Valorization of food waste-derived aqueous phase from hydrothermal carbonization as liquid fertilizer: a case study for plant growth and soil fertility

Considering the valuable resources in food waste (FW), using hydrothermal carbonization has been proposed as an effective way. The liquid product aqueous phase (AP) is rich in nutrients and artificial dissolved organic matter (DOM), has been identified as a promising liquid fertilizer. However, the effects of FW-derived AP fertigation on plant agronomy and soil fertility have not been thoroughly investigated. Herein, tomato and lettuce cultivations were conducted in substrate and sandy soil with diluted AP, alongside tap water (CK) and chemical fertilizer (CF). In this study, AP retained a large quantity of organic and inorganic nitrogen and diluting it 150-fold eliminated its phytotoxicity. While AP yielded tomato seedling biomass comparable to CF in substrate; In sandy soil, AP significantly increased lettuce biomass by 16–26.7% across three seasons. Soil fertilized with AP exhibited greater soil nitrogen content, higher enzyme activities and increased soil microbial biomass, without causing sodium accumulation. Regarding soil DOM, AP increased the number of formulas by 50.8–62.8%, but decreased the molecular weight by 21.8–24%, compared with CK and CF. AP significantly elevated the levels of lignin- and carbohydrate-like compounds in soil DOM, stimulating both soil humification and carbon lability. AP decreased bacterial abundance but increased fungal abundance in the soil. AP attracted rhizosphere microorganisms like Proteobacteria and Actinobacteria in bacteria and Basidiomycota and Ascomycota in fungi, which facilitate organic matter degradation and nutrients cycling. This pilot case study confirms that AP is both technically and economically feasible for use as a liquid fertilizer.