Compost and biochar synergy enhances photosynthetic efficiency and antioxidant responses in leafy vegetables under sub-optimal nutrient supply

• Compost and biochar synergy in leafy vegetables was investigated under sub-optimal nutrient supply. • In both spinach and rocket, C+B synergy under limited fertilization optimized N–C metabolic balance. • C+B treatment optimized photosynthetic function in spinach under low fertilization. • C+B synergy improved chlorophyll concentration and stress-related amino acid synthesis in leafy greens. Biochar and compost have been extensively studied as soil amendments useful for improving the physical and chemical characteristics of soils. However, due to their heterogeneous nature, they may induce different productive and physiological responses depending on the crop. Therefore, in this article, two growth chamber experiments were set up to test the short-term effects of compost and biochar applied alone and in combination (C+B) on the yield and morpho-physiological characteristics of spinach ( Spinacia oleracea L., exp 1) and wild rocket ( Diplotaxis tenuifolia D.C, exp 2) under optimal, sub-optimal, and non-fertilization conditions (Full, Half, NoFert). The two experiments showed that the C+B combination, under sub-optimal fertilization conditions, contributed to improving the balance of nitrogen and carbon metabolism. Specifically, the C+B mixture increased total chlorophyll concentration and promoted the synthesis of amino acids involved in osmotic regulation and nitrogen recycling in both rocket and spinach. The efficiency of the photosynthetic apparatus under sub-optimal fertilization conditions improved thanks to the combination of C+B in spinach, while the addition of compost alone led to excessive nitrate accumulation in the leaves. The total antioxidant activity of both crops improved due to the application of compost and C+B, especially in spinach, where plants treated with Half x C+B shoed a doubling of phenols. The smaller variation observed in wild rocket is likely due to higher baseline levels of phenols and antioxidants in the soil, which provide constitutive protection. In conclusion, the specific responses of the two crops are linked to intrinsic differences related to the species, highlighting the need to implement management strategies tailored to the crops.