Hydrochar amendment in weathered soil: impacts on fertility, plant growth, and herbivore fitness in the soil–plant system

Background and aims The application of hydrochar (HC) to soil has beneficial effects, but few studies have evaluated its impact on soil–plant-insect systems. This study investigated the effects of HC derived from sugarcane vinasse and bagasse on soil properties, dissolved organic matter (DOM), plant development, and the performance of a key herbivore. Methods HC application was performed on maize and tomato crops in an Ultisol at doses of 5, 15, and 30 t ha −1 . Plant growth parameters (biomass, length, height, and chlorophyll index), soil β-glucosidase activity, and the quantity and quality of leached organic matter were measured, along with Spodoptera frugiperda (fall armyworm- FAW) fitness traits (larval viability and weight). Results In the short term, the lowest HC dose significantly enhanced tomato growth (p < 0.05). The responses of β-glucosidase activity and soil DOM indicate that enzymatic stimulation depends on both the HC rate and the presence of mineral fertilization. Furthermore, HC induced plant resistance, as evidenced by antibiosis effects on FAW (p < 0.05), reducing larval viability by up to 44% when insects were fed maize leaves, in addition to sublethal effects such as decreased larval weight, and for tomato, an average reduction of 22.96% in viability. Conclusion Therefore, the application of HC as a soil conditioner, in addition to its effects on soil fertility and plant growth, has been shown to have an antibiosis effect on the fall armyworm (FAW). Further investigations, such as the expression of genes and metabolites of the plant defense system, need to be conducted to elucidate the mechanisms of action involved.