Partial replacement of mineral nitrogen with vermicompost supports crop productivity and soil fertility in carbon depleted soils

The increasing instability of fertilizer supply chains, together with the progressive decline of soil fertility, is making nitrogen management a critical issue for intensive cropping systems. This challenge is particularly relevant in carbon-depleted soils, where low organic matter limits nutrient retention and reduces nutrient-use efficiency. In this context, this study assessed the effects of integrated vermicompost–mineral nitrogen management on soil fertility and plant performance in a lettuce–tomato cropping sequence. A controlled soil-based experiment was conducted on a sandy loam soil characterized by low organic matter (0.98%) and total nitrogen (0.078%). Six fertilization regimes were compared, ranging from full mineral nitrogen to complete substitution with vermicompost, under equivalent nitrogen inputs (210 kg N·ha−1 for tomato and 90 kg N·ha−1 for lettuce). In lettuce, marketable yield was maintained across fertilized treatments, reaching 1.45 kg·m−2 even at high substitution levels. In tomato, yield increased from 3.3 to 7.4–7.5 kg·m−2 under mineral fertilization and 25% substitution, whereas higher vermicompost proportions reduced yield to 5.1–6.0 kg·m−2. Nitrogen source affected nitrogen assimilation and plant physiological responses. Integrated fertilization enhanced nitrogen assimilation, with total free amino acids in lettuce increasing from 25.14 μmol·g−1 FW under full vermicompost substitution to 81.76 μmol·g−1 FW under V50-N50. In tomato, total amino acids reached 259.2 μmol·g−1 FW under mineral fertilization compared with 122.8 μmol·g−1 FW under full vermicompost substitution. Vermicompost application improved soil fertility in a dose-dependent manner. Under V100-N0, soil organic matter reached 1.6% in lettuce and 2.1% in tomato, while total soil nitrogen reached 0.14 and 0.18%, respectively, clearly exceeding both V0-N0 and V0-N100. These results show that the optimal substitution rate was crop-dependent. In tomato, V25-N75 and V50-N50 provided the best compromise between productivity, economic return, soil improvement and Sustainable Fertilization Performance Index (SFPI) performance, with values up to 0.73. In lettuce, higher substitution rates, including V75-N25 and V100-N0, maintained marketable yield while enhancing soil fertility, with SFPI values up to 0.91. The techno-economic analysis confirmed the need to balance short-term profitability with long-term soil restoration through crop-specific fertilization strategies. Therefore, integrated vermicompost-mineral nitrogen management offers a practical approach to reducing dependence on synthetic fertilizers, recovering soil fertility, and sustaining crop performance in intensive horticultural systems.