected agricultural soils in China suffer from acidification and secondary salinization due to long-term excessive nitrogen fertilization, hindering green sustainable development. Alkaline thermal hydrolysis (ATH) liquor, a high-value liquid fertilizer derived from sewage sludge, is enriched in protein-derived nutrients (proteins, peptides, amino acids) and biostimulants. It not only enhances crop quality but also realizes municipal sludge valorization. This study explored ATH liquor’s nutrient profile, environmental safety, and effects of its partial chemical fertilizer substitution on greenhouse tomato yield, fruit quality, and soil fertility, to determine the optimal ratio. Results showed that ATH liquor is abundant in proteins, peptides, amino acids, and organic carbon, with heavy metal contents well below the safety limits specified in agricultural standards and no detectable phytotoxicity. Among all treatments, 50% ATH liquor substitution yielded the highest tomato yield and net economic return, increasing by 21.48%-26.12% and 22.88%-24.23% respectively compared to the full chemical fertilizer treatment (M0) across the two-year field trial. For fruit quality, 30% ATH liquor substitution was optimal, with significant enhancements in key nutritional constituents. Additionally, the substitution of chemical fertilizer with 30%-50% ATH liquor effectively enhances the activities of soil dehydrogenase and carbon-degrading enzyme (soil β-glucosidase), as well as the soil quality index (SQI). Comprehensive assessment identified 50% ATH liquor substitution as the optimal regime, synergistically improving agronomic productivity, economic benefits, and ecological sustainability. These findings support scientific ATH liquor application for facility tomato production and soil quality improvement, promoting sludge resource utilization and green development of protected vegetable cultivation.